Wednesday, September 17, 2014

Assembling the mcHF transceiver boards - getting ready.

If you remember last time, a few of the parts were still on their way - but since then, all have arrived and I have assembled one set of boards and gotten the transceiver operational.

But I'm getting ahead of myself.

Starting the build:

As noted before, there are 480-ish parts - most of them being 0805-sized surface-mount resistors and capacitors - these being somewhat smaller than grains of rice - so I set about putting together my assembly area by buying a shallow baking pan.

The thing about these tiny parts is that no matter how cheap or expensive they are, they are equally invisible when they fly away accidentally -  or just fall off the board, and even if the part costs less than a cent, if you don't have any more, it will then cost you several dollars to get more of them.  (For the really inexpensive parts, just get several times the quantity that you need!)

I lined the bottom of the baking pan with white paper for both electrical and thermal insulation and started putting parts on the UI (User Interface) board.

At this point I should say that having put it together, I learned something that anyone putting one of these together should consider:

A few mistakes:

Assemble the RF Board FIRST, then assemble the UI board!

The reason for this is quite simple:  There are no really tricky parts on the RF board and by the time you finish it, your SMD soldering skills - whatever state they might have been in when you started - will be much improved - and this will come in handy when you solder the fine-pitched 100 pin microcontroller on that main board to it!

I didn't do that.

In following the recommendation of someone else who'd put one of these together, one of the first things that I'd put on the board was, in fact, the microcontroller.  Although I have soldered a fair amount of SMD before, it had been several months since I'd done it and I was a bit rusty.

Also, when I started soldering the microcontroller, I'd not noticed that the PC board was somewhat warped, probably a result of having been through the post from the UK, and that after I'd finished soldering the pins down a large percentage of them did not actually make contact to their pads because of the warpage!  Had I my wits about me I would have removed the microcontroller, clamped the board down to flatten it and done it again, but I didn't do that:  Instead, I went around, pin-by-pin with an extremely fine-tipped iron and tacked each one down, laboriously checking it and its destination for continuity until I'd finally gotten them all - a process that took over an hour and actually resulted in my ripping one of the pads off and a bit of microsurgery on the board to make the connection.

To reiterate:

1) Build the RF board first to get practice at soldering SMD components
2) Before soldering the microcontoller, clamp the board down to assure that it is perfectly flat!

Equipment:

I employed several methods in populating the boards.

The first thing that I did was to wash the boards in soap and water, scrubbing fairly vigorously then patting them dry, followed by wiping them down with denatured alcohol to remove adhesive tape residue from the packing and residual oils - followed by another wash with soap and water and a good rinse and dry.

For soldering equipment, I would recommend, at a very minimum,  the following:
  1. A TEMPERATURE-CONTROLLED iron.  The iron should have a closed-loop feedback in it to maintain a constant temperature and not merely a dial to make it hotter or colder.  DO NOT attempt to solder this sort of thing if your iron does not have the ability to maintain its temperature to within 10 degrees C (about 20 degrees F) or so of its setpoint or you can damage the board.  Such irons are pretty cheap and plentiful these days so there's really no excuse for not having one if you are building these sorts of projects!
  2. Fine (small) solder braid.  This is the small, thin solder braid (the trade name being "Solder Wick") that is roughly 2.5mm (1/8") or so wide or smaller.  It really needs to be this small in order to fit between pins of SMD devices to suck up extra solder.
  3. Small-diameter solder.  This would be the solder that is anywhere from the diameter of a hair to the diameter of #24 AWG wire.  In the U.S. you can still get the 60/40 or 63/37 Tin/Lead solder - which melts at a lower temperature and has better longevity and chemical stability - but good-quality lead-free solder will also work.  If you do not use rosin core solder, read the instructions with the solder carefully as some of the "organic" fluxes MUST be immediately washed off after soldering to prevent electrically conductive residue and/or corrosion from occurring!
  4. Good-quality tweezers.  I have both metal and bamboo tweezers that are helpful for both picking up parts and holding them down during soldering.  The bamboo tweezers, bought for less than $1.00 on EvilBay (including shipping), are nice in that they are insulating both electrically and heat-wise, the latter making it easier to solder down parts.  Flat toothpicks also work very nicely for holding down components for soldering.
  5. Magnifiers!  Unless you have extraordinarily good close-up vision, you will want some good magnifiers.  These can be in the form of very strong glasses or binocular goggles - some of which have extra flip-down lenses to change magnification - for hands-free use as well as jeweler's loupes and strong magnifying glasses for extreme close-up examination of solder joints and circuit board traces.
  6. Volt-Ohm Meter.  An absolute must for troubleshooting, measuring voltages and resistance to determine if something is amiss.  It need not be fancy, but a cheap, $5 digital meter is probably better than a $10 automotive-type analog meter!
A few very nice things to have:
  1. Solder Paste.  Using a dispenser - often in a small, hypodermic syringe with a large-ish gauge needle, this contains both paste and flux.  This is applied to the pads and then the part applied into the paste and then heated with hot air - see below.
  2. Hot air rework tool.  These are essentially a miniature heat gun, but with fairly precise temperature control and the ability to be fitted with different tips to heat various areas and shapes.  If you need to remove a multi-pin chip such as the 100 pin microcontroller, this is going to be the ONLY way that you going to be able to do it without damaging either the chip or the board!  This also allows you to solder many components at once if paste has been applied to their pads and then the components applied to them.
  3. Flux pen.  This looks like a felt-tip pen, but filled with liquid solder flux.  It is a good idea to apply this to each solder pad on the board to make it easier to solder, making better and more-reliable joints.
 Methods:

There are (literally) hundreds of web pages and YouTube videos on how to solder - specifically, how to do surface-mount soldering, so I won't explain the exact methods - just name them.

 The "dab, place and solder" method:

For the individual, small, passive components such as resistors, capacitors and inductors I generally did the "dab, place and solder" method in which I would apply the solder paste to the pads of 4-8 components, place the components on those pads, and then, with the fine-tipped iron and either a toothpick or the bamboo tweezers, hold the component down while soldering.

When doing this it important to only place 4-8 components at a time because:
  • It is best to place the component in paste when it is still "wet" so that it not only sticks.
  • Doing a batch of 4-8 components is more efficient than doing one at a time, but not so many that you are likely to forget where each component that you just place is.
This last point is actually quite important because one tends to place all of the same types of components at a time.  For example, say your project has 25 0.1uF capacitors in it, scattered all over the board:  If you just place 4-8 of them (I would always place 6 components - an easy number to remember) then those 6 components are easier to find, particularly when a lot of components have already been placed on the board and it is getting harder to tell, at a glance, one that is soldered from one that has not been soldered!  By counting up to 6 each time (or fewer, if you didn't have 6 of whatever it is to solder) you can be sure that you found them all!

After soldering them down, I would then run my finger over the board to discover any "loose" components - that is, ones that were stuck to the board with paste, but not soldered, and hopefully I'd be able to quickly discover where it was supposed to go!

For soldering multi-pin ICs, there are many methods using soldering irons.  Most of these involve tacking a corner pin or two and going from there, sometimes soldering each pin at a time if the pin pitch isn't too small.

One of the more popular methods involves "flooding" the pins with solder, often by dragging the tip across them all to distribute it to all of the connections and then sucking up the excess with solder braid:  You can find web pages and YouTube video describing this "Drag" method in detail.

Of course, if you don't have solder paste, the use of very fine wire solder works fine as well with the appropriate adaptations of the above techniques.

The hot air method:

The above method works well if you are soldering same-value components scattered all over the board, or if you are attaching components with larger pin counts.  If, however, you are able to populate a lot of components in a small area of a board it will be quicker and easier if you were to use the hot-air tool.

In that case, you would apply paste to the pads and then place the components into the paste - this time, for (possibly) many more components.  Then, very gradually, heat up the area with the components until the solder flowed completely.  In this case, with the components concentrated in one small area rather than all over the board, it makes sense to spend the time to bring up the temperature gradually.

One point of concern with the paste - particularly if it is heated too quickly - is that the liquids within it can flash-vaporize and bubble within:  When these bubbles pop, it can cause the part being heated to jump off the board, or at least out of position, requiring you to stop what you are doing and relocate the part.  If you are finding that the parts jump around when you are heating them, you are probably heating them too quickly!

For soldering larger devices such as multi-pin ICs, the typical methods include laying a thin line of solder paste along the line of pads rather than a dot on each pad:  Methods for doing this may be found on many web pages and YouTube.

One advantage of having a hot air gun is that even if you do use a soldering iron, if the solder masks were applied properly, reheating components that were previously soldered with an iron will often make them re-align and straighten up and make their joints look more uniform.  It also has the advantage that if you happened to miss soldering a joint to which you'd applied paste, it will now be soldered!


Next time on this topic:  A few pictures and comments on the actual assembly!


Tuesday, August 19, 2014

YAFP - The mcHF SDR HF Transceiver

Those who know me are already familiar with a four-letter acronym that I've used for many years:

YAFP

Which stands for:

Yet
 Another
  Fu__n_
   Project

What, exactly the "F" stands for is up to you, but the sentiment is the same, regardless:  Too many fun projects, too little ($#!+) time!

Falling into this category is one that a friend of mine, Brett, N7KG, made me aware of and that is the "mcHF", a small, (relatively) inexpensive SDR-based HF (10 watts, 80-10 meter, SSB-CW) transceiver, based around the STM32F4 processor, designed by U.K. amateur Chris, M0NKA - see the web site:

http://www.m0nka.co.uk/

This radio is definitely NOT a kit.

What you can get from from Chris is:

  • Two circuit boards
  • A full-color TFT LCD display
And from the web site:
  • Documented source code
  • Information on how to use the bootloader
  • Where to get the compiler and the various utilities.
  • Pre-compiled binaries
  • How to compile your own binaries
And that's it!

If you want to build the radio, you must get - on your own - all of the other parts to populate the board - which numbers around 480 or so!  Fortunately, the vast majority of these (numerically speaking) are things like surface-mount resistors and capacitors that are on the order of a penny or two each.


The web site above contains a "BOM" (Bill Of Materials) but for the U.S. amateurs, this is a bit awkward to use as it uses mostly Farnell U.K. part numbers.

Figure 1: 
The boards and LCD supplied by M0NKA.  At the bottom-left is the RF board while
the upper-right is the CPU/User-Interface board.
Other than source code, instructions and some binaries, everything else is up to YOU, including
the case which you must figure out how to get made using someone's 3-D printer!
Click on the picture for a larger version.
For the convenience of others, I've gone through the pains of cross-referencing those parts to Mouser Electronics part numbers (in most cases providing several alternates for the same part) and where the part was not available from Mouser, offered a Digi-Key alternative and some may even be obtained on Ebay:  This alternate Bill of Materials may be found in the "KA7OEI" folder of the FILES section on the M0NKA mcHF Yahoo Group which requires membership access, or you could send an email to me and I could pass it along.  Even if you were to choose not to obtain the parts from Mouser and/or Digi-Key, at least you can figure out the specifications of these parts and figure out what, exactly, they are and where to get them.

There are a few parts, notably the ferrites, RF output transistors that are not available from either Mouser or Digi-Key, and these may be obtained either via Ebay or other suppliers.  There are a few specific parts, however - the 3.5mm connectors for the audio input, output and key connectors and for the DC power connector - that do NOT seem to be available from ANY U.S. supplier and unless one "kludges" alternatives must simply be ordered via Newark/Farnell from their U.K. distributor!

There is some "support" in the form of a Yahoo group mentioned above and some exchange of ideas on the website forum itself, but this is, as they say, open source in terms of both hardware and software.

At the moment the only source of circuit boards is from Chris, M0NKA himself and a set of these, shipped to the U.S. along with an LCD will run about $68 (as of August 2014) - if he has them in stock.  As of this posting he does not currently plan to make the board layout files available, but since the schematic diagrams are published, there's nothing stopping anyone from laying out their own board!

This sort of project is definitely NOT for everyone as this is very far from "plug and play":  The software is a bit rough around the edges and the feature set presently extant is a bit sparse, but it is reportedly functional and open-source and a quick look at the source code reveals that it shouldn't be too terribly hard to add user-interface bells and whistles while the addition of other features like additional audio filter bandwidths shouldn't be too difficult, either.

What would take a bit more work would be the addition of things like DSP noise reduction, other operational modes like AM and FM and the addition of stand-alone support for digital modes like PSK-31, RTTY (to name but two) and the self-copying of CW, for example, although this sort of thing should be possible... in theory...

Where it stands:
 
Figure 2: 
The box into which the accumulating parts for the
radios are being tossed as they arrive!

Myself, Brett and Bryan, W7CBM are putting together four of these radios between us (I'm building two) and we have almost all of the parts on hand, missing only a few components on the RF board:  Those are on their way from Mouser or from the EvilBay vendor.  Meanwhile, the 3.5mm and power connectors are reportedly on their way from the U.K. at this moment.

The next step - which might be the most difficult of all - will be finding time for the three of us to get together and get these small parts on the board and do some assembly and testing!


As I said before:

YAFP!

Monday, August 18, 2014

Completely containing switching power supply RFI

In the old days, radio amateurs were concerned with (or should have been) energy from their transmissions getting into devices unintentionally, the classic being televisions, phonographs, telephones, hi-fi sets, and the like.

A few years ago hams' hackles were raised with the prospect of BPL - Broadband over Power Line - a system by which the already-extant infrastructure used to convey electrical power would be used to transport data all about the land.  While it did work (sort of) it had the potential to cause a great deal of interference to amateur radio operators.

A lot was written and to their credit, some designers/operators even designed their systems to avoid putting energy within the HF amateur bands - to varying degrees of success.  While this wouldn't have really helped the causal shortwave listener, it did still not address the fundamental problem that the power lines were simply not suitable, low-loss, low radiation transmission media for radio frequency energy.

What we really should have worried about was not BPL...

Figure 1:
The computer power supply making RF noise up and
down the HF bands.
As it turns out, when it comes to worrying about devices that had the potential to clobber our HF bands, we really should not have worried too much about BPL - which, as hindsight has proven, wouldn't have gotten anywhere, anyway, but rather devices that are right under our noses:  Switching power supplies - particularly the cheap, lightweight ones that are now supplied with everything that we buy and even put in our own shacks!

These inexpensive "wall warts" used to consists of a small, iron and copper transformer - often with a rectifier and capacitor.  These devices would plug into the wall and operate, typically for 5-10 years until whatever it is that they were powering wore out.

Unfortunately for them, they would consume 1-5 watts all of the time just sitting there doing nothing, even when the device was "off" - the so-called "phantom loads" or "power vampires" and many locales/countries have legislated them out of existence in favor of the newer, much more efficient switching-type devices.

All would be good except for two things:

The first of these is that many of these cheap switching-type wall warts last only 12-24 months before dying - usually a victim of an inferior quality capacitor and/or poor design.  What this means is that more often than not, the device to which they were attached is often thrown out as well.  While this new-style switching-style wall-wart may take less power to operate, it is my guess that considering that its premature failure caused a premature product replacement, it never actually saved any energy - since it probably took a lot of energy to make the device in the first place - or saved the consumer any money!

Stepping back off the soapbox, these switching supplies - even if well-built and long-lasting (if you are lucky enough to encounter one) bring us to the second of the two problems concerning us about these devices:  The generation of RFI, or Radio Frequency Interference.


Such was the case with one of these devices that I use on my TV to run a small multimedia computer.  This computer, obtained surplus, did not come with its original supply so I found an OEM Dell laptop supply of  reasonable quality and suitable ratings - about 19 volts and 3 amps.  There was one problem:  It seemed to radiate a low-level RFI signal that got everywhere on HF.

Figure 2: 
Configuration showing the interconnects and where the RF circulating currents are flowing.
The conduction of RF currents onto the AC power, speaker and outside antenna leads
assured that it was being radiated far and wide!
Now part of this was how it was connected - See Figure 2, above:
  • The power supply was connected to the AC power line.
  • The power supply was also connected to the TV through the video/audio cables.
  • The TV was connected to the high-power stereo system which, in turn was connected to speakers in different parts of the room.
  • The TV was also connected to a coaxial cable that went to the rooftop antenna.
What this meant was that this power supply was, itself, indirectly connected to both ground - via the power line - and several forms of antennas, via the TV, TV antenna, and speakers.

Whatever low-level RFI was being produced by this power supply had exactly what it needed to be conducted out into the world and cause problems:  A complete path in and out of the power supply!

What is sounded like:

Typically, switching power supplies sound like a "buzz" every 30-60 kHz - the power supply's switching frequency - up and down the bands, usually worse on lower bands, but not always.  This buzz is usually modulated at twice the power line frequency (120 Hz in the U.S., 100 Hz in most locations in Europe, Asia and Africa) but this modulation is usually very "dirty" and full of harmonics:  If the radio is switched to "AM" mode the "buzzy" nature of the modulation becomes much more apparent!

It is often the case that the 30-60 kHz intervals at which the interference is "bunched" together are more apparent at lower frequencies such as the AM broadcast band and 160 through 80 meters (1.8-4 MHz) - that is, one can more clearly hear the distinct switching supply "carriers".  As one moves up in frequency the interference may sound like it is decreasing, but this may not actually be the case as these "bunches" of energy often get spread out, changing from a fairly sharp "buzz" as you tune across the switching harmonic to more of a "hiss" and in severe cases - and on higher bands - these "bands of hiss" may actually run together.  In the latter case, it may not, at first glance, sound like a switching supply at all, but rather just an elevated noise floor!  In this case it may not be until one switches to AM and notices that this "hiss" has a powerline frequency AM component to it and/or that it disappears when the power is removed from the supply that it is, in fact, from a switching supply!

The latter was the case of the power supply depicted in Figure 1:  On 160 meters it could be heard every 60 kHz or so as a "dirty" buzz, but on 40 meters it was an indistinct rise in the noise floor of about 2 S-Units that was about 10 kHz wide while on 20 meters it just seemed to raise the noise floor by 1-2 S-Units everywhere that, to the uninitiated, didn't even resemble noise from a switching power supply!

It should be pointed out that I'd already modified this power supply to reduce its conduction onto the AC and DC power leads and that had solved one problem - bothering a receiver that was located next to it - but the lower-level, HF frequency energy that was induced across the power supply between its AC input and DC output was much more difficult to manage as that was not a matter of either shielding or direct power line conduction.

Since I'd already gone out of my way to add bifilar chokes to both the AC and DC leads of this power supply, I'd likely reduced its potential to emit energy by a significant amount, but here, we are talking about residual amounts that are being coupled into what amounts to antennas that are connected to my TV system and being picked up by a sensitive HF receiver.


Before we continue on, let me say a few things about what won't work to fix this.

What will NOT work to solve this problem:

Ferrite beads and snap-on chokes!

Ferrite beads and snap-on chokes will not likely solve this sort of problem because what is needed is to prevent the egress of the RF energy from the switching supply one or more of the following:
  • Very high series reactance to block RF energy
  • Shunting of RF energy to a common path on the input and output of the power supply to prevent it from circulating elsewhere.
Simply put, a simple, ferrite bead or snap-on ferrite cannot practically introduce enough inductive reactance to effectively knock down the RF energy to the degree that we might like.  While it may reduce the energy by a few dB, it is often the case that we need to reduce the RFI by a few 10's of dB and more aggressive filtering is usually required to do this!

Ferrite beads and snap-on chokes are better at minimizing the ingress of energy to reduce its potential of being bothered by external RFI than they are at eliminating the emission of RFI in the first place.  In other words, the reactance that they add to the interconnect leads gives whatever built-in RFI immunity the device already has more of a chance of working.  They are much less effective in quashing the emission of RFI by that device in the first place.

To get enough inductance to present a high inductive reactance at the lowest desired frequency it is often required that many turns be wound on a piece of ferrite, but the size of the core, the diameter of the wire - and even the length of the wire - usually precludes putting more than a turn or two on all but the largest core.

In our case, I'd already have installed additional filtering in the power supply - and it wasn't enough - so we are going to attack this problem using the second of the above techniques:  Shunting the RF energy to a common path.

I knew now that I had to do the complete "filter job" on this power supply.

Having had to do this before on other power supplies, I gathered the necessary parts - this time, documenting my efforts for this blog:
  • Dead PC power supply, complete with case and power cord.
  • Two brand new low-ESR electrolytic capacitors of suitable voltage for the DC power supply, capacitance between 100uF and 1000 uF.
  • Two monolithic 0.1uF ceramic capacitors of suitable voltage for the DC power supply.
  • Terminal strip.
  • A piece of perforated prototype board.
  • Misc. screws/hardware for standoffs.
  • A piece of plastic for a shield - see text.
  • Four self-adhesive rubber feet.
  • Some soldering skills.
  • A bit of common sense!

Before I go on I must spout out a few weasel words of warning:
  • This project involves hazardous/lethal AC power/mains voltages!  DO NOT undertake this project unless you have experience with such voltages and the necessary safety procedures in dealing with them!
  • Please observe the safety regulations and requirements for your locale noting that the methods described here may not be suitable for your area!
  • You MUST make certain that the components that you use are rated for the voltage/current at which they will be operated!
  •  YOU are responsible for your own safety.  I cannot be held responsible for damage, injury, accidents or even death that might occur by following - or failing to follow - any instructions or recommendations on this page!
  • If you do not feel comfortable working with high voltages and currents or do not have familiarity with wiring procedures and safety related to such, PLEASE do not even think of doing so!
  • Figure 3: 
    The discarded PC power supply case, stripped of its insides leaving
    only the power receptacle and the on/off switch.
    Click on the image for a larger version.
  • YOU HAVE BEEN WARNED!!!

Gathering parts:

The first thing to do is to gut the PC power supply, leaving in the case the connector for the power cord and the on/off switch if it has one.

Please be aware that the input capacitor of the power supply may retain voltage even if it has been powered down for a long time - check and discharge it if necessary.

The picture shows several of the parts that you will need from the power supply:
  • Bifilar input choke.  This could either be toroidal, or look like a transformer.  Make certain that you identify the two "halves" of the inductor:  AC power will flow through each half, separately.  These inductors will have values of 100uH to 50 mH per half, depending on the source.  Those depicted in Figure 4, below, measured about 4.5 mH per half.
  • Common-mode capacitor.  This will typically have a value between 0.047uF and 0.22uF and will be connected directly across the AC line - usually located right next to the bifilar input choke.  In the U.S., these capacitors are typically 0.1-0.22 uF.
  • Two identical high-voltage bypass capacitors:  These connect from each side of the AC supply and go to the case ground.  These are typically blue or yellow and have values from 1000pF to 4700 pF (e.g. 1nF to 4.7nF).
  • The safety fuse(s) from the power supply - if they are not blown.  In the U.S., there is typically only one fuse found on the "Line" (black wire) side of the AC input.
  • Figure 4: 
    Parts needed for the AC input filter, found on the discarded PC
    power supply:  The fuse, the common-mode
    capacitor (the yellow block), the common-mode choke (the
    toroidal inductor with two halves) and the two blue disk-
    ceramic capacitors.
    Click on the image for a larger version.
  • Another Common-mode capacitor.  If you have another PC power supply to scavange - or if the power supply that you have has one, get from it another common-mode capacitor of the same description as above.  This is is optional.

Comment:
  • It has been noted that some REALLY CHEAP and/or "suspected origin" power supplies have been spotted that have none of these RFI suppressing components - or even a fuse - even though their cases had a "UL" and "FCC" certification sticker on them!  In this case, it was probably just as well that the power supply was pulled out of service as their were neither safe or compliant with regulations! 

Warning:
  • All of the capacitors should have on them explicit AC voltage ratings consistent with those of the mains voltage in your area.
  • DO NOT use any capacitor unless it has printed upon it the proper voltage rating!  The capacitors typically used for these applications are usually blue or a light yellow color and have printed on them an AC voltage rating.
Note:
  • You may be able to find a pre-built filter unit that has a standard IEC (e.g. "Computer Plug") connector on it that you can mount to the power supply case, saving you the trouble of building a filter.  These units may be found both new and surplus.  You can also find such filter units pre-built as depicted Figure 5, below
Figure 5:
Various styles of bifilar inductors that may be found in scrapped
switching power supplies - plus a complete, self-contained
AC RFI filter built into an IEC power connector
in the upper-left corner.
Click on the image for a larger version.
Constructing the filter:

 The schematic diagram of the filter is shown below.

The filter is of the so-call "Brute Force" type and it is a common-mode low-pass filter that removes high frequency content from the AC power line.  Because our main goal is to contain the RFI within the box, any RF energy from the switching power supply first hits the common-mode capacitor which forces it to be equal on both sides of the AC power line.  The RF energy then hits the bifilar RF choke which then cancels out any energy that is equal on both sides of the AC power line - a condition that was just enforced by the common-mode capacitor.

Any RF that managed to make it through the bifilar choke will now be greatly dimished and it is now shunted by the two capacitors to the metal case to ground while the (optional) common-mode capacitor on the AC input side reinforces the equilibrium of any RF energy that might be on that common-mode choke.
Figure 6: 
The completed AC input filter, constructed on a piece of phenolic prototype board.
This one has a common-mode filter on both the input and output.
Click on the image for a larger version.

The filter shown was built on a piece of phenolic prototyping board, maintaining at least 1/2" spacing (12mm) between any two points that carry mains voltages or between a mains voltage and/or a ground point.  On the bottom, short pieces of solid bus wire were used to interconnect components and to make the loops used to solder the interconnecting wires.

As can be seen, the power supply's original fuse was retained and used on the "line" (hot) side of the AC input of the filter as a matter of safety.

Figure 7: 
Schematic of the AC input filter.
Note:  Typically, a 100k-1Meg "safety" resistor is connected across the mains (on either side of
the inductor) to discharge the capacitors should it be disconnected while the AC sine wave
is at either peak.  This is not shown in the above diagram since this resistor was already present
in  the power supply to which it was connected.
The phenolic board was mounted on the side of the PC power supply case, but to protect it from items protruding into a vent hole and causing a short or electric shock, a piece of heavy plastic larger than the perfboard was cut out and mounted against the case.  This piece of plastic was cut from a discarded "blister pack" that had contained items bought at the store and was fished out of the trash can:  It just so-happened that there was a large enough portion of flat plastic to accommodate my needs and it made a nice, durable and free shield!

The board was mounted using 6-32 screws and spacers as standoffs to hold it about 1/4" (5mm) or so from the side of the case.  For a ground connection, a ring lug was put under one of the screws and soldered to the ground connection on the filter - and also soldered to the ground connection on the AC power plug which, itself, was also connected to the case.

Using the original on/off switch and wire from the scrapped power supply, the filter was wired to the AC mains and then over to the power supply.  Some push-on pins were found that mated snugly with the power supply's AC input connections and connected to its AC input, but it would have been possible to cut off the original AC power cord and wire it in.  Some RTV ("silicone") adhesive was then used to secure the push-pins on the power supply's AC input as well as to hold it to the bottom of the case - either of which could be removed later, if necessary.

DC output filter:

Figure 8: 
Schematic diagram of the DC output filter.  The "ground" of this filter was firmly attached
to the metal power supply case using the ground lug of the terminal strip seen in Figure 9, below..
With the AC line input now being completely filtered, we still have to isolate the other end of the path through which the low-level RF currents can flow - the DC output.


Inspecting the PC power supply again, I noticed that there were two toriodal inductors and I removed them both.  One of them had several different wire gauges and was set aside, but the other consisted of a pair of wires wound in parallel, connected in parallel on the circuit board - and a quick check on the inductance meter showed its value to be around 43 microhenries - plenty good for our purposes.

Figure 9: 
Output filter components mounted on a terminal strip with the ground
lead of the capacitors being wired to the mounting lug..  One of the
two yellow monolithic ceramic capacitors can just be seen
behind the closest terminal strip.
Click on the image for a larger version.
Had neither toroidal inductor been suitable as-is, I would have picked the one with the heaviest-gauge wire and removed all but the winding with that wire:  Most of these power supplies use toroids with yellow or green cores and a dozen or two turns on these typically yield inductances well above 10 uH - more than enough to block HF energy when bypassed with good-quality capacitors!

On a terminal strip I mounted the inductor and two low-ESR electrolytic capacitors, as shown, bypassing each one a 0.1uF monolithic ceramic capacitor.  The use of these low-ESR capacitors rather than "normal" capacitors is important as these types are specially-designed to remove the high-frequency components.  Once you get above a few hundred kHz, however, many electrolytic capacitors start to lose their efficacy so monolithic capacitors such as the ones shown take over, shunting the RF to the case ground.

Important construction notes and comments:
    • Again, use ONLY LOW ESR capacitors for the output filter.  These capacitors are almost always rated for 105 degrees C, so if the capacitors that you have say "85C" on them, they are probably not low ESR - but their having "105C" on them that doesn't guarantee that they are low ESR, either!
    • While the output capacitors of PC power supplies are (ostensibly) of the low ESR type, it is often the failure of these capacitors - along with the fan - that causes these power supplies to fail, so don't count on a failed power supply to be a usable source!  Unless you have an ESR meter, don't count on a capacitance meter to tell you if a capacitor is any good, either:  It can still read the proper value, but have terrible ESR!
    • If a terminal strip cannot be found, a small piece of copper-clad circuit board could be used, instead, with the components mounted "dead bug" on it, using the copper itself as a ground plane.  The circuit board materal would then be mounted using screws, to the metal case, assuring a solid ground connection.
    • Be sure to ground the output filter directly to the case near the point where the DC cable exits the case rather than run a wire to the AC input filter's ground point!  One of the ways to maximize the effectiveness of the filter is to minimize the length and impedance of its ground/common connection, and the best way to do this is to utilize the case itself!

    Figure 10: 
    The input filter mounted on the wall of the power supply case.
    Note the clear plastic shield behind the phenolic board to
    prevent accidental shorting/contact through the vent holes.
    Click on the image for a larger version.
    Note that if you use ordinary 0.1 uF disk ceramic capacitors instead more modern monolithic ceramic units be aware that many of these can have rather low voltage ratings (e.g. 16 volts) unless otherwise marked.  Also note that they cab lose effectiveness at high frequencies so they should be bypassed with 0.001uF capacitors.

    This terminal strip was mounted to the case using a 6-32 screw and a "star" washer.  The DC output cable of the power supply was then cut and wired to the terminal strip, using the ground lug as a "common" and passing the DC through this filter which effectively shunts any RF to the case ground.

    Finishing it up:

    Once all wiring is completed, ohmmeter tests should be made to verify continuity (or lack thereof) as appropriate and stick-on feet should be applied to the bottom to prevent it from sliding around and scratching whatever surface it rests on.

    Figure 11: 
    Filters and power supply mounted within the case.
    The power supply itself was affixed using RTV
    ("silicone") adhesive.
    Click on the image for a larger version.

    How well does it work?

    At HF frequencies this filter's effectiveness is seemingly absolute in that the power supply within cannot be detected from outside the box, even with a portable shortwave radio held within a few inches!

    It should be noted that it is not the "shielding" of this box to which one would attribute its effectiveness, but simply the fact that the AC input and the DC output share a solid, common RF ground.  Any RF currents on the AC input and DC output simply circulate on this common ground (e.g. the metal case) rather than radiate on the AC power leads and/or the wires connected to the DC output - or the things connected to it!

    Were this same circuit arrangement constructed on a flat piece of metal without a shield cover, it would have worked nearly as well:  The 100% cover of the case is there mostly to prevent accidental electric shock and shorting of the otherwise exposed AC mains connections.  With the cover removed, the RFI can radiate from the power supply, but unless a wire/conductor is placed within the box such emissions would go only a very short distance.

    Figure 12: 
    The completed, enclosed, power supply.
    Click on the image for a larger version. 

    This blog entry may be considered to be a follow-up to the December 8, 2012 entry about "Reducing Switch Supply Racket racket (RF Interference) - Link

    On a related topic, see also the September 4, 2013 entry, "Quieting High-Current switching supplies used in the ham shack - link



    Saturday, July 19, 2014

    Replacing the filters in the Drake SP75 Speech Processor

    A few months ago I fired up my old Drake TR-7, getting on the air with some friends on 40 meters after not having used the radio for a while.  It seemed to work fine - until I switched the audio through the matching Drake SP75 Speech Processor as some of those on frequency were having a little bit of trouble hearing me and started getting the following comments:

    Figure 1:
    The front panel of the Drake SP75 speech processor.


    "Ughh!"

    "That sounds terrible!"

    "Your 'lows' are completely missing!"

    "Your audio sounds 'restricted'"

    "Turn it off!"

    I obliged, of course, but I also knew that in the past I could switch in the speech processor, setting it to a very low level of clipping, and no-one could really tell the difference between it being switching in and switched out, so I knew that something had definitely changed!

    Later, I used some available test equipment (computers and software) to see what had changed, setting up the following:
    • Using another computer (a netbook) I ran the Audacity  (link) program, a free, open-source audio editor.  Using that program, I generated 5-10 minutes of white noise and set it to play back that white noise as a loop.
    • The generated white noise from that computer was fed into the "Tape" input of the speech processor with the audio level set just high enough to properly drive it.
    • The audio output from the SP75 was fed into another computer running the Spectran (link) program - also free - which does audio analysis.  Another program that would work, but is more difficult to use with a much steeper learning curve, is "Spectrum Lab" (link).
    The result of this was that I was able to compare the "flat" white noise input to the speech processor by the netbook with the audio spectra coming out of the SP75.  Ideally, the SP75 would not appreciably "color" the audio - that is, the the frequency response of the SP75 should be pretty "flat", not rolling off either the lows or the highs, at least in the frequency range used for speech (e.g. 100-300 Hz to 2700-3000 Hz or so).  Unfortunately, I can't seem to find the screen capture of that spectral plot or else I'd include it here.

    By this point, I was fairly sure that I already knew the answer - and the above technique of "sweeping" the audio passband using white noise verified it:  The "low end" audio frequencies (below approximately 700 Hz) were being rolled off significantly - by 6-10 dB and more, explaining why my audio sounded so bad!

    How the SP75 works:
     
    Before we go on, a few words on how the SP75 works.

    This is a combination AF/RF speech processor and it works by first routing the input audio through an XR2216 audio compressor.  Then, the audio is double-sideband modulated at around 459 kHz, filtered to produce a lower sideband signal using a pair of 455 kHz ceramic filters, RF clipped, filtered by another ceramic filter, and then demodulated back to audio.  By applying the clipping at RF, the distortion products are (largely) generated out-of-band rather than at audio.

    By applying both audio compression - to assure a consistent amount of RF produced for the SSB modulator - and RF clipping, the "best of both worlds" in terms of audio processing can be applied in terms of improving the "peak-to-average" ratio for speech while minimally increasing the amount of perceived distortion.

    What had gone wrong:

    Because of the loss of low frequency audio, I figured that one of three things had happened:
    • One or more electrolytic capacitors in the audio path had dried out and decreased in value causing the loss of low frequency response.
    • The BFO, nominally at 459 kHz, had gone off-frequency and caused the audio passband through the filters to shift.
    • One or more of the 455 kHz ceramic filters had gone bad.
    With the white noise applied to the input and still using the Spectran program to check the audio spectra I went though several of the audio test points noted in the manual, applied them to the audio input of the computer running Spectran and observed that at least to the balanced modulator, the audio was completely flat, ruling out the likelihood that a capacitor had gone bad.

    I then fired up an SDR (Software Defined Radio) - an RF Space SDR-14 - and started probing around inside the SP75, noting that the BFO was, in fact, where it should have been:  within a few 10's of Hz of 459 kHz, ruling out the second probability of the above.

    Connecting the input of the SDR-14 to test point 11, through a 2k resistor to minimize circuit loading, a location after all of the filtering and clipping and on the output of an amplifier stage, I centered the SDR on the passband - while still sending white noise through the SP75 - and looked at the resulting display and saw that it was anything but flat, indicating that one or more of the ceramic filters in the unit had, in fact, gone bad!

    Identifying the ceramic filters:

    In looking at the filters themselves they were clearly made by Murata, marked with "CFW455" followed by what looked like an "I6" printed in white ink while the schematic diagram simply called out a part number of "CFW455I".  In doing a bit of research on the web, I determined that the Murata "CFW455I" had the following specifications:
    • Center frequency:  455 kHz
    • Input/Output Impedance:  2 kohms
    • -6dB bandwidth:  +/- 2 kHz
    • Stop bandwidth:  +/- 7 kHz (at -50dB)
    • This device was a 6 pole filter
    What was also apparent was that this particular filter was no longer being manufactured, so I started looking around for a replacement.
    Figure 2:
    The new filter (left) and the old filter (right). 
     All but one of the leads lines up with the original.
    Click on the image for a larger version.

    What I did NOT want to do was use "new-old" stock because of age-related degeneration with these parts.  Typically, these parts have silkscreened, silver-plated electrodes on the surfaces of their ceramic elements, but even though these are usually fairly well sealed, they gradually degrade for whatever reason, either due to slow corrosion of the potting compound that protects them, ingress of moisture from the environment, or possibly due to electrolytic degradation due to chemical reaction and/or voltage applied to their terminals.

    Whatever the reason for their degradation, I decided that I did not want to get "new" 10-20 year old parts and risk having them be out of spec!

    In perusing the various catalogs, I noticed that Murata does still make a part that is electrically identical - the CFWLB455KJFA-B0, available from Mouser Electronics - so I ordered some.

    Installing the replacement parts:

    Figure 3:
    New holes that need to be drilled into the board to accommodate the different pinouts of the new filters.
    Note that this picture was taken before the old holes were completely cleaned of solder.  On the far right
    hole, carefully avoid the adjacent trace - both when drilling, and when later soldering the jumper.
    Click on the image for a larger version.

    I'd ordered the CFWLB455KJFA-BO filters knowing ahead of time that while they were electrically identical, they were NOT mechanically identical, so with the new filters now in hand I set about modifying the SP75 circuit board after carefully removing the three original Murata ceramic filters using both a "solder sucker" and plenty of "Solder Wick" (tm).

    Figure 4:
    The trace at the center filter position (FL2) that
    inevitably lined up with one of our newly-drilled holes.
    Click on the image for a larger version.

    As can be seen in Figure 2, the new filter is slightly smaller than the old one - and the pinout is slightly different, as well, but fortunately there is only ONE pin (the "output" - but these filters are bilateral, so it doesn't matter which is used for the input or output) that is actually in a different physical location which means that we need to drill just one hole for each of the filter locations.

    Referring to Figure 3, above, you will note that the new hole is in line with an intersection of two imaginary lines drawn from other pins, so the new hole just needs to go, as shown in the picture, "above" the now-abandoned hole and inline with the "input" pin.

    Of course, Murphy has to intervene as shown in Figure 4 where the extra hole drilled for FL2 ended up going right through through a trace on the top side of the circuit board.

    Fortunately, we have the technology (a soldering iron, solder, and wire) to relocate this trace and get around this problem literally!

    Both ends of the trace that ran under the original filter were sliced with a sharp knife and the original trace was heated with a hot soldering iron so that it lifted off the board.  The ends of the trace were then scraped clean of the green coating and a short piece (some #30 wire-wrap) of wire was soldered into placed, used to route around where the filter would be placed as depicted in Figure 5.
     
    Figure 5:
    The removed and re-routed trace using a short
    piece of #30 "wire wrap" wire.
    Click on the image for a larger version. 

    Having done this, the board was now ready to receive the three new filters.

    Because only the lead with the drilled hole does not match the original pinout, they may be (mostly) soldered as normal.  For that "other" lead, a short piece of wire - a trimmed component lead, for example - may be used to make the connection to the original, now-unused hole to the new pin as seen in Figure 6, below.

    Figure 6:
    On the bottom side of the board, the installed filters and the jumpers to the leads that connect
    to the positions with the newly-drilled holes.
    Click on the image for a larger version.

    Meanwhile, on the top side of the board, the filters look like this:

    Figure 7:
    The new filters as viewed on the top side of the board.
    The rerouted trace (the yellow wire) may be seen just to the left of the middle filter.
    It is interesting to note that these "CFWLB455KJFA-B0" filters are actually marked "W455I"!
    Click on the image for a larger version.

    Getting the SP75 back into working order:
     
    Firing up the SP75 after replacing the filters I noticed immediately that its audio didn't sound right - very "tinny", even worse than before.  Putting the white noise back into its input and connecting the SDR-14 to TP-11 I noticed immediately that the 459 kHz BFO frequency was entirely outside the passband of the 455 kHz filters.

    What had happened?

    From what I can tell, one of two things might have changed:
    • These new filters (CFWLB455KJFA-B0) are slightly narrower than the original CFW455I ceramic filters used by Drake.  In this scenario, the BFO and the edge of the audio passband would have been "moved" entirely outside the filter.
    • The original Drake filters were marked "CFW455I6" - a designation that doesn't seem to be correlate with anything in a catalog that I could find.  Perhaps the "6" indicates a center frequency of "456" kHz?  If this is the case, that would imply that the original filters were specially-selected for the higher center frequency and, perhaps, had matched bandwidths.  Based on what I was seeing, having the passband of the filter shifted up 1 kHz to 456 kHz would have put it in about the right place.
    In either case, the 459 kHz BFO frequency would not be suitable for the new filters, so how to change the BFO frequency?  The BFO frequency was originally set with a quartz crystal - a rather expensive component to get custom made - but there are two easy alternatives:
    • Slightly reworking the oscillator to use L/C components such as a 455 kHz IF "can" (transformer) as frequency-determining elements.  This would, at the very least, involve adding a series DC-blocking capacitor were this route taken. A bit of care would need to be taken to assure that this arrangement was temperature stable to within a few hundred Hz over the expected frequency range.
    • Using an inexpensive 455 kHz ceramic resonator - also available from Mouser.
    I chose the latter since I had several of those on hand, plus they had the double advantage of being quite tunable over the range of several kHz and they are fairly frequency-stable, likely to move only a few hundred Hz, at most, over the temperature range that one might experience.  As for the original 459 kHz crystal:  I have wrapped it in paper and plastic tape and secured it inside the SP75 case in case I need it for some reason in the future.

    Figure 8:
    A Murata 455 kHz ceramic resonator used in lieu of the original 459 kHz crystal and a 180 pF capacitor in parallel with the
    frequency trimming capacitor - both components being mounted on the bottom of the board.  As noted in the text, the value
    of this fixed capacitor was determined experimentally using the methods described in the text.
    Click on the image for a larger version.
    Simply dropping the 455 kHz resonator in place of the 459 kHz quartz crystal yielded a tuning range of about 460-463 kHz (the frequency range will vary depending on the nature of the resonator) so I had to "pad" C44, the 7-62 pF tuning capacitor with a 180pF capacitor - the value having been experimentally determined - to get it into the correct frequency range for the filters.  What I ended up needing for my filters was a BFO frequency of 458.7 kHz - easily within the tuning range of the 455 kHz ceramic resonator, but your specific BFO frequency may vary, depending on the ceramic filters that you end up with.

    Comment: 
    There's no real reason why LSB (lower sideband) must be used when picking the BFO frequency as these filters are symmetrical.
    If you use an L/C network for setting the frequency, pick the frequency that gives the best results using the methods described below.  It so-happens, however, that ceramic resonators are easier to move up in frequency than down as this requires just series capacitance, so using a "high side" BFO and LSB is just easier in this case!

    How to determine the correct BFO frequency for your filters:

    To determine the correct BFO frequency I used the same method that I'd used to determine that the original filters had gone bad in the first place, that is:
    • Insert a white noise source - at just high enough audio level to drive the SP75, but low enough to avoid any overload or clipping - into the input of the processor.  The correct level is that which is just high enough to make the front panel "audio" light turn on solidly, and then just a little more.
    • Using a program like Spectran to observe the audio spectra, note the "flatness" of the audio output taken from the speech processor and fed into a computer.
    By varying the BFO frequency one can see the effects of the filter's bandpass:  Too low a BFO frequency and the upper edge of the filter starts to cut off the low audio frequencies (the processor generates lower sideband, remember!) and too high, the intermodulation products of the clipping can start to affect audio quality if both sidebands are recovered and demodulated.

    Because these ceramic filters are considered to be "low cost" they do have a bit of intrinsic ripple (their specifications are for +/2 dB of ripple)  and they do not have a "brick wall" response, so don't expect a superior "shape factor" - that is, a very abrupt cut-off, but rather a fairly gradual cut off over the span of several hundred Hz or a kHz.  If you are a purist, you can order several extra filters so that you may pick and choose which one(s) give the best, overall response - but note that the circuit board cannot take very much soldering/unsoldering, so you would want to install sockets or some other temporary connections were you swapping filters in and out frequently!

    By carefully adjusting the BFO frequency, one should be able to get a fairly flat frequency response down to 200 Hz or so and up beyond 3000 Hz, fully encompassing the frequency range of any transmit audio source that you'd be likely to use!

    Consider the result below:

    Figure 9:
    The "sweep" of the audio output, using the "Spectran" program, of the SP75 after replacing the filtersusing a white noise source
    on the input and sampling the audio on the output.  This is about as good as one can expect using inexpensive ceramic filters
    like this, but listening  on the output with an audio amplifier, it sounds quite good.  This "sweep" was with Spectran analyzing a
    white noise input an averaging over 96 samples to help "smooth" out the result.  Even so, the "roughness" is still evident due to
    the fact that we are, in fact, measuring the spectral energy of white noise as our original signal source!  Not shown in this
    "sweep", the actual frequency response of the SP75 extended to nearly 5 kHz with gradual rolloff above 3 kHz.
    Click on the image for a larger version.
    Figure 9 shows a pretty flat audio output - within a few dB - from the SP75.  As noted in the caption, above, listening to an audio source inputted through the processor via an external amplifier, it does sound pretty "flat" to the ear.  As can be seen, there is a bit of roll-off below 1200 Hz and this is entirely due to the ceramic filters themselves, but this could, in theory, be corrected with a simple R/C network:  The roll-off below 100-200 Hz seems to be intentional by the designers of the SP75 and occurs mostly in the output stage, although a bit of it is, in fact, from the "edge" of the ceramic filters' response.

    Wrapping it up:

    Overall, I'm pleased with the result, even though the project was a bit more involved than I'd expected it to be.  Up to a clipping level setting of 6-9 dB, there is hardly any noticeable distortion added to the audio - just as it used to be when the SP75 was new!

    Of course, a speech processor is one of those things that should be used sparingly.  Under normal conditions with good signals it is probably not needed at all and when conditions start to get a bit rough, the added compression - if not taken to a ridiculous level - it should add more "punch" to a signal than it would degrade the audio due to excess compression, clipping, distortion and/or coloration.  This particular processor's "clipping level" control goes all of the way to 20 dB - a ridiculous amount that yields results that may be intelligible, but are likely to be unpleasant, so that setting should never be used in any but the worst possible band conditions - if even then!

    Note: 
    It is worth paying very close attention to the SP75 manual in setting up the input and output levels for the SP75 for the microphone that you plan to use. Unfortunately, if you use several different microphones with the SP75 and they each have wildly different output levels, things get very complicated as only the microphone on which the SP75 was originally set up will be driven properly unless you have outboard attenuators on the microphones to assure that they all have about the same levels!
    If the input level is too high, there may be too much audio compression in the XR2216 stage while if too low, the efficacy of the processor itself is reduced.
    Also, the output level control should be set so that the audio level is the same when the processor is switched out (e.g. bypassed) and in with clipping set to 0 dB.

    Monday, June 30, 2014

    A "Helical" resonator to increase isolation of 20 meter stations during Field Day

    As with many groups, the Utah Amateur Radio Club (UARC) operates both a CW and SSB station on 20 meters during Field Day.  Being that 20 meters is typically one of the "prime" bands where most of the contacts are made during daylight ours, it would make sense to optimize operation on that band as much as possible.

    Despite separating the antennas as far as is practical with the available geography - about 300 feet (approx. 100 meters) - and arranging the Yagis north-south of each other so that they are pointed generally broadside each other (east/west) for U.S. coverage, occasionally we encounter a bit of interference between the SSB and CW stations on the same band.  This isn't too surprising because, at times, the transmit frequency of one station is just 100 kHz away from the receive station of the other and with stations so close together - in terms of frequency and proximity - they are bound to "interfere" with each other, at least occasionally!

    Unfortunately, "normal" filtering schemes that one might initially consider don't really apply here.  The methods and means of filtering that one might see applied at HF stations include:
    • Tuning stubs using pieces of coaxial cable - typically in 1/4 and 1/2 wavelength segments.  These are typically used to "notch out" signals from, say, 20 meter operations that are bothering a 15 meter station.  This can happen because many radios filter the band(s) below the rather poorly on receive and/or a 15 meter transmitter will often generate low-level noise on 15 meters and all bands below it which can often be problematically radiated by multi-band antennas such as Yagis.  The "noise problem" is mentioned in more detail, below.

    • Commercially-available bandpass filters (e.g. "Dunestar" ).  This filter would be used to mitigate the same sorts of problems as the notch/stub coaxes mentioned above, but in smaller, more convenient packages.
    • Tunable L/C traps using transmitter variables and standard inductors.  Using the sort of components that one might use for high-power antenna tuners, it is possible to construct L/C filters through which one can transmit that could be used to tune notches to eliminated "problem" signals.  Unfortunately, these are somewhat complicated to use and rather large, so they are rarely seen.
    The reason why the above really aren't useful (at least for transmitting through) is that they simply don't have a high enough effective "Q".  All that can really do is keep one band (say, 20 meters) out of another band (say, 15 meters) - That is, you couldn't expect it to affect, say, a signal at around 14.050 MHz and have minimal effect at 14.150 or vice-versa since it is impractical to make tuning that sharp using the methods above.  What's worse, if you do build a narrow enough filter to remove a station that is, say, 100 kHz away from you, it has fairly high insertion loss:  This may work for receive if you can separate the two signal paths in your radio, but you certainly can't transmit through it!
    Figure 1:
    The two 20 meter "Helical" resonators.  The one on the left is a "notch" only used
    on the SSB station to remove the CW transmitter's energy and the energy of
    the SSB transmitter from the CW passband while the one on the right
    is a passband filter used no the CW station, both to keep the SSB station's
    energy out of its receiver and to prevent low-level noise from the CW transmitter
    out of the SSB station's passband..  The loss caused by the use of
    either one of them is on the order of 1dB or less.
    Click on the image for a larger version.

    What's more is that most transmitters produce a low-level, broad-band noise spectra when keyed up that can blanket the entire band - not to mention, sometimes, low-level spurs related to who-knows-what that can sometimes land on the other station's receive frequency.  Even if these spurs are 100 dB down, they may still be strong enough to be an annoyance to the other station on the same band!

    The situation can seem hopeless, then, if you can't manage to get a huge amount of geographic isolation and/or higher-end rigs that purport to have (or actually have) ultra-clean transmitters and receivers with super-high dynamic range.

    Or is it?

    A few years ago I decided to see if it was possible to make a "20 meter Helical Resonator" so I dug out the formulas and observed that such a filter would be possible with practical (and relatively cheap!) materials - just!

    Figure 2:
    The coil wound on the glass jar along with the nearby
    coupling probe.  This particular filter was lined with copper
    foil, but it turned out not to make any difference, so
    I didn't bother lining the next one!
    Click on the image for a larger version.
    I went to one of the big-box home improvement stores - Lowest-Depot, I think - and got several one-gallon, metal paint cans.  Noting that their very thin metal construction made them a bit flimsy, I decided that they would require a bit of reinforcement to make them mechanically stable but the main requirements - being cheap and 100% metal - were satisfied.  Technically, they were just a bit undersized for a true 20 meter helical resonator, but I figured that if this didn't work, it would be a relatively inexpensive failure.

    A friend of mine gave me a chunk of #6 solid copper wire - but copper tubing of a similar outside diameter would have worked just as well.

    I wound it on a smaller form - some plastic pipe that was 3-1/2" (approx. 9 cm) diameter - and then forced it over an empty "Adams" (tm) peanut butter jar which was just about the right height and diameter to fit inside the 1 gallon can and being made of glass, was already a low-loss material that would have no trouble at all standing up to the high RF voltages that would be present.  Initially taping the wire to the glass, I used RTV ("Silicone") adhesive in several places to secure the turns, avoiding excess use of it as I wanted to keep the losses as low as possible.

    Comment:  The dimensions of the glass form - the peanut butter jar - are approximately 4-1/8"(10.5 cm) outside diameter and 7-1/2" (16.5 cm) tall.  The coil was wound on approximately 4-1/2" (11.5 cm) of this height.

    Having initially wound the wire over a smaller form caused it to fit fairly tightly over the larger, glass form, so it "took" well to staying in place while I carefully separated the turns equally and secured their positions.  Note:  After the RTV had cured, the tape that had been used to temporarily hold the turns in place was removed.
    Figure 3:
     The mounting of the SO-239 connector on a piece of aluminum
    plate for a more stable platform.  This is riveted to the side of the
    paint can, but it could have been screwed in place.  This
    particular connector was attached at the seam of the can to
    take advantage of the thicker metal there.
    Click on the image for a larger version.

    Winding more turns than I expected that I would need, I set the coil aside for a day or two to allow it to cure and constructed a capactive coupling probe from a copper plate that was bent to a radius that (more or less) matched that of the coil - See Figures 2 and 6.

    For a connection to coaxial cable I cut a piece of 3/16" aluminum plate that was about 2 times the length of an SO-239 connector and about 1.5 times its width, formed it to the same radius as the paint can and then attached to its center the connector, and then pop-riveted the assembly over a hole that I'd made in the side of the paint can that aligned with the top of the coil.  See Figure 3.

    Inside, I then connected the center conductor of the SO-239 connector to the copper radius plate using a length of #12 AWG solid copper wire, forming a capacitive coupling probe which was adjustable in position relative to the coil by virtue of bending both the radius plate and the rod connecting it to the SO-239 connector.  Picking the same spot on the opposite side of the coil, I duplicated the connector/coupling probe arrangement and put one there as well.

    Now, the tricky part:  Setting the frequency tuning range!

    Placing and centering the coil in the paint can, the bottom end was grounded via a hole that I'd drilled in the can for that purpose.  I also placed the coupling probe about 3/8" from the top of the coil as a starting point for determining resonance and coupling, knowing that this was likely to change.  Using an antenna analyzer, I swept the frequency up and down and saw a very prominent deflection that I believed to be the self resonance of the coil at around 12 MHz in my case - and placing my hand near the top of the coil and causing the frequency to shift downwards and confirmed this.  Taking a fraction of a turn off at a time, I soon moved the frequency up to around 14.5-15 MHz.
    Figure 4:
    A tuning disk of glass-epoxy circuit board material that was cut
    using a hole saw that was soldered
    to the threaded rod.  This disk must be as flat/parallel to
    the lid as possible and remain so during rotation
    for smooth, even tuning.
    Click on the image for a larger version.

    In the center of the paint can lid I drilled a hole large enough to accommodate a 1/4"-20 nut and de-burred it.  On the top side of the lid I soldered a brass 1/4"-20 nut using plumbing solder and a hot soldering iron rather than a torch as the latter would have likely ruined the can's plating and made soldering much more difficult.

    For the paint can lid I cut a round piece of wooden paneling about the diameter of the lid with a hole saw and made a hole in its center large enough to accommodate a 1/4"-20  nut.  and used RTV to attach it to the top side - this, to stiffen it - see Figure 5.

    Onto some threaded rod (brass rod is easiest to work with) I soldered a disk of glass-epoxy circuit board material to form a capacitive plate that would go up and down to allow tuning, making this disk as close to a right angle to the rod as possible (e.g. parallel to the lid) so that when rotated, the disk maintained an even distance to the top of the coil as it went up and down.  If the disk isn't perfectly "flat", the frequency will be seen to "wobble" up and down while adjusting the tuning.  This same sort of thing can happen if the disk isn't perfectly round and the hole isn't centered, but cutting it with a standard hole saw solves those problems!  Note:  If single-sided circuit board material is used, place the copper side down, toward the coil.  The metal from which this disk is cut isn't really important as it could be also be aluminum or even steel:  It just needs to be stiff, flat and round!  If it cannot be soldered to the threaded rod - which may be the case if steel/stainless threaded rod was used -  then the disk may be electrically "connected" using 1/4"-20 bolts and some lock washers as the protrusion of the bolt through the center should have minimal effect except for the fact that the bolt on the "top" side will prevent the disk from being adjusted as close to the lid of the paint can and slightly reduce the tuning range.

    The rod was then threaded through the bottom of the lid of the paint can and a lock (e.g. "split") washer and a flat washer along with another nut used as a "jam" nut were spun onto the rod to give it a bit of tension and the threaded rod was then lubricated with a drop of oil to make its operation smooth.  As can be seen in Figure 5, a piece of scrap wire from the coil was soldered to this nut and secured with small straps to maintain the tension on this jam nut.

    Putting the lid back on I set the disk as high as it would go (against the lid) and re-checked the frequency and found that it was again low due to the capacitance of the metal lid itself as well as the disk and then did more trimming of the coil, a process that required repeatedly taking the lid off, trimming, and putting it back on again to check.

    Eventually, I got to the point where I could tune through the 20 meter amateur band and was ready to do some initial testing.  Using an HF transmitter set to just a few watts and a VSWR bridge and combination power meter on one side and a 50 ohm dummy load and another power meter on the other side, I tuned through resonance and noted that I had both a high amount of insertion loss and a high VSWR at resonance.
    Figure 5:
    The top of the can, reinforced with a disk of thin paneling/
    plywood that had been glued to the top with RTV to make the
    thin metal top of the paint can much more solid.  Soldered
    to the lid - but not visible - is a brass 1/4-20 nut through which
    the tuning rod is threaded.  There is a flat and split ("lock") washer
    used to set the tension and the piece of copper wire seen in
    the picture above is used to prevent the jam nut from
    spinning as the knob is turned.
    Click on the image for a larger version.

    I moved both of the the coupling probes equally closer to the coil itself and taking a bit more of its winding off - as the proximity of the probes actually lowered the resonant frequency.  After a few more iterations of trimming the coil I found a point where I was able to get around 80% of power to pass through the filter at resonance (about 1dB loss) and a reasonable VSWR - less than 1.5:1 - and achieve a bandwidth of only a few 10's of kHz:  This would be my CW station bandpass filter!

    Notes: 

    - Had I cut too much wire off and raised the frequency too high I would have simply soldered - using a very hot iron - a short piece of pre-bent, to match the curve of the glass - section of #12 or #14 wire, to extend the top of the coil:  At the very top, the thickness of the wire in the coil at the top ("high voltage") end is less important in determining its loss characteristics and a few inches/centimeters of smaller-diameter wire here will have no ill effects.

    - Avoid the temptation to increase the coupling too much to reduce losses much below 1dB or so.  If you do this, the resonator will be over-coupled and its filtering effects in the SSB portion of the band will be reduced.  Experimentation showed that a coupling of about 1 dB yielded 15-20 dB of attenuation at 14.150 MHz and above when the bandpass was tuned to around 14.050 MHz. If the transceiver has a built-in antenna tuner, by all means, use it!

    - It is in the "narrowness" of the filter that the importance of using large outside-diameter conductor for the coil becomes important.  The first attempt at a bandpass filter used #12 wire and dramatically inferior results were experienced with the filter offering only 3-6 dB of attenuation 100 kHz away on 20 meters.  The #6 wire used in these coils was probably a bit overkill and #8 AWG would have probably been fine, as would copper tubing of similar outside-diameter (e.g. 3/16" or approx. 4mm outside diameter.)  Remember:  RF flows (pretty much) only on the outside of the conductor, so there's no real need to use a solid conductor for the coil - but make sure that you use only clean copper or silver-plated material for it!  Beyond a certain point, however, the "loaded Q" - that is, our coupling (via probes) into the coil to put our transmitted energy into it - and then take it back out again - becomes dominant and "improving" the coil itself even more - by using still-larger wire, for example - reaches the point of diminishing returns.


    Once I was satisfied that I'd gotten the tuning where I'd wanted it I carefully attached the glass peanut butter jar to the bottom of the paint can with plenty of RTV and to the bottom of the same can - on the outside of the bottom - "RTVed" another disk of scrap wooden paneling that I'd cut with a large hole saw (or freehand) to prevent the thin, metal bottom from wobbling and adjusting the tuning and set this assembly outside in the hot sun for a few days to let the RTV cure.

    Note: 
    As can be see in Figures 2 and 6, I lined one paint can with copper foil.  The other filter, I left alone, with its original gray protective paint inside the pain can:  There was no difference in apparent "Q" or performance!
    This result was not unexpected as the current is rather widely distributed along the inside of the "cavity" and the ohmic losses there are of less importance than those of the coil itself. Any future duplications of these filters will not use copper foil lined cans.

    I then replicated the above - but this time, I built a filter with just ONE coupling probe:  Instead of a bandpass filter, this would be a "suck-out" (e.g. "Notch") filter.

    For adjusting the coupling of the notch filter, the easiest way is this:
    1. Connect, using a UHF "Tee" connector", to a radio and a 50 ohm dummy load with a VSWR meter connected between the radio and the Tee connector.
    2. Set the notch at 14.050 MHz (or the desired 20 meter CW frequency) using an antenna analyzer.
    3. Tune the radio to 14.150 and connect it inline with the notch filter.
    4. Transmit into the dummy load with the notch inline and note the VSWR.
    5. If the VSWR is lower than about 1.3-1.5:1, move the coupling probe closer to the coil, noting that you'll have to re-tune the coil - possibly removing some wire.
    6. Go back to step 2
    Note that a VSWR of 1.3-1.5:1 will not adversely affect operation - particularly if your radio has a built-in tuner - but it does indicate that the notch is just starting to have an effect.  If you have the means to do so (e.g. tracking generator and spectrum analyzer or signal generator and high-sensitivity power sensor) you can verify that at the notch frequency, there is at least 15 dB of attenuation. From experimentation, it has been determined that at about 100 kHz away, when the notch filter causes a VSWR of about 1.5:1 to occur, it will notch out the energy at its center frequency by 15-20 dB.


    How the two filters are used at a Field Day site:

    With both a bandpass filter and a notch filter available at a Field Day site, the two are used in combination thusly:

    Figure 6:
    A closer view of the capacitive probe and its connection to
    the SO-239 connector.  This arrangement is somewhat
    flimsy, mechanically, and should be re-thought.
    Click on the image for a larger version.
    • The Bandpass filter is placed on the CW station, in series with the transceiver.  With its usable passband of +/- 15 kHz or less, it covers a reasonable chunk of the 20 meter CW passband, but it is easily retuned by a operator who simply turns the knob on top while watching the reflected power and will attenuate energy from the SSB station at 14.150 or above by 15 dB or more.  This passband response will reduce not only the signal level from the SSB station to minimize the possibility of receiver overload, but will also attenuate any broadband noise that the transmitter might produce that could degrade the 20 meter SSB operation.  Most CW operation occurs within a 10-15 kHz of a central "spot" - particularly if one "runs" a frequency - so frequent retuning is usually not required.
    • The "notch" filter is connected "across" the SSB station's transceiver coax with a UHF "Tee" connector, but is tuned for the center of the CW passband where such operations will take place.  At SSB frequencies, it has practically no effect whatsoever, but at the CW frequencies, it will attenuate by 15-20 dB, preventing both overload by the CW station and also notching out broadband noise that might be produced by the SSB transmitter itself that might degrade reception at the CW frequencies.  As long as the SSB operator doesn't transmit anywhere below 14.125 (which should never happen!) the VSWR will be practically unaffected.


    Does it work?


    Yes, actually.  While we haven't needed it every year (we don't know why we can get away without it some years even though we have been using the same rigs and antennas for years now...) when placed into service, it does completely remove - or very much knock down - inter-station interference between 20 meter operations.
    Figure 7:
    The bottom of the paint can, reinforced with a piece of thin
    plywood/paneling to keep the glass coil form from wobbling
    about.  This piece of wood is attached using RTV adhesive
    and allowed to cure for several days before handling.
    Click on the image for a larger version.


    Notes on these filters:

    • Paint cans are quite flimsy.  As you can see from the pictures, several measures had to be taken to "beef up" the paint cans to overcome their inherent mechanical instability.  Without these steps, tuning  could be radically affected by just touching the top of the filter, the weight of the coax hanging on the a connector, or just bumping it while handling it!  Even after doing all of these things they are still quite fragile!
    • They are a pain to tune the first time - and a bit of a pain to set up at each Field Day!  After the initial tune-up, you need to have on-hand an antenna analyzer to know where the notch is and a reflectometer (e.g. VSWR meter) is imperative for the CW station to know when the bandpass filter is centered on the operating frequency.
    • There's no practical reason why similar filters couldn't be constructed for the 10 and 15 meter bands - or other frequencies in between.


    Comments on the rigs that UARC uses for Field Day:

    The rigs that UARC has used for the 20 meter stations for several years have been old Kenwood TS-450SATs - the ones with built-in antenna tuners.  We have "standardized" on these since they seem to be relatively clean and able to withstand strong, nearby signals - even on the same bands - as compared to other rigs.  We have "discovered" a few interesting things, however:

    Most radios produce "Below Band" noise spectra when keyed up:

    Almost all modern, solid state rigs from all manufacturers tend to put out a noise spectra that blankets all HF frequencies on and BELOW the band on which they are being operated.  What this means, is that if you are operating on 20 meters using a multi-band antenna such as a Windom or Yagi, when that radio keys up on 20 meters, those operating on 20 meters and lower will experience an increase in the noise floor - even if there is no modulation!  This noise is limited on bands above the current band by the low-pass filter, but depending on the specific band, it could also affect the next band up (e.g. it could be present on 17 meters when operating on 20 meters if those two bands use the same low-pass filter, depending on the radio's design.)

    In the case of the TS-450s with the built-in antenna tuners, we have found that simply enabling the antenna tuners - even when operating into a 50 ohm load and the tuner isn't really needed - will attenuate this noise by 15-40 dB on all other bands by causing the tuner to function as a low-Q bandpass filter for both receive and transmit and for this reason, there is now a label on all of our TS-450s admonishing the operators to always keep the antenna tuner inline!  After all, no-one really notices the extra 0.25 dB or so (measured!) loss that it causes at or near 50 ohms!

    As noted above, other ways to eliminate transmitter-related noise and QRM on other bands include:
    • The use of 1/4 and 1/2 wave notch/stub coaxial filters.  These are band-specific and notch ("suck out") the frequencies for which they are tuned will leaving other frequencies unaffected... more or less.  One must make sure, however, that when changing bands, that the notch is removed and (if needed) replaced with the appropriate notch for another band to prevent a different interference problem or, above all else, you must make sure that you do not transmit on the band for which the notch you currently have installed is tuned!
    • The use of commercial, high-power bandpass filters such as "Dunestar" (tm).  These are fairly broad, bandpass filters that cover (more or less) an entire amateur band.  These do a decent job of knocking down out-of-band energy on both receive and transmit, but they can be quite expensive to implement - particularly if you have a set of these at each station!  As with the notch filters, they must be changed when you change bands!

    A notable exception to this seems to be rigs with tube finals.  This isn't too surprising as the Pi network output of these rigs is inherently narrowband - as is often the preselector/driver - so they are practically incapable of producing broadband noise!  Because of the receiver's preselector front end that these same radios typically have, they also seem practically immune to QRM from operation on other amateur bands as well!


    Certain radios are banned from our field day site!

    The one radio that is explicitly banned from our Field Day site is the Icom IC-706 and its close Icom relatives of the same/similar vintage.  When this radio first came out, one was set up as the 20 meter SSB operating positions and within a few minutes of Field Day having started we had to take it off the air because it made operation of the 20 meter CW station - as well as the 40 meter SSB station - completely impossible - plus its receiver was completely demolished by the other transmitters' on the air as well, no matter what band they seemed to be on!


    I don't remember what we replaced it with - our old "backup rig", a Kenwood TS-820, I think - but after that, all was well.


    The realities of Field Day:

    (That is, we use the rigs that we have!)

    Having said all of the above, I'm sure that someone reading this will say "You wouldn't have any of these problems if you got a bunch of (fill in the blank) radios!"

    I do know of a few higher-end radios that do seem to co-habitate with each other without causing mutually-assured QRM, but unless you have a bunch of club members that happen to bring those same radios every time, or unless the club just happens to own such radios - either donated to them or because of a large enough budget - it must make do with what one has onhand.

    That is the case with most of "us hams", isn't it!