Sunday, November 23, 2014

My first coherer

Recently the topic of the meeting of the Utah Amateur Radio Club was on "old" radio.  Really old radio, which is to say "radio" in the days before vacuum tubes and even crystal detectors.

One of the detectors used in these old days was the Coherer (see this link.)  "Discovered" more or less by French physicist Eduoard Branly in the 1890 and based on phenomenon observed by others, this device does not detect the presence of radio signals in the "conventional" way, but it goes from a high resistance state to a low resistance state in the presence of a radio signal - and then it needs to be "reset" (by mechanically tapping it) before it can detect again - a short of "one-shot" detector.

The most common type of detector (the "Branly" type) consists of metal filings located between two conductors, typically in a glass tube.  As it turns out these filings can be different types of metals, often described as "imperfect" conductors", but one should not confuse this as meaning that these are "semiconductors" like diodes, but rather referring to (possibly) semi-oxidized bits of metal of various sorts.

Back in the heyday of the coherer - before it was made obsolete in 1907 with the advent of the crystal and electrolytic detectors (yes, the "crystal" detector was a vast improvement over the coherer, which tells you how "bad" it actually was!) the most common type of transmitter was the spark gap which emitted a short, high-energy repetitive burst of radio signal.

How, exactly, a coherer works is still not well understood even today, but it is believed that there may be some sort of "micro-welding" of the particles on the molecular level in the presence of an RF signal that causes them to form a path that will carry DC that, after detection, is then broken (literally!) by tapping the coherer, mechanically, with a device called a "de-coherer" that rearranges the particles within.

Making a coherer:

There are a number of articles to be found on making a coherer, but nothing terribly specific that I found on the rather short notice caused by my rush to build a coherer:  The club meeting was later that evening and I was in a bit of a hurry to throw together the coherer at the last minute.  I found a web page by Neil, the " http://sparkbangbuzz.com/  " that included a portion on making a coherer that said, in effect that about any sort of metal filing that he'd happened to use did work, at least after a fashion - although some things worked better than others.

So, I went to work.

A few weeks earlier I'd ordered some inexpensive 25mL glass burettes from American Science and Surplus and from one of these I cut up a few lengths, each about 2-3/4" (approx. 7cm) long using a diamond cutoff saw and flame-polished the sharp edges with a mapp-gas torch - but just enough to make it safe without causing the diameter to narrow.

Standing one of these the pieces of burette on a square of cardboard I placed a number of short pieces of flux-free, silver-bearing, lead-free plumbing solder and then, carefully (!) heated the glass with a propane torch.  Since the burette was made from "Pyrex" type glass, it was not likely to shatter and soon, the molten solder filled the piece of glass - but it took 3-4 times before I managed to get it right:  I either knocked it over or, in its liquid state, it would run out of a small gap in the bottom!  Eventually, I managed to carefully press it against a paper towel to seal the bottom to keep it from leaking out and I soon had a "plug" of solder the same diameter as the inside of the glass burette!

After this had cooled I wrapped it in a paper towel and then proceeded to (carefully!) break free it from the glass burette by hitting it with a wrench, liberating the metal plug within.  From that, I cut away the dross that had formed at what had been the top end and in the middle, cut it diagonally, polishing the two pieces with small needle files once I was done.
Figure 1:
My first coherer, of the Branly type, made from a piece of a glass burette, solder, filings from contacts of
defunct circuit breakers and some epoxy!  It works, but not very well!

At this point I had to reduce the diameter of these two pointed "slug" ends slightly so that they would fit easily into the other piece of burette that I'd cut:  If I'd attempted to force it into either end of the burette piece, it would have surely shattered the glass!  Once I had two pieces that I could slide into at least one end of the burette segment, I then soldered short pieces of wire to the flat sides of these two slugs using a very hot iron and pre-tinned wire with 60/40 solder:  A bit of skill was required since I was soldering solder to solder!

Having done that I then set to "make" the filings that go in between the two electrode "plugs" that I'd just prepared.  In other reading I'd found accounts that indicated that nickle-silver coins made good filings so I proceeded to attack the contacts of some defunct circuit breakers that I'd rounded up with some needle files.  Working over a piece of white paper, after about 20 minutes and three pairs of contacts I had a nice pile of filings ready.

One of the pieces of glass that I'd reserved was the "bottom" end of the burette - the one with the nipple - for the body of the coherer itself and I threaded with wire through that end.  Mixing some 5-minute epoxy I worked it into the hole and moved the plug back and forth so that it made a complete seal around the plug to prevent any filings from finding their way around its sides and then stood it on end to prevent the epoxy from running out.  Very carefully "hitting" the assembly with a bit of heat from the propane torch, the epoxy hardened almost instantly - a fact tested by poking it with a piece of wire - and I poured the filings into the open end.  Carefully lowering the other electrode/plug I oriented it so that the two "slopes" of the electrodes met toward the middle and then put more epoxy into the other end to seal it as well - and waited for the epoxy to set.

After 15-20 minutes, I carefully applied a bit of heat which accelerated the curing of the epoxy to the point of it being safe to handle and I now had a completed coherer, ready for testing.

Or so I thought...

The way that a coherer is supposed to work is that its DC resistance is supposed to be very high (if not close to infinite) until the presence of a fairly strong RF signal, at which point its DC resistance will drop - and stay that way - until it is reset by being mechanically disturbed.
Figure 2:
An old Ford coil being used as the source of the "Spark Gap" transmission.

In this case, whenever the filings were between the two conductors, I was getting just a few ohms of resistance, essentially making it useless as a detector.

Drat!

With very careful fiddling I discovered that I could arrange the filings so that they barely touched one electrode or the other and then, if I set off a spark on the nearby Ford coil, the resistance would go from, say, 40 ohms to 20 ohms - enough to demonstrate that it did work, but not very well.


With the demonstration at the club meeting coming up in a few hours I figured that this would have to do, so I set it aside for a while to do other things, but when I came back in an hour or so I found that I could much more easily arrange the filings and now get a few hundred ohms, trigger the spark, and then get a few ohms or tens of ohms:  The only thing that I can figure is that the freshly-filed, silver-laden particles had somewhat tarnished after that time and the "imperfect" connection was now (slightly) easier to attain.

Figure 3:
Diagram of the coherer-based detector.  Not shown in this diagram are the particles between the sloped
electrodes of the coherer, or the "de-coherer"!

Using the coherer I wired up the circuit shown in Figure 3, above.  This has an LED in series, putting only about 1.1 volts across the coherer itself with a pair of 1k resistors "isolating" the the device at RF, allowing the wires connecting it to act as sort of an antenna if desired.  For determining if the coherer had been triggered, one could look at the LED's brightness and/or monitor the voltage drop across one of the 1k resistors with either a voltmeter or and oscilloscope.  When "setting up" the coherer it was a matter of fiddling about with it until the red LED lit dimly.  When the spark "triggered" the coherer, the LED would then illuminate much more brightly until the glass body of the coherer was lightly tapped with the plastic body of a ball-point pen, reverting back to its "dim" state.

When it came to demonstrating the operation of the coherer at the club meeting I simply connected my digital oscilloscope across one of the 1k resistors.  As it turns out, my 'scope has a VGA output that I connected to the club presentation room's overhead projector so that everyone present could see the trace on the 'scope change DC level with triggering of the coherer when it detected RF from the sparking of the nearby Ford coil, and then be reset when I "de-cohered" it by gently whacking it with a ball-point pen!

* * *

How sensitive was this coherer?

I never did try to check the range.  The "transmitter" was just the Ford coil in Figure 2, just as depicted with the two short #12 AWG wires connected to it being the "antennas", but the coherer seemed to reliably trigger at a distance of about 6 feet (2 meters) which was the maximum distance that we tried being that this was the size of the table on which we'd set up our demonstration!

Considering the size and shape of the wires in Figure 2 I figure that the this spark gap transmitter is actually operating in the VHF/UHF/Microwave range, much like that of the original Hertizan Wave apparatus of the late 19th century!  - see this link.

So there you have it, a coherer!

* * *

It's been a few weeks now since I built this coherer and its resistance is still too "low" in that I have to fiddle with it to to get it to work, but it does seem to be a bit less "finicky" in getting it set up to be triggerable thanit was when I'd first built it, so whatever is going on with those metal particles to make them work better still seems to be happening, albeit much more slowly!

At some point I may try building another one or two coherers using different materials, once I have done a bit more research on the topic (I still have some pieces of burette left!) but that will have to wait for another day!


Wednesday, October 15, 2014

The "Pointless" 10 meter DSB QRP transmitter

Figure 1:
The slightly-modified "Shake Light" and the transmitter.  The flashlight portion
still works!
Click on the image for a larger version.
The annual Homebrew meeting of the Utah Amateur Radio club meeting was coming up and while I was going to show off what I'd done thusfar on my mcHF transceiver - link I wanted something else that was a bit more "fun" and a very quick build, preferably one that could also give a signal to hear on the transceiver at the same time.

After a bit of brainstorming, something simple and pointless occurred to me - the so-named 10 meter DSB (Double SideBand) QRP transmitter in the title of this article.

I'm not sure where the idea came from but the germ was, no doubt, related to a previous article in this blog, "A Mechanically-powered Capacitor Flashlight" (May 13, 2012) - link  where I describe the inner-workings of the "shake lights" that were the objects of the annoying commercials that had been playing at the time.

These commercials have (thankfully) long since disappeared from TV and the flashlights have disappeared from the stores but I bought several of them on clearance a few years ago.  As noted in the article linked above, while these particular flashlights actually do have electronics in them that will generate electricity in response to mechanical motion, not all of them do as some are apparently electrically inert being marked as a "Toy" as the article "What Light Through Yonder Flashlight Shakes - link describes.

The flashlights that I got on surplus aren't as good as the somewhat expensive ones mentioned in the article above, but I was curious how well they might power a simple, low-power transmitter.

The transmitter itself is very simple and is depicted in Figure 2, below.

Figure 2:
The diagram of the "Pointless" 10 Meter QRP DSB Transmitter.  C4 is connected directly in parallel with the
0.22 Farad capacitor inside the "Shake Light" which means that the flashlight's "run" time is also increased!
Click on the image for a larger version.



This transmitter is almost as simple as it could be!
  • The RF energy comes from a 28.570 MHz crystal oscillator module that I found in my box of "Crystals and oscillator modules."  At 5 volts it consumes 5-6 milliamps and it seemed to function down to about 3 volts.  I picked this frequency because it was in an amateur band on a frequency where SSB was legal.  The fact that it was 10 meters was a plus since that made the antenna a bit more efficient.  In testing other oscillators, some drew over 20 mA, so it I stuck with the lowest-power device that I found.
  • U1 is a doubly-balanced mixer that I pulled from a piece of scrapped equipment.
  • T1 is a 1k-to-8 ohm transformer from Radio Shack, part number 273-1380.
  • The Microphone is a standard electret microphone available about anywhere.  I think that it came from a junked telephone handset, but I'm not sure.
  • The 1 Farad, 5.5 volt capacitor came from a junked piece of computer equipment that had used it for a memory backup power source.
  • Some telephone cord - the sort with embedded cloth woven in - used to connect the flashlight to the transmitter.  This cord is very lightweight and able to flex repeatedly without fatiguing and breaking - plus it was free, having been cut from a junked telephone handset.
  • No RF bypass capacitor on the V+ lead is shown as the oscillator module seemed to have one internally, so I didn't bother...
How it works:

Assuming that capacitor C4 is already charged up to something around 5 volts, pressing switch SW applies power to the oscillator, the microphone, and microphone amplifier, Q1.  Q1 boosts the audio from the microphone by a factor of about 100 or so and transformer T1 converts the rather high impedance from the amplifier down to something closer to the 10's of ohms that U1, the mixer, wants to see on its diodes.

Figure 3:
View of the back side of the transmitter.  The 1 Farad capacitor,
C4, dominates the view!
Click on the image for a larger verion.
When current from the audio transformer upsets the diodes in U1, the mixer, it gets out of balance and RF gets through:  The more out-of-balance from the higher audio level, the more RF gets through.  Interestingly, if the audio goes positive, the phase of the RF goes one way, but if the audio goes negative, the phase of the RF goes the other way, providing symmetry in both amplitude and phase.  In the process it generates "Double Sideband" (DSB) which is the same as single sideband except that both USB and LSB are generated at the same time.

The output of the mixer goes through L1 which, in this case, is a 3.9 uH choke.  For this device I used a telescoping whip about 26.5 inches (67.3 cm) long and this amount of inductance "approximately" resonates it at this frequency - very approximately, but better than if I'd just fed the output of the mixer directly to the whip!  The RF output power has not been measured, but it is likely on the order of 1-2 milliwatts PEP.

It is difficult to tell from the picture in Figure 3, but the entirety of the plastic-bodied BNC antenna connector is actually insulated from the grounded aluminum bracket to which it is mounted and both its "shield" and center are connected together.  This allowed me to "bypass" any matching network on an antenna that I might try and use the telescoping whip antenna that I'd happened to grab from my box of VHF/UHF whip antennas.


A few comments about the design:

Messing with the mixer:

When I used the RMS-11X mixer - which is surface-mount - you'll see from careful scrutiny of Figure 4  that I used tinned (#26 AWG) hookup wire to mount it upside-down on the prototype board, bridging the three common "ground" connections and taking them to the bottom of the board along with the other three connections, namely the LO (Local Oscillator), RF (typically the signal input or output) and the IF.

As is typical I connected the LO to the crystal oscillator via C3, the capacitor being used to block the DC component of the oscillator and reduce the current consumption since it is a DC short.  Meanwhile, the RF side was connected to the antenna with the "IF" port going to transformer T1 as the audio source.  I was, however, surprised when I got no modulation at all and decided to investigate.

Figure 4:
The Top of the "Pointless 10 meter DSB QRP transmitter.  U2, the oscillator,
is the square can while U1, the mixer, is the white, square block to the left
of it, just above the transformer.
Click on the image for a larger version.
Typically, these doubly-balanced mixers are connected such that the connections to the diode "ring" inside of them are brought out via the "IF" port - which makes since since, in many cases, the "IF" can include audio, or even DC, but when I measured the "IF" port I found a DC "short".  Re-checking the data sheet, I verified that I'd not messed up the pin-out - something easy to do when you are using a device upside-down - but then I remembered something in the back of my mind from a previous time that I'd used one of these RMS-11X mixers and checked another connection, finding the (approximately) 0.225 volt bi-directional "Shottky" diode drop on the "RF" pin instead - something that I'd apparently noticed in the past, but only dimly just remembered!

Why the data sheet shows the connection in the what that it does is beyond me.  Perhaps it is an error, but it could also be because the "RF" port is sometimes so-designated because it may that which that has the greatest amount of isolation to the "LO" port over most of the frequency range of the device - a fact implicitly bolstered by the fact that "DC" is not included in the "official" frequency range of any of this mixer's ports!

Swapping the positions of the "antenna" and "audio" leads was easy enough and upon doing so I suddenly had modulation - but not very much, so I added the Q1 amplifier - a simple, common-emitter, self-biasing design that draws around a milliamp, but provides a lot of gain:  Now, instead of having to yell at the transmitter to drive it into clipping and not get much power, I can talk fairly quietly, up-close, drive it into clipping, and not get much power!

Fighting with Farads:

As noted in the May 2012 blog post, the only reason that these "shake lights" were usable to the consumer at all was that they actually contained a pair of lithium coin cells!  If you removed those coin cells, the magnet and coil assembly did actually work, but it took a few minutes of shaking to bring the voltage from zero up to the 2.4 volt or so just to get the LED in the original flashlight to shine very dimply - and another couple of minutes to get enough charge before it would shine brightly for, perhaps, a few 10's of seconds.

Even though this entire circuit drew less than 10 milliamps I noted that it dragged the original 0.22 Farad capacitor from a 5 volt charge (provided by an external power supply) instantly down to about 2.25 volts due to its internal resistance, requiring the addition of C4 - a physically-larger capacitor with much lower internal resistance, dropping about 0.15 volts under load which meant that not only would I get better run time, but charging would likely be a bit more efficient since less power would be lost in resistance within the capacitor itself!

How long does it take to charge the entire thing from zero volts?

I'll admit that I've not that I've never run this completely down and then tried to charge it up again using the shake light (I'm not a teenager anymore!) but I did do a bit of experimentation and have determined that for every 5 seconds of talking, it takes about 60 seconds of shaking to restore that amount of charge on the capacitor, once it it has already been charged to 4-5 volts.  Based on those numbers it would probably take at least 15 minutes of vigorous shaking with a completely discharged capacitor before one would start to get usable results!

Is this transmitter, powered by a "shake light" practical?

No, not really! If you wanted a means of mechanically powering low-power transmitter (or even a transceiver!) - one that could produce much power power than this - you would be far better-off using one of those crank-powered flashlights and scavenging power from its far superior means of energy production, instead!

Concluding thoughts:

This transmitter project was built in just a couple of hours in a single evening using parts entirely from my junk box:  The only part that I had to buy (but I had on-hand, anyway) was the 1k-to-8 ohm Radio Shack audio transformer.  The entire reason for building it was to demonstrate that the mcHF transceiver was capable if "hearing" a signal - and to get some laughs at the UARC Homebrew meeting.

One of these days I'll see how far it will transmit.  I suspect that it will go, perhaps, a couple of thousand meters (yards) using the whip antenna - particularly if the antenna matching is optimized, but being that it is capable of only a few thousandths of a watt output, it will never break a DX pileup on its own!

Wednesday, October 1, 2014

What to do if your IFR 1200 Service Monitor display just shows "@" signs on the display - and if your deviation meter goes non-linear...

Figure 1.  A properly-working IFR-1200 in receive mode.
A few months ago the old IFR FM/AM-1200S service monitor on the bench at work started getting flaky.  If you turned it on, cold, it would just display a string of "@@@@@@@@@@@@@" ("at" signs) on the display and would not do anything else.

If it was allowed to sit in this state for a while and warm up, another power cycle could often bring it to life, but eventually this trick stopped working and it was completely dead.

Because we had another IFR 1200 kicking around, we used that for a while - but it had a nagging problem with its deviation meter in that it didn't read quite correctly so, eventually, I decided to tear into them both and fix the problems:  More on the deviation meter problem, later...

Fixing the "At sign" problem:

Pulling the Processor board I noticed immediately that there were exactly FOUR electrolytic capacitors - at least two of them were the typical power supply bypass capacitors, but one of them was related to a line that had something to do with the CPU's startup/reset - in this case, C6, the one near the upper-right corner.  This particular capacitor, if it becomes leaky, tends to hold the CPU's RST line high and the internal pull-down resistor cannot take it out of reset and you get it out of the "@@@@@@" mode.

Since, on old equipment, you can almost always blame the capacitor - and be right much of the time - I decided just to replace them all.  The picture below shows the board and the four capacitors:

Figure 2
The Processor board from the IFR FM/AM-1200S (the older "A-3" version).  The culprit is C6, the capacitor just to the left of the CPU near the upper-right corner.  The newer version of the board looks completely different and may/may not have a similar problem.  If it does, it would be due to C3, a 4.7uF capacitor along the top edge of its board.
Click on the picture for a larger version.
The four capacitors are easy to spot:  Three of them are clustered together near the bottom-left corner and the other (C6) is just to the left of the CPU, the large chip in the upper-right corner of Figure 2.

All four of these capacitors should be replaced while you are at it.  Note that your circuit board may also be of the "blue" variety and if so, it may be a bit fragile:  When removing the capacitors use a temperature-controlled soldering iron and proper techniques as these types of boards (e.g. the "blue" ones) are reportedly easy to damage.  I didn't have any trouble when working on this board, but this is good advice, nonetheless!

I replaced these capacitors with 47uF, high-temperature 105C devices.  It is important to note that the capacitor that you use to replace it must be either:
  • Shorter than the height of the tallest chip+socket on the board, or
  • If the capacitor is taller, you must use extra lead length and lay it on its side, using spaghetti tubing to insulate the extra leads.
The reason for this is that there is very limited clearance between this board and its neighbor:  If the capacitor is too tall it will hit the next board, either making the installation of this (or the adjacent) board impossible, or the aluminum end of the capacitor will short out components on the other board!

By the way, the capacitor that causes the problem with the version of the board shown in Figure 2 is C6, the one in the upper-right corner!

* * *

About that problem on the "other" IFR-1200S:  Non-linearity on the deviation meter.

It was noticed that the deviation reading on the other IFR-1200S wasn't "linear".  In other words, if I did a first Bessel Null with a 905.8 Hz tone and set the deviation meter for a reading of 2.18 kHz, it would read higher than 5.00 kHz at the second Bessel null of the same 905.8 Hz tone.

Interestingly, the deviation as displayed on the oscilloscope was correct, but the deviation meter was "off" by an ever-increasing, non-linear amount as the deviation went up.  I then noticed that this was NOT true if I switched to the "Medium" bandwidth mode - which is actually the "Wide" filter with a low-pass filter in the audio path.

What it turned out to be was one or more of the 10.7 MHz crystal filters on the IF on the "10.7 MHz Gen/Rec" board - and the reason that I determined this was that I noticed that the audio coming from the FM demodulator was variously distorted in the "FM Narrow" mode, but not distorted at all in the "FM Mid" or "FM Wide" modes.

Because, as we know, the "Mid" and "Wide" share the same filter (a fairly wide ceramic filter) we could rule out the demodulator itself as the culprit for the source of the distortion.  I also noted that the actual amount of distortion varied with the amount of deviation:  Because the location and amplitude of the sidebands of an FM signal vary with the amount of deviation (and modulation frequency) this also told me that there was something in the signal path that was asymmetrically disturbing this signal as it passed through - and it could only be one thing:  The "FM Narrow" IF filter!

Fortunately, I was able to use an off-the-shelf 2-pole ECS 10.7 MHz 15 kHz wide monolithic crystal filter to replace it with, retune the filter for lowest distortion (as indicated on the IFR's own distortion meter) and this fixed the problem!  Fortunately, these components are fairly cheap and as of the time of writing, still readily available.

For reference, these are Mouser part:  520-107-15B  - This comes as a matched pair of two 2-pole filters, which is exactly what you need - in other words, you buy "one" of these items and get two devices that have been matched at the factory. The Digi-Key part number for the exact, same item is:  X704-ND.

Because these new filters operate at 1.8k ohms instead of the higher impedance of the original crystal filters I had to parallel the input/output (R4 and R10) resistors with resistors to achieve the 1.8k source/termination impedance to properly match these filters:  For this I used 3.0k resistors in parallel with R4, R80, R6 and R81.

The specified ratings for the above pair of filters are 15 kHz at the -3dB points and at least -40dB at +/- 25 kHz.

The input/output termination transformers for these filters should be readjusted (using a non-metallic tool to prevent breakage of the core) according to the manual, although adjusting for minimum distortion with a 1 kHz tone set for 5-6 kHz deviation (using the unit's own distortion or SINAD meter, which also measures distortion/noise, but presents it differently) and then do similar for the "discrimination" coils on the demodulator, as well as slightly adjusting the various bandpass transformers along the signal path as well.  Please note that you will need to check/readjust the deviation for all three ranges of the deviation meter when you are done.

After I did this the deviation meter, once recalibrated according to the manual, behaved normally!

The two filters related to the FM signal path are YFL1 and YFL2.  These are the two metal-can crystal filters located closest to the top of the board (nearest to the connectors) in the center, un-sheilded area.

Note about the AM signal path and filter: 

The "AM" signal path uses a still-narrower filter that is not used in FM demodulation.  If this goes bad a suitable substitute may possibly be one of the narrowband FM filters of the same product line as the above.

It won't have the exact same bandwidth as the original filter, but the input/output impedance of these filters appears to be the same as the original.

The Mouser part number for the single device is 520-107-7.5A while the matched pair is 520-107.7.5B.  The Digi-Key part number for the X701-ND for the single device and X702-ND for the matched pair.

The devices noted above, when used as a pair, are rated at 7.5 kHz bandwidth at the -3dB points and -40dB at +/- 14 kHz with an input/output impedance of 1.8k.

The AM filters are YFL3 and YFL4 and these are the "crystal cans" located just below YFL1 and YFL2 noted above.  There may be small chip ceramic capacitors connected directly to the leads of these crystal filters on the bottom side of the board:  When you remove them, note exactly where they went.  Unless you have the test equipment to "sweep" the AM IF filter precisely, to not worry about putting these capacitors back into place as they were matched with the original crystal filters and the properties of those specific units when they were new.

Since the aforementioned replacements are slightly wider in bandwidth, and since AM is generally more "forgiving" than FM in terms of and phase imbalance across the passband when measuring things like distortion (but not necessarily frequency response) - and the fact that many amateur radio operators would not really be using it for AM very much anyway - just plopping in new filters to replace obviously-defective units will probably result in reasonable performance, anyway!

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, you should 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 - and lessons learned:

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!

Additionally, be very careful when handling the microcontroller:  DO NOT handle it by the pins - not because of the static, but because they are very easy to bend and once you bend them, they can be extremely difficult to get back into their required sub-millimeter, precise (in all three axes!) position - so just DON'T BEND THEM!

Figure 1:
The UI (User Interface) board - the side with the MCU on it, before adding the connectors, switches and the connector along the top edge.
You might notice that I installed both U5 and Y1 along with C93 asnd C94 as I wasn't watching what I was doing:  You need only purchase either Y1 or U5 (U5 gives a bit better sample rate accuracy and stability, but Y1 is probably perfectly adequate).   If you use U5, you do not need to get Y1 or C93/C94.
Click on the image for a larger version.

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. A TEMPERATURE-CONTROLLED iron.  This is so important that I thought that I'd say it twice!
  3. A really fine tip for your soldering iron.  This tip should be almost needle-thin at its point - and that is no exaggeration!  This tip would not be used for soldering many of the larger components - for that you would be using just a "small" tip:  You would change to this really fine tip when you need to do a bit of re-work, such as tack down a single pin (or two) that didn't flow properly, or use it to "wick up" a bit of solder that might have bridged some pins.
  4. 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.
  5. 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!
  6. 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.
  7. 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.
  8. 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!
You SHOULD NOT start construction until you have ALL of the items above!

Figure 2:
 One side of the the partially assembled RF board.
Click on the image for a larger version.

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.  If you are in the U.S. you can get a small amount (but more than you need for this project) for a reasonable price from Cash Olsen, KD5SSJ.  (Google him for more info, or see the link 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.  These units are not terribly expensive, available for much less than $100 U.S. for a decent one if you look around.  I paid about $90 for one about 10 years ago from Circuit Specialists and they have become cheaper and better since then, with many different types available from both Circuit Specialists, EvilBay and many other places.
  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 sticks the component to the board better.
  • 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 exactly 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, and you can look at the page of Cash Olsen, KD5SSJ - http://www.kd5ssj.com/   (Note:  I have not pecuniary interest in Mr. Olsen's web page or offerings other than I did buy some solder paste from him several years ago and was satified with his product.)

Figure 3:
 The "other" side of the RF board, also partially assembled.
Click on the image for a larger version.
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 can 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