A (simple) WWVB loop amplifier for radio-controlled clocks

Note:

The techniques described below should work - with only minor adaptation - for any "Longwave" time signal used by these radio-controlled (non-GPS) clocks - not only WWVB, but DCF77, MSF, BPC and both JJY signals as well.

* * * * *

Last year I moved a bunch of SDRs (KiwiSDRs, RTL-SDR) and a bunch of network gear to a new shelf in my shack, but this placed them much closer to the wall on which I'd previously mounted the two "Atomic" (e.g. radio-controlled) clocks which had been there - and operating - for years.  Since then, they hadn't been able to reliably synchronize to the 60 kHz WWVB signal out of Fort Collins, Colorado.

Figure 1:
The two radio clocks surrounded by
the four-turn loop, near a number of
pieces of "noisy"equipment.
The top clock is set to UTC and
the bottom for local time.
Click for a larger version.

While annoying, I wasn't terribly surprised.  There are several switch-mode power supplies involved in the aformentioned gear and it's not uncommon for them to operate in the 30-60kHz range, offering the potential of "jamming" the receivers.  As both the location of these clocks - and the nearby gear - is convenient, I wasn't too inclined to move them again and initial efforts to "filter" the switching power supplies didn't really help - but I wasn't surprised about this, either, since it's likely direct coupling of their magnetic fields that is the culprit rather than any electrostatic field as the clocks themselves use ferrite loopstick antennas sensitive to just the H-field.

A solution

Many years ago a friend came to me to solve a similar problem in a downtown Salt Lake office building where the WWVB clocks in a conference room never synchronized and I constructed the remote loop and amplifier/coupling system, described here:

• Getting "Atomic" (WWVB) clocks to work indoors and in weak signal areas - LINK 

In short, a rooftop loop antenna amplified the signal and it was conveyed into the room with the clocks where it was further amplified and then, using inductive loops placed in the proximity of the clocks.  This is how the WWVB signal was coupled to them.  To my knowledge, this system worked for many years (well over a decade) and for all I know, it may still be in use.

 Why revisit?

I've tackled this type of problem before - but I decided to revisit it as the circumstances are slightly different:  I already had a signal source as noted below plus I wanted to see if I could do this with more commonly-available components in a simpler manner.

While I don't have a WWVB loop on my roof, I do have a dedicated LF E-field whip antenna - a 40 year old LF Engineering LF-400B with integrated low-pass filter.  This antenna has been on the roof wherever I have lived almost continuously since I purchased it in the mid-late 1980s and with a few repairs over the years, it still works well, having been on the roof of my current house for several decades.  Its use for LF reception as described on the following page:

• A (semi)-typical suburban E-field whip receive system for the 630 and 2200 meter amateur bands - LINK 

The fact that I already had an LF/VLF receive antenna system meant that I had a "clean" source for WWVB, and other devices that receive signals below 500 kHz (e.g. LF receivers for 630 and 2200 meter operation and my Blitzortung "Blue" receiver) and I decided to add one more to the list.

Other types of outdoor antennas 

Note that the circuit described here should work well with other types of active antennas including E-field types such as the PA0RDT "Mini-Whip" and the DX Engineering ARAV3 - to name but two.  An amplified loop such as the Wellbrook and similar will work, provided that it is not oriented such that the desired time station's transmitter is not in its nulls.

Buffer/Amplifier

Through back-of-the-envelope calculations I figured that the already-amplified signal from the active whip needed another 15dB or so of boost and it could then be applied to a loop of wire around the WWVB clocks on my wall.  One thing that helps greatly is that the WWVB signal is extremely strong here in northern Utah - on the order of 5mV/meter or so - and connecting an oscilloscope to my LF-400B whip's signal output showed that the amplitude-modulated time code of the 60 kHz signal from WWVB was visible among the many others.

What I needed to do was to tap off the signal (e.g. "bridge" the connection) from the existing coaxial cable without affecting was was being sent to the other devices using it, amplify it. and apply it to the loop - and I did this "tap" using a BNC "Tee" connector on my antenna feed.

The circuit diagram below gives more details:

Figure 2:
The schematic of the loop buffer/amplifier/driver showing the isolation from the power supply
via L1, the buffer circuit of Q1 and the amplifier and loop driver of Q2.
Click on the image for a larger version.

Circuit description

Of high importance is L1, a common-mode choke, liberated from a failed switch-mode power supply somewhere.  This particular unit has an inductance of about 1mH per winding meaning that it has about 377 Ohms of impedance at 60 kHz and helps to prevent a ground loop and the coupling of noise from the power mains.  If you replicate this circuit I would strongly suggest that whatever you use for L1 have at least a similar amount of inductance.  On either side of L1 are electrolytic capacitors (C1, C2 - preferably of low ESR types) to offer low impedance and a degree of reinforcement of common-mode rejection through L1 while C2 and C3 provide RF bypassing for the circuit itself.

A buffer amplifier consisting of Q1 - with a high-impedance input, but no actual gain - couples the signal from the existing antenna:  Having several k-Ohm of input impedance, it is unlikely to appreciably load the existing antenna system.  On the feed from the E-field whip, I simply installed a coaxial "T" connector to allow me to bridge across the signal feed rather than split the signal, which would have been complicated owing to the fact that the DC power for the whip was also being carried on that same cable.  The connection to the amplifier in Figure 3 was made using a very short piece of coaxial cable (about 2 feet long - less than a meter) and since this whip is not used for reception above about 500 kHz, neither its presence or that of the added amplifier had any discernible effect on the other received signals.

Coupling from the existing antenna are series components L2 and C4, selected to resonate at about 60 kHz:  The resonance is extremely broad, so finding a capacitor combination precisely equal to the "ideal" value of C4 - according to the formula below actually calculating as 0.007uF (7000 pF) - is unimportant.  This series resonant circuit is probably not essential and a simple coupling capacitor of 0.01uF (10000 pF) could be used (omitting L2 entirely) but I chose built it with L2 to broadly filter off-frequency signals - something that might be important if your E-field whip antenna doesn't have a low-pass filter to remove AM (Mediumwave) signals as mine does as well as to block any stray coupling of HF signals when I transmit.

Figure 3:
The circuit of Figure 2 built on a piece of prototyping board
in the case.  Bifilar choke L1 is on the right with the BNC
connector (J1, input) and output to the loop (J2) on the left.
Click on the image for a larger version.

The buffered signal from Q1 is then passed to amplifier Q2 which is configured to have "about" 15dB of signal gain.  This circuit is, perhaps, slightly more complicated than it needs to be, but with its feedback, it is very stable and tolerant of large signals.  The use of electrolytic capacitors for C5 and C6 is, perhaps, overkill (0.1uF ceramic would probably suffice) but I used them as they were handy.

As the signal from WWVB is quite strong at this location, there is only one stage of amplification shown in Figure 2, but if I lived more distant, greater overall system gain might be required.  Replicating the circuit involving Q2 (R4-R8, C5-C6) and cascading it with the existing amplifier would add yet another block of gain to boost the absolute signal level - but this would presume that whatever active antenna you were using outdoors to pick up the WWVB signal was working well, providing a "clean" signal and that the deficit was just in signal strength at the clocks rather than than signal-noise ratio.

Due to the smallness of the project box that I chose I couldn't mount the bifilar choke "through" the prototype board so it was mounted on the edge to minimize height.  To hold it in place I used UV cured resin along the edge to prevent it from breaking the pin connections mechanically:  UV cured resin is very handy as it's about a strong as epoxy, but it is cured almost instantly meaning that it's able to be handled immediately.  For the BNC connector, the one that I found in my parts bin didn't have its matching mounting nut, but more UV-cured epoxy did the job for that, too!  As can be seen in Figure 3, I didn't bother "mounting" the board in the box, letting it hang about on its own wires.

Indoor Coupling loop

The "coupling loop" - visible in Figure 1 and shown on the schematic - is just a loop of wire - and it is used to inductively couple the signals from the outside antenna to the clocks.  In my case, I measured a rectangle that would encompass both of the wall clocks and found a cardboard box with similar dimensions and on it I wound four turns of 22AWG hookup wire.  Connecting this loop to the amplifier, I used some shielded microphone cable:  Coaxial cable would have been fine as would just some single-pair speaker wire as this frequency is not all that much higher than audio!

Neatly forming the individual conductors, I used small "zip" ties to hold them together and with four screws, attached it to the wall, placing the clocks inside the loop of wire.  Within the loop, signals from the amplifier would be strongly coupled into the ferrite loopsticks in the clocks themselves - but being very small in terms of the 60kHz wavelength, this loop is unlikely to radiate more than a few feet/meter outside it.

To improve efficiency of the coupling loop I wanted to series-resonate it at around 60 kHz as this would increase the amount of energy transferred to the loop from the amplifier somewhat, effectively providing "free" signal gain.  Measuring the inductance of the loop I found that it happened to be about 22uH and using this simple formula, I calculated the value of C7 - the resonating capacitor in Figure 3:

LC = 25330/(F_MHz)²

Where:

LC is the product of the inductance and capacitance (e.g. Capacitance in pF * Inductance in uH)

F_MHz is the desired resonant frequency in MHz (e.g. kHz/1000)

Knowing that we have 22uH of inductance in the coupling loop and a frequency of 60 kHz (0.06MHz) we end up with "LC" being equal to 7036111.  Dividing this value by the known inductance of our coupling loop (22uH) we get  the capacitance, as in (7036111/22) = 319823pF, or 0.319uF.

Figure 4:
The finished amplifier in its box, hanging
out below the loop - connected, and in
service.  (It's just visible in the bottom
of Figure 1)
Click on the image for a larger version.

As 0.33uF (330000 pF) is the closest common capacitor value, I used that for C7.  Again, as with C4 and L2, the resonance is very broad and precision isn't too important.  The article linked near the top of this page goes into more detail on how one would construct and resonate a coupling loop.  Based on this formula, if I wanted to resonate the same loop for use with DCF77 at 77.5 kHz I would have picked a 0.18 or 0.2uF (180000 or 200000 pf) capacitor, instead.  Similar changes could be made to accommodate longwave time signals on other frequencies (e.g. 40 kHz, 50 kHz, 68 kHz).

The formula above can also be used to calculate the value of C4 with the 1mH (1000uH) L2 inductor:  If your interest was for another frequency, such as DCF77 at 77.5 kHz, C4 would be 0.0047uF (4700 pf), instead.

It need not be said that this loop should not be placed very close to whatever outdoor receive antenna you are using - but more than about 10-15 feet (3-5 meters) should suffice:  If they are too close to each other, feedback (oscillation) could occur - but as this loop is only around 0.01% of a wavelength in circumference it does not radiate efficiently at all - and since it's inductive, its signals won't efficiently couple to an E-field antenna, anyway.

In the diagram, C7, the resonating capacitor for the coupling loop, is shown at the amplifier - but it could have been placed at the loop itself.

Power supply  

First off, do not use a switching power supply for this device!

As noted, common-mode choke L1 was used to "decouple" the power supply from the amplifier - and also from the coaxial cable of the LF antenna.  To power this loop amplifier I would strongly recommend using ONLY a transformer-type DC power supply and NOT any type of switching power supply for the simple reason that the switching power supply will be comparatively noisy, and it - its harmonic - will likely operate at/near the frequency of WWVB or whatever time signal you are trying to receive.

This power supply does not need to be regulated:  Simple capacitor filtering with low-ish ripple (a few hundred millivolts) will suffice and any voltage between about 11 and 16 volts will work which means that about any old "wall wart" in that voltage range - regulated or not - would be fine.

Conclusion

Having had the parts on hand it took only a bit more than an hour to piece this together and almost as long to put it in the box seen in Figure 4.

When I forced both clocks to re-acquire WWVB's signal for syncing they immediately set themselves to the correct time and date - and since it had been the start of daylight saving time the night before but had not been able to synchronize prior to this - they "knew" the new, correct time, too!

* * * * *

This page stolen from ka7oei.blogspot.com

[END]

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 money.  Whether it actually saved much energy overall is debatable since it probably took a lot of energy to make (and ship!) the device that the failed supply powered in the first place!

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 a genuine (not counterfeit!) 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 problem was due to how and to what 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 its cable, 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 and on to conductors that could act as antennas!

What it 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 (and all noise reduction is turned off) 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 occurs are more obvious 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 amplitude 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 and 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 just 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 seem resemble noise from a switching power supply - at least until one switch to "AM" and observed that the background noise seemed to be modulated with twice the mains frequency.

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 will not be enough.

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 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 the probability of the device in question from 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 to keep RF out of it.  They are much less effective in quashing the emission of RFI emitted 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.

As noted before, in this case I'd already have installed additional filtering in the power supply that was orders of magnitude more effective than simple snap-on ferrite devices - 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 with values between 100uF and 1000 uF, inclusive.
• 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.
• An AC line filter - or parts to make one.
• 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:

• If you don't have an AC line filter on-hand, you'll need to get the parts for one and the first on the list is a 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, enough inductance to be effective down to a few hundred kHz.
• 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. where 120 volts is used, these capacitors are typically 0.1-0.47 uF.    Make sure that the capacitors that you use have "X1" or "X2" marked on it somewhere, indicating that it is both safe and designed for this purpose.
• 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).  Make sure that the capacitors that you use have "Y1" or "Y2" marked on it somewhere, indicating that it is both safe and designed for this purpose.
• 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, but a fuse on each side of the AC line may be required in other parts of the world.



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 scavenge - 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 they 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 AC voltage rating!  The capacitors typically used for these applications are usually (but not always) blue, light yellow or white in color and have printed on them an AC voltage rating.

• Make sure that the mains-connected capacitors that you use have an "X1", "X2" "Y1" or "Y2" mark on them indicating that they may be safely used for power mains filtering.

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.  Such a pre-built filter unit is depicted in the upper-left of Figure 5, below:  If you find one of those, by all means, use it!

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 diminished 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.  So-called "safety" capacitors were used in the above filter which are designed to not fail catastrophically:  The smaller-value capacitors - C1 and C2 are "Y" class, which do not fail shorted (a failure could connect the "hot" lead of the mains supply to ground) while the larger-value capacitors (C3, C4) are of the "X" class type which are designed to fail safely - going open rather than shorting which could otherwise cause them to melt, burn or explode.

Figure 7: 
Schematic of the AC input filter.
Note:  Typically, a 100k-1Megohm "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 permanently 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 a 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.

Errata:
 Please note that "C4" is also a low-ESR electrolytic capacitor, not "C2" as
indicated in the text, above.

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 junked PC power supply again I noticed that there were two toroidal 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 with a 0.1uF monolithic ceramic capacitor.  The use of these low-ESR capacitors rather than "normal" electrolytic capacitors is important as these types are specially-designed to remove the high-frequency components.  Once you get above a few hundred kHz and into the MHz range 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 and "seem" to be good, 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" or "Manhattan style" on it, using the copper itself as a ground plane.  The circuit board material 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 broad metal plane of 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 these ordinary disk types 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) after having already been attenuated by the chokes 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 and it is likely that a shortwave receiver would have detected it at very short range (e.g. within a few feet/a meter) but (importantly!) the "grunge" would not be conducted on either the AC input or DC output leads:  The 100% cover of the case is there mostly to prevent accidental electric shock and shorting of the otherwise exposed AC mains connections and that there is no chance at all of any radiation of noise from this power supply - even over very short distances! 

It's worth nothing that the "shielding" by the metal box is NOT what is containing the RFI, but rather the fact that the input/output RF currents from the power supply are shunted to a common conductor (the box) which effectively eliminates any differential RF on the in/out leads:  If the cover was left off the aperture of the open box would be to small to effectively radiate RF - at least at HF - and the over is mostly for the purposes of aesthetics and safety.

Figure 12: 
The completed, enclosed, power supply, the DC output lead seen emerging
via a grommet.
Click on the image for a larger version. 

Other related articles on this subject:

• The December 8, 2012 entry about "Reducing Switch Supply Racket racket (RF Interference) - Link 
• See also the September 4, 2013 entry, "Quieting High-Current switching supplies used in the ham shack - link
• The article "Minimizing VHF and HF RFI from electronic ballasts and fluorescent tubes - link - from December 17, 2015 may also be useful in quashing interference from these sources.
• The December 1, 2017 article "Containing RF noise from a sine wave UPS" - link - also details how interference from an RF-noisy device may be suppressed.
• Quieting an insanely noisy LED floodlight - link.  This describes how a constant-current LED supply that produced enough interference to quash HF reception was quieted down to the point of undetectability. 
• Quieting a 150 watt Samlex sine wave inverter - link.  This page talks about making an otherwise RF-noisy sine wave inverter quiet at radio frequencies.
  

[End]

This page stolen from ka7oei.blogspot.com

Reducing switching supply racket (RF Interference)

Note:

There is a follow-up to this posting in the August 18, 2014 blog entry - link - where there are details given to contain the switching supply noise even more!

Switching power supplies are ubiquitous these days - and for several good reasons:

Figure 1:
Typical laptop-type switching
power supply - the very unit
that was modified!
Click on the image for
a larger version

• They are more efficient than plain old iron transformer power supply with a linear regulator.
• They can be much smaller and lighter than their transformer/linear counterparts.
• They are cheap by comparison since they use less material overall - particularly iron and copper - in the transformer.

 They do have several real drawbacks:

• Most tend to be less reliable than their old heavy iron counterparts.  I've observed that the typical switching-type "wall wart" (plug-in power supply) seems to last just 2-4 years whereas the old-fashioned iron types would usually outlast the device to which they were connected.
• They can generate some terrible radio interference!

On the first point, I often wonder if the amount of power they save due to their efficiency is outweighed by the fact that they often fail after just a few years, often causing it and the device it powered to end up in the trash because of the failure of less than $1 worth of components - but that's another topic of discussion!

Shown in figure 1, above, is a typical power supply of the sort used on a laptop computer.  As far as switching supplies go, this is one of the better-built units, now used to power a small form-factor PC that I have attached to my TV to watch digital/online media - and because of this, it's plugged in pretty much all of the time.

Note:  I have since plugged this supply into a "smart" power strip.  This strip has a sensing circuit that detects when the TV is turned on and only then are the "switched" outlets powered up, saving energy by powering down those devices that are never used when the power is off.

Figure 2:
Typical "Common Mode" AC line filter.   The capacitors force RF
to be "common mode" so that the bifilar inductor (in the middle) can
best do its work!

As it turns out I could hear some (admittedly weak) harmonics of a switching supply on my HF receivers, but I generally ignored them until I happened to tune across the AM band on my newly-repaired Marantz receiver (see the previous blog entry) and heard some very strong hum-laden carriers every 30-50 kHz across the broadcast band that blotted out most of the local stations.  Unplugging nearby mains-powered devices soon revealed that the source was (mostly) the power supply pictured above, located only a few feet away from the receiver.

Taking this as a challenge - and an excuse to take some pictures and do a write-up for this blog - I set about to make this power supply much less obnoxious, RF-wise, so I put the power supply on the bench and popped it apart.

Figure 2:
Inside the power supply - the AC input on the left side.
The original bifilar RFI filtering choke (upper-left)
has the green/yellow wire wound onto it.
Click on the image for a larger version.

The usual warnings about high voltages:

• This power supply - and others like it - operate from potentially lethal line voltages.
• DO NOT attempt to open or modify a power supply unless you are thoroughly familiar with the proper techniques and safety precautions when working with these voltages.



Figure 3:
A close up of the RFI limiting components.  Below the bifilar
choke (the device with the green/yellow winding) is the
capacitor that forces RF energy to be "common mode."
Click on the image for a larger version.

• If done improperly, modifications to the power supply may make it unsafe to use and become a fire and/or shock hazard, so do not do this sort of work unless you know exactly what you are doing!

The main RFI suppressing components of the power supply may be seen in figure 4 with the AC input on the left - namely the black device with the green and yellow wire wound on it (a bifilar-wound inductor) and a capacitor - the black rectangular box (marked with "104") below it.

A switching power supply is really a powerful oscillator with the voltage being transferred to the load with a small transformer - the size reduction compared to the old-style "wall warts" being permitted because the power supply operates at a frequency much higher than that of the line voltage's 60 (or 50) Hz, and at several 10's of KHz, usually in the 30-60 kHz range for most of these types of power supplies.  This higher frequency of operation is also the reason why switching power supplies often cause interference issues to radio receivers:  It is the harmonics from this high-power oscillator that are more likely to be conducted to the outside world via the AC power connector and/or the DC output.

In Figure 2 is the diagram of a typical "common mode" AC line filter.  Looking similar to a transformer is the bifilar choke that is doing most of the work of filtering the high frequency components of the switching supply plus it also can do a pretty good job of actually isolating the power line at these higher frequencies so that not only are those spectral components generated inside the power line contained therein, but also that the supply itself won't supply a path to conduct RF energy from whatever it is that is being powered by the supply (a computer, set-top box, modem, etc.) into the power line itself.

The way that this works is that any RF energy on one side of the choke will get coupled to the other side of the choke equally.  Since this bifilar choke is a choke, its inductance will form a series impedance to block higher frequencies from passing through, the effectiveness being related to the inductance of the winding itself.

Key to this working properly is that any RF energy on one side of the bifilar choke must be exactly equal to the other side or else the imbalance can actually cause more interference as unequal RF energy from one side would be induced on the other side.  To force the RF energy to be equal is the job of the two capacitors shown - one on the input, and the other on the output.

This particular power supply had only a capacitor on the load side of the power supply - where the noise was being generated.  While this will do most of the work, it does help to have a capacitor on both sides, but this is often not done as a cost-saving measure.

Figure 4:
Inductance of the original coil.
With only 268 uH per side:
That's not much filtering at
AM or the lower HF bands!
Click on the image
for a larger version.

Since the yellow-green wired inductor didn't seem to be adequate, I removed it from the power supply and measured it (see figure 5).  Noting that the inductance is a mere 268 uH, I thought that I could do better with some other line-filtering inductors that I happened to have in my junk box - this one (figure 6) measuring about 4.594 millihenries (4594 uH) which  is about 17 times as much inductance which also means that it will, ideally, offer 17 times as much impedance to RF energy that might escape from the power supply via the AC power line.

Since the original choke was 268 uH, let's find out how much equivalent series resistance that amount of inductance offers at, say, 1 MHz - in the middle of the AM broadcast band.  The formula for inductive reactance is:

Z = 2 * Pi * F * L

Where:
   F = Frequency in Hz
   L = Inductance in Henries
   Z = Inductive reactance in Ohms

So, plugging 268 uH at 1 MHz into the above we get 1683 ohms - not too bad, actually.  By replacing this choke with the 4.594 millihenry version our impedance scales up proportionally to 28.850k ohms at 1 MHz!  In addition to the bifilar action of the choke, this significant amount of inductive reactance will go a long way toward both keeping the RF energy from the switching supply off the power line, but it will also keep the power supply itself from acting as a pathway to couple potential interference from the devices connected to it to/from the power line.

Comment:

It is common to attempt the use of ferrite beads to suppress RF Interference of this sort, but it's very unlikely that it will help much - particularly at lower frequencies (e.g. lower HF bands such as 160 and 80 meters, not to mention the AM broadcast band) because these devices simply cannot add enough inductance to add a significant amount of impedance:  At these frequencies (say, below 10 MHz) it takes multiple turns on a chunk of ferrite to add enough reactance to make even a small dent in the amount of conducted interference!

Cramming this much larger component into the same space as the original bifilar choke was a bit of a challenge, but laying it on its side and using "flying leads" to connect the inductor to the circuit board made it possible to fit it inside the case.

For good measure I also added another capacitor (a 0.047 uF device) to the "other" side of the inductor (the side opposite the black capacitor mentioned above) to better-equalize any RF currents that might occur across it (the small green capacitor in figure 7).  Just to be safe, I also put some polyimide (a.k.a. Kapton^tm) tape on the aluminum heat sink (visible in figure 9) to make sure that the windings of the coil could not touch (and electrify) the heat sink itself or other nearby components.

Figure 5:
Inductance of the new coil.
With 17 times the original
coil's inductance, it's likely
to provide better filtering
at lower frequencies!
Click on the image
for a larger version.

Having installed this new bifilar inductor I still had the original bifilar device (the one with the green and yellow wire) on hand so I decided to put it on the DC output to further contain any RF energy emitted by the power supply - and why not, since it was "free"!  Using its color coded windings, I connected it as shown in figures 7 and 8 with heat shrink tubing to insulate the soldered connections on the power supply's DC output cord.

Putting everything back in the case I carefully re-checked the clearances and insulation to make certain that not only would everything fit, but also that nothing could short out - especially when everything was smashed together when the cover was put back on.  While I could have glued the two halves of the cover back together, I decided to use some of the same polyimide tape mentioned above as it has a very strong adhesive - and I would be able to easily take the power supply apart should there have be a problem.  After reassembly, I then re-checked the DC polarity of the output connector to make sure that I didn't accidentally reverse it when connecting the output choke.

The result?

While I can still "hear" the harmonics radiated from this power supply on the AM radio that's just a few feet away, they were now weaker that most AM stations instead of being "extremely loud" and clobbering much of the AM dial - this fact indicating a reasonable amount of success.  While the intent was not to attempt to completely "clean up" the power supply's spurious radiation, the radical difference indicated that all spurious radiation from this particular power supply was likely to be very much reduced.  Elsewhere in the spectrum, I can no longer hear even a hint of this power supply on any HF band!

Figure 6:
"New" inductor with added 0.047 uF capacitor.  It is
connected with "flying leads" to provide connections
into the circuit.  Not seen in this picture is additional
insulation added between the body of the
new choke and the heat sink.
Click on the image for a larger version.

Since these types of power supplies are seemingly everywhere, it should come as no surprise that there are several of these in my ham shack and I've applied the above techniques to those other power supplies that were found to cause interference on the HF frequencies.  Depending on the power supply and the amount of extra room inside the case, one may (or may not) be able to add as many additional inductors and capacitors as was needed to quash the RFI emitted by the power supply, so in several instances I've added filtering outside the case,  typically inserting capacitors and a bifilar inductor on the DC lead (but close to the power supply) where it would be safe to do so.

Figure 7:
The original bifilar inductor, now connected on the DC
output to provide additional filtering.  Heat-shrink
tubing was used to insulate the output DC connections.
Click on the image for a larger version.

Ideally, one would put such filtering (e.g. inductors) on both the AC and DC leads, but it's worth remembering that these power supplies pollute the RF environment largely by conducting the harmonics of the switching frequencies through the input and output leads:  If one blocks the RF energy from being conducted on just one lead or the other (e.g. the AC input or the DC output) the circulating currents carrying this energy through the power supply (e.g. in on the AC side and out on the DC side) are significantly reduced and adding such blocking can considerably reduce emitted RFI.  Also worth mentioning is the fact that many switching-type DC supplies - particularly "wall-wart" types - have minimal or no common-mode filtering (e.g. using a bifilar choke or two separate series chokes) on their DC output, probably because it's a bit more expensive to do it this way.

I've noticed upon opening the case that some switching power supplies - perhaps of dubious origin and quality - are completely missing the RFI filtering components.  In these same power supplies it is often apparent that there is a position on the circuit board for these components, but they are either empty (in the case of missing capacitors) or jumpered over (in the case of missing inductors) - clearly a cost-saving measure and probably illegal in some countries.  For these power supplies the addition of any RFI suppressing components will likely have a significant effect on reducing interference that they may generate!  I've also observed that many of these same supplies of unknown pedigree often use the cheapest-possible components and it may well be that they will not prove to have a long lifespan!

Figure 8:
The modified power supply with the reconfigured filtering
and placed in the bottom half of the original case.
Click on the image for a larger version. 

Where does one get these bifilar inductors?  Most computer-type power supplies have these on their inputs and they may be found in most reasonably-quality switching supplies.  Remember how I mentioned that these switching supplies often die after just a couple of years?  These dead supplies may be a ready source of components to better RFI-proof the supply that may be causing interference to you!

Figure 9 shows, in the highlighted portions, the bifilar inductors - and some associated capacitors - found in some typical junked power supplies.  On the left is a typical PC power supply  where one can see what looks like a small transformer next to the AC power line fuse.  On the right is a power supply from a junked VCR with the bifilar inductor also very near the AC power line fuse.

Note:  If you raid junked power supplies for components, make sure that they are unplugged (obviously!) and that the large, high-voltage capacitors filter have been safely discharged.  If you are unsure about how to do this, please seek advice and help from someone who does know before engaging in a project dealing with potentially deadly AC power voltages!

Figure 9: 
Examples of RF filtering components found in junked
switching power supplies.  On the left is a PC (computer)
power supply while on the right is a power supply from
a VCR.
Click on the image for a larger version.

These two bifilar chokes - while somewhat different in style - are split in two with one side of the AC power line on one side, and the other side of the AC power line on the other.  Being wound on a common core, their winding are very tightly AC-coupled (at radio frequencies, at least!) which is how they function to prevent conduction of this energy onto the AC power line.  Before removing them from the board, verify with an ohmmeter from the original AC power connection that the inductors you spot are, in fact, in series with the power line - with one half on one side, and the the other half on the other side!

You'll also notice that these two power supplies have something in common:  There are capacitors very near the bifilar inductor.  In the case of the PC power supply (on the left) there is a large, yellow rectangular capacitor on the AC input of the power supply and on the opposite side, there are two blue disk-ceramic capacitors (one of them covered with heat-shrink tubing).  In the case of the VCR power supply (on the right) you'll see even more filtering:  There are several blue capacitors sprinkled throughout, but also the orange-red capacitors next to the bifilar inductor itself.

It is quite typical for there to be blue capacitors on the inputs of power supplies for filtering - these being "safety components" that are specifically designed for both filtering, and for reliability so that their failure won't inadvertently cause the case of the device to be connected to the dangerous AC line voltage!  The other capacitors - the big yellow one on the PC supply and the two orange-red ones on the VCR supply - actually do much of the filtering.  The one thing that all of these capacitors (blue, yellow and orange-red) have in common is that they are specifically rated to withstand the AC line voltage!  Careful inspection of these components will reveal not only their capacitance value, but also their voltage rating.

If one is reasonably careful, discarded switching power supplies can offer a ready source of components - both inductors and capacitors - to help reduce their conduction of switching energy and the interference that it may cause.