Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes

Several years ago we started replacing the old, "iron" ballasts with T-12 fluorescent tubes at my work with T-8 tubes and electronic ballasts.  The program to do this was along these lines:

• When a T-12 tube or ballast failed, immediately retrofit the fixture.
• As soon as practical, retrofit those lights that are on all of the time, or "most" of the time.
• Gradually retrofit other fixtures as needed/convenient.

The lamp sockets (known as "tombstones") for T-8 and T-12 sockets are, for all practical purposes, interchangeable.  (Yes, there can be some voltage rating differences...)  Since, in most cases, we were working with what had originally been 4-tube fixtures - two of which (e.g. one ballast) had been disconnected years ago to save energy (we didn't miss the extra light)  we had extra tombstones on-hand to replace those that were broken, discolored or damaged and in some cases we even replaced the bracket and tombstone with those of the brand new "official" T-8 variety.

Figure 1:
Typical "iron" ballast used for T-12 tubes.
Click on the image for a larger version.

We were also able to get T-8 ballasts of known-good brands (Advance, Triad, Sylvania, etc.) for very good prices from an number of sources - far lower than "retail" - and we were sticking only with these brands because we felt that they would offer good reliability and efficiency.  In the years since we have started, the majority of fixtures have been retrofitted and we have yet to replace even a single electronic ballast and we've only replaced a half-dozen tubes - most of those within a week or so of being installed - all of this while providing at least as much light with lower power usage and heat load.

Before we go on, here are a few "weasel words" of warning:

• The devices discussed present hazardous/fatal shock hazards:  Do not even think about doing any such modification unless you have experience with such things.
• There are no guarantees, expressed or implied, that the modifications will work for you, are suitable or will meet electrical code requirements in your area.
• If you do such modifications you take any and all risks related to it, including damage to person or property, injury, fire, or liability.
• All RF interference situations are unique:  There is no guarantee at all that even when performed as described that the steps described will mitigate problems that you might be experiencing.
• Please use care and common sense!
• You have been warned!

The rise of interference:

There is one place where we have had problems with the T-8 tubes and accompanying electronic ballasts and that is the electronics shop.

Soon after retrofitting the fixtures in the engineering shop we noticed that we had difficulty hearing local 2 meter and UHF repeater when listening in that room, the noise floor having risen up by 10-20dB - depending on frequency and exact location within the room - with 120 Hz modulated noise and the fact that it disappeared when the lights were turned off pointed directly to the cause.


Differences between magnetic and electronic ballasts:


For a typical 4-foot (approx. 102 cm) tube, good, old magnetic ballasts are, at least when powered from 120 volts, transformers that boost the voltage to that high enough to sustain conduction on each cycle of the 120 Hz sinusoid.  When starting up there are special windings that will activate the filament and/or boost the voltage such that this conduction - which is higher when the tube is "cold" (both physically and has not been lit) but when the current is established as the tube lights, these diminish. To limit the current through the tube to a safe value it is typical that there is a large amount of series inductance which, by virtue of its reactance, sets the maximum current consumed by the lamp.

For more general information about how fluorescent tubes and ballasts work, go to "Sam's F-Lamp FAQ" - link.

For various reasons (core, skin-effect, magnetic and resistive losses, etc.) these old "iron" ballasts are somewhat inefficient, but even so, this efficiency of the linear, fluorescent tube made the combination far more efficient than practically any other tungsten (incandescent) light source.  Nowadays, the skinnier "T-8" tubes are used along with electronic ballasts and these skinnier tubes can, for various reasons, produce at least as much light as the larger T-12 tubes, but in order to maintain such efficiency and to provide reasonable longevity they must be driven with an electronic ballast.

Unlike the old-fashioned "iron" ballast, modern, electronic ballast are essentially high-power oscillators that produce a lot of voltage (perhaps much as 1kV peak, open-circuit and while starting the tube) at a fairly high frequency - say, 30-60 kHz.  Using this higher frequency allows much smaller, lower-loss magnetics to be used and driving the tube with a higher frequency than that of the mains improves its "power to light" conversion efficiency even more.

Electronic ballasts and interference:

Even the old, "iron" ballasts with the fluorescent lamps could cause RFI (Radio Frequency Interference), largely due to significant nonlinearities in the conduction of the tube with respect to the applied voltage resulting in the generation of harmonics.  Furthermore, a malfunctioning ballast could generate interference in other ways, such as internal arcing. Typically, such interference, if present, occurred on the mediumwave (AM broadcast) frequencies and, occasionally, in the HF (shortwave) range and, most typically, either very short range - within a few feet/meters of the lamps themselves - or conducted via the power connections.  Usually, replacement of the defective tubes and/or ballast remedied this situation.

With electronic ballasts the situation is quite different.

With a high-power (10's of watts) electronic oscillator on-board there are already problems at hand:

• With frequencies in the 10's of kHz, harmonics appearing in the mediumwave and HF are of much lower order.  In other words, the 30th harmonic of the switching frequency in an electronic ballast puts you squarely in them mediumwave range while the 30th harmonic from an iron ballast is still within the frequency range of hearing!
• The rise/fall times of the waveforms from an iron ballast are quite slow in comparison with those from an electronic ballast which, by necessity of design, can be in the order of 100's or 10's of nanoseconds.  Having these high switch rates on the transistors inside the electronic ballasts puts energy squarely in the MF, HF and even the VHF or UHF range!
• The length of a common 4-foot (1.2 meter) tube/fixture is commensurate to the wavelengths within the VHF/UHF range.  This means that the fluorescent tube itself is perfectly capable of acting as a radiator at many frequencies - not to mention various other parts of the fixture and contained wiring.

Unless one were to scrap the electronic ballast altogether and go back to using the old, iron ballast, if you experience interference issues there are a number of things that you will have to do.

Conduction of high frequency energy onto conductors and radiators:

In the case of our electronics shop the biggest problem appeared to be direct radiation of energy at VHF frequencies, apparently from the tube and connecting wiring.  One way to minimize this effect would be to reduce the harmonic energy emerging from the ballasts and finding their way onto the tube and wiring.

The most practical way to do this with minimal risk of deleterious effects (e.g. loss of efficiency, electric shock, fire, reduction of operational lifetime of the ballast and/or tubes) is to apply a bit of series inductance to the leads that feed the fluorescent tube.  While we are at it, we might as well apply some common mode filtering to the AC (mains) leads as well to minimize conduction of "grunge" that might find its way out via that path, as well.

Figure 2 shows the typical connection of the tubes and electronic ballast.

Figure 2:
Electronic ballast and typical connection to tubes.  In typical "room temperature" installations the filament heater is not used, except as a simple emitter of electrons:  In the example, above, "load sharing" is afforded by the individual, series capacitor for each tube and starting ionization for each tube is provided solely by high-voltage excitation from the ballast.
Not depicted, ballasts are available that have four wires to each tube to provide heater current for starting the tube(s) in cold environments, but most "indoor" ballasts have only a single wire connected to each end of the tube.
Click on the image for a larger version.

As can be seen, the tubes are in parallel with the output of the high-power oscillator, typically using a series reactance (capacitor) to limit each tube's current and to provide a mechanism for load-sharing and offer operational stability in the presences of the tubes' negative resistance characteristics.  Many electronic ballasts allow different numbers of tubes to be connected:  3-tube ballasts will allow 2 or 3 tubes while 2-tube ballasts will usually allow just one tube to be powered, and using these series reactances is a simple way of keeping the tube current more-or-less constant, despite the number of tubes connected - either at the time of installation or due to later tube failure.

Figure 2 also indicates something else:  On the wires marked "A" and "B" are carried the high-frequency currents from the ballast's oscillator.  As we discussed before, not only are these same currents very non-sinusoidal and contain many harmonics, but the tubes themselves tend to badly distort the currents, further-increasing the harmonic content on these leads.  Finally, at VHF and UHF frequencies, the very lengths of wires "A" and "B" and the tubes themselves, "T", can make them capable radiators in their own rights!

The distortion added by the tubes themselves is likely not significant one gets above MF and HF frequencies as the mechanism is generally too slow in its own right, but we still have the harmonics from the oscillator itself to contend with which can be carried, as current, by the ionization of the gas within the tube itself which will form an antenna and as we know, as long as there is some current flowing, we will have electromagnetic radiation.

Inductors used for reducing the harmonic content on the tube current:

Our best bet is to reduce the harmonic content on the tube current itself and the easiest way to do this with minimal intrusion and risk to the user is to insert a small amount of inductance onto the connecting leads "A" and "B".

Figure 3:
A pile of toroidal inductors pulled from some scrapped power supplies.
In the upper-left is a bifilar choke used for mains filtering, also found
in higher-quality switching supplies.
Click on the image for a larger version.

We had a number of junked switching power supplies in the "boneyard" and we pulled a pile of toroidal inductors (see Figure 3) from some of them and did a quick analysis.

The most common type of toroidal inductor is the sort typically found in PC power supplies, usually used for output filtering, wound in either a yellow or yellow-and-white core.  The permeability of these cores isn't extremely high, typically achieving somewhere in the area of 20-30 microhenries with 10-15 turns, but they are not particularly suitable for use much above the range of 100-200 kHz:  Higher than this they get quite lossy - but we don't really care too much about that since we are trying to quash this energy, anyway.

The "Yellow-White" cores appear to be "26 Mix" Iron Powder types that are specified for use from DC to approximately 800 kHz.  The aforementioned "Yellow" cores would have the color code for "6-mix", but when they were measured, their permeability was closer to that of "26 mix" indicating that whoever made them wasn't precisely following color convention!

The sizes (outside diameters) of these inductors varied, from approximately 1 inch (25.4mm) in diameter to over 2 inches (50mm).  If you were to translate these to "store bought" toroids these could equate to sizes from "T94-26" to as large as "T200-26" or "T225-26".

Another inductor that appeared to be useful was one that was simply painted light gray and it, too had permeability roughly equal to that of the yellow/yellow-white ones - that is, 10-20 turns yielded something in the area of 20-100 microhenries.  I've found no obvious reference to "light gray" painted toroids in the typical references so I don't know what "mix" they might be, but they appear to be vaguely similar to the "Yellow-White" ones in general properties.

When either the gray or the yellow/yellow-white inductors were broken, it appeared that they contained some sort of metallic powder, pressed and molded into shape and covered with paint. 

The third type of inductor, often used for filtering the the AC mains input, appeared to be some sort of very high permeability ferrite and putting 10-20 turns on these yielded hundreds of microhenries or even millihenries of inductance:  These are not suitable points "A" and "B" and should be set aside for now - but we will be using them later.  When broken these particular cores were either a black compound (e.g. ferrite) or, in a few cases, wound with some sort of thin, ferromagnetic tape as in the case of the red-cored inductors depicted in Figure 3.

Which toroids to be used where?

Let's take a look at a redrawn version of Figure 2:

Figure 4: 
A fluorescent ballast/fixture with added ferrite to minimize RFI by virtue of "isolating" the fluorescent tubes at
radio frequencies.
For "cold weather" ballasts that use filament current for starting, each "pair" of wires to each tube would be treated as a single conductor through L1-L3.
See text for explanations!
Click on the image for a larger version.

First, let us take a look at inductor L1 which is in series with the "common" lead "B" that connects to one side of both tubes.  With the inductors that we had we are able to put approximately 15 turns of the original wire into this core which yielded an inductance of 10-15uH (value not critical!)  In running the numbers we can see that at 30 kHz, a typical operating frequency of an electronic ballast, this inserts approximately 3 ohms of reactance into the circuit.  Considering that for a 32 watt, T8 fluorescent tube that the average current will be around 300 milliamps (see reference, here - link) we can calculate that if the current were perfectly sinusoidal (it is not!) the effective loss would be a volt.  Because this is a reactance, we would not experience I^2R losses in the same way that we would if we were dropping the same voltage with an ohmic loss (a resistor) but even if we were, this would amount to only 1/3rd of a watt or so.

If we were to translate this same inductance (15 uH) to the middle of the AM broadcast band (1 MHz) we would see a reactance of around 94 ohms - significantly higher than at the operating frequency, reducing the potential current through the tube, at that frequency, by a factor of around 30 which corresponds to 10's of dB reduction.  Since only a fraction of the oscillator's energy is at harmonics of this magnitude a negligible amount of power is being blocked by this inductance.

Taking this to an extreme, let us consider a frequency of 150 MHz with this same 15uH of inductance where we find the reactance to be approximately 14000 ohms - extremely high and, for all practical purposes, completely "blocking".  In reality the effective series resistance would not be nearly this high as not only would this particular core material ("26" mix) not be effectively offering such inductance at this frequency, but there would also be capacitive (shunt) coupling across the inductor/windings itself and between various conductors (wires) located under the cover of the ballast shield.  Nevertheless, the impedance would be significantly increased due to the inductance present and the core losses at this frequency.

Taking another look at Figure 4 we can see that we'd need to install similar filtering in the other leads connecting to the tubes as well (e.g. the "A" leads) and the similar math applies.  By installing such inductors in both leads we effectively "isolate" the tube at high frequencies by virtue of the series inductance while leaving the frequencies at which the tube is powered - around 30 kHz - virtually unaffected.

How do we know that this does not affect normal operation?  A bit of emperical measurement can (and did) verify this assertion:

• A "Lux Meter" placed below the fixture showed no discernible difference in light output before and after modification.
•  The added inductors did not get perceptibly warm:  If they did, this would indicate power loss!

A core not to use at points "A" and "B":

In an experiment I placed one of the high permeability ferrite cores (not one of the yellow cores) in place of L3:  It immediately got too hot to touch, indicating both high losses and a significant amount of inductance.  When checked on an inductance meter I found that approximately the same number of turns as I'd placed on this core when tested in the fixture (15 or so) yielded around 200 microhenries - a reactance of around 38 ohms at 30 kHz - a significant amount at 300 milliamps of tube current!

The upshot:  When trying an core, verify that it does not warm up by an appreciable amount before deciding to use it!

If it gets (noticeably!) warm after several seconds/minutes, it is not suitable and could pose a hazard due to heat:  This heat could, in theory, melt insulation which could pose an electrical/fire hazard or the heat itself could melt or cause combustion in its own right!

Don't worry - we still plan to use this core!


Additional modification:

Figure 5: 
Added inductances on the leads marked "A" in Figure 4,
above.  The inductor on the far left is a multifilar choke -
see the text below for an explanation.  There are actually three
small yellow/white toroidal inductors, one being obscured.
Click on the image for a larger version.

If you look at Figure 5 you will see, on the right side, the three, smaller yellow/white inductors (one is obscured.)  In this particular case this is a 3-tube fixture, the third tube used to provide extra light as it was placed over the workbench area.

Because it was directly over the workbench, it was typically much closer to the gear being used and tested which made the noise emitted by it a bit more problematic by proximity.  To reduce this noise still-further an additional inductance was added in the form of the large, yellow inductor on the left.  This particular core was taken from a scrapped PC power supply (almost all of them have such a device) and the three wires were wound, in parallel - keeping the same number of turns - through the core to fill it up to make, in this case, a "tri-filar" winding.  Doing this forced an additional series inductance on the "A" leads depicted in Figure 4 as "F1" but it forces the currents through the tubes to be equal - not too difficult a job considering that L1 and L2 (depicted in Figure 4) have already presented a high impedance (at high frequencies) on those leads already.

While doing the initial work on the first fixture to be modified, a Yaesu FT-817 - an all-band, all-mode amateur radio transceiver - was laying on the workbench immediately below it and it was noted that by grabbing a wire of the unmodified fixture I could couple some of the noisy RF (at around 144 MHz) into my body which would then be re-radiated as evidenced by the the noise from the FT-817 increasing.

As a rough indicator of the efficacy of the added inductances I observed that after installing L1-L3 I could grasp the wire "after" these inductors (on the "tube" side) and hear no difference, but if I did so on the "ballast" side of the inductors I heard the noise increase, indicating to me that they were doing their job in removing a significant amount of "grunge".  While I was doing this I noted that I could also hear "grunge" if I grabbed around the AC mains wires as well, indicating that some of this energy was being conducted via that route.  Being a commercial building, the mains input is routed via metallic conduit and it was noted by poking around in the "drop" ceiling and placing the antenna of the FT-817 near-ish the power conduit that relatively little of the noise was emanating from the conduit itself, but since I had a plethora of chokes on-hand I decided to try an experiment.

Using the high-mu (lossy) cores:

Using one of the same "lossy" ferrite cores that got hot when tested, I parallel-wound, in bifilar fashion, as many turns of the AC mains wire of the ballast as I could fit on the core.  As with the milti-filar core, above, it is important that these wires be kept parallel and that the exact same number of turns be used for each of the two mains conductors so that the "common mode" attenuation be maximized.

When wound in a bifilar fashion - and on leads that have only a residual amount of RF energy - we are using these ferrite cores in the manner intended with no fear of their getting hot:  Because both wires of the AC mains go through in parallel, their magnetic fields cancel and the core does not "see" it - it is only that small amount of RF energy that is not supposed to be there that is being blocked by the bifilar winding on the core!  It is important to note that in order for this type of winding to work, you must wind equal numbers of turns of both wires, preferably in parallel or gently twisted together:  It is by virtue of the equal inductance and coupling between these two wires that this sort of filter works.

After doing this I did the "grab" test with the wire again:  If I grasped the wire on the "ballast side" of the newly-added bifilar inductor I could hear an increase in the noise on the FT-817, but if I did so on the "mains" side I could not.  In other words, this choke was keeping the "grunge" from the ballast out of the mains.

While it may not have been particularly important on VHF/UHF to add this extra bifilar inductor on the mains power leads, it was pretty easy to do since I had the fixture "open" already - and I decided that every little bit helps!  Also, it is worth noting that on MF and HF that it will likely be via the mains power leads that the majority of noise will be conducted while at higher frequencies like VHF and UHF, the wavelengths are small enough that energy can be emitted directly from the lamps and fixtures themselves as their very size is a significant portion of a wavelength.

Efficacy of the modifications:  Did they work?

Of the 6 fixtures in the shop, only the two directly above the workbench where they are closest equipment under test now have both the small chokes in each of the leads in series with the tubes (e.g. L1 and L2).  All of them have an choke on the "common" side of the tube (L3), the common-mode choke on all of the tubes (F1) and the bifilar choke on the mains side (F2).

Did this modification have any effect?  The quick answer is yes and here are the ways that it was measured.

• A portable HF/VHF/UHF all-mode transceiver with a whip antenna (an FT-817) was placed underneath/near the fixture, the distance of the plastic diffuser.  The "S-Meter" reading of the noise of the modified fixture was compared with an unmodified fixture with the same complement of tubes and ballast type/model and the modified (filtered) fixture was found to radiate far less than the unmodified one based on observations of the S-meter and audible noise.
• Two spectrum analyzers were set up on the workbench in the electronics shop connected to small whip antennas and sweeping in the range of 100-200 MHz.  After all of the fixtures in the shop were modified, the amount of noise indicated on the analyzers was 10-20dB lower than before the modification, depending on frequency.  It was not completely gone, but significantly reduced.
• Using a portable handie-talkie transceiver, several VHF (2 meter) repeaters were checked for receive signal quality within the shop before and after fixture modification.  All but the strongest were inaudible in the shop prior to modification, but all were easily audible afterwards.  A slight amount of extra noise is apparent when the lights are turned on, but there is no trouble in finding a location that provides a suitably noise-free signal for testing/monitoring now.

Final comments:

The addition of screening

Prior to the installation of the chokes in series with the lamps some experimentation was done with metallic screening of the fixture.  The particular fixtures that we are using are light-gauge Lithonia fixtures that have spot-welded seams and appear to be likely to provide a reasonable RF-tight seal on the back side:  It was via the open front, through the plastic diffuser that RFI was radiating - apparently from the tubes themselves.

In an experiment we decided to cover the front of the fixture with aluminum foil.  While blocking the light, it would serve to help us determine if this was a viable means of containing the RFI within the enclosure.  We placed, lengthwise, two parallel sheets of foil over the diffuser, but it happened that there was a very narrow (1/4" inch, approx. 6mm) gap between the two strips running along the long dimension of the fixture.

We noted that with the foil in place that the interference actually got worse until we bridged the narrow gap with small pieces of scrap foil in 3-4 places.  We surmised that the two pieces of foil were acting as independent radiators until we connected the two, at which point the noise level was reduced - but it was still significant.  Clearly, the foil itself was coupled to the the noise energy within the box and re-radiating a significant percentage.

We then bridged the foil to the steel box itself, causing a further reduction in noise - but the amount of reduction depended strongly on where, exactly, we made this connection.  If this was done in each corner of the box (e.g. four places) there was a significant reduction.

Practically speaking one would not use aluminum foil to provide such shielding as it would clearly block the light.  Aluminum window screen would also not be recommended as there is not guarantee that there is an electrical connection across its entire face as each wire is not "bonded" to its neighbor - plus it, too, blocks a significant amount of light.

This leaves the use of so-called "expanded metal" or "hardware cloth" - both of which are electrically-connected across their plane as options.  Of the two, the "hardware cloth" - typically galvanized steel mesh is, by far, the cheapest and least "light blocking" alternative - plus, it is possible to solder to the zinc coating with relatively little difficulty.  This material is available in different mesh sizes and for VHF/UHF frequencies the larger sizes (around an inch or several centimeters) would be adequate.

The problem would be providing a firm, electrical bonding of this screen to the case of the light fixture.  On the "hinge" side of the diffuser cover one would use very short, wide metal straps to make the connection to the case, but for best efficacy it would also be preferable to make a similar connection on the side opposite the hinge where the cover opens - but aside from the installation of "finger stock" or very short plug/receptacles, this is rather complicated to do!

About LED replacements:

I am aware of LED drop-in replacements for T-12 and T-8 fluorescent tubes, but has, for the time-being, dismissed these owing to issues of cost, reliability, efficiency, and "light quality" (e.g. color rendition or "CRI").  For an in-depth report on this subject see "Performance of T18 and T8 Fluorescent Lamps and Troffers and LED Linear Replacement Lamps" - link.  Since this report was published significant improvements have been made in LED efficiency, but similar issues - particularly those related to quality and longevity - remain unless one uses devices made by reputable manufacturers.

As far as EMI/RFI issues related to LED replacements for T-12 and T-8 tubes:  Based on reports in the amateur radio press and on forums such as the EEVBlog (examples here - link and another link) I would expect similar - or worse - problems to occur, depending on the manufacturer - as they would also have issues with high slew rates on switching regulators and/or semiconductor-related transients.  While it is very likely that common-mode mains filtering would also be recommended on these devices as well, it may be more difficult to remove noise along the entire length of a series or series-parallel fed array of LEDs that was nearly 4 feet (1.2 meters) long as this is a significant portion of  a wavelength at VHF/UHF frequencies and could easily radiate on its own right!

If you do wish to upgrade an existing "troffer" or similar fluorescent fixture to LEDs, I would strongly recommend that you obtain JUST ONE and carefully analyze it before you risk ruining your HF, VHF and/or UHF reception with little means of mitigation!


[End]

This page stolen from "ka7oei.blogspot.com".

An L/C audio bandpass filter using cheap audio transformers

Figure 1: 
The transformer to be used
as an inductor!
Click on the image for a larger version.

After modeling the active bandpass filter mentioned in the previous post (link) - which was a response to a question asked by Bill, N2CQR in his SolderSmoke blog (link) - I got to thinking again about L/C filters.

I'd initially dismissed passive L/C filters because one would need to use inductors somewhere in the range of 10 millihenries to 1 Henry to operate at audio frequencies with reasonable source and/or termination impedance, and such chokes aren't particularly cheap or easy to find these days.

Or are they?

I decided to rummage around in my collection of small transformers and found three 8 ohm to 1k-center tap audio transformers sold by Radio Shack (P/N:  273-1380 - link).  These transformers aren't terribly expensive ($2.99 each in 2013) and certainly available if you have a store or three nearby.  What about using the windings of these as a inductors for our filter?

I measured the inductance of the three possible wire combinations:  The 8 ohm windings, the entire 1k winding, and on each side of the center tap of the 1k winding.  I was pleased to note that the three transformers - two of which were bought recently and one had been bought several years ago - were pretty consistent in their inductance (about 15% spread amongst the three) and also that the inductance on each side of the center tap was extremely close (within 1%) - but that would make sense for a center-tapped winding! 

With the amount of loss, self-capacitance and effects of the other windings, it was a bit tricky to get a good reading of the inductance, but I was able to determine that the average of the three transformers - at audio frequencies - was around 498 milliHenries - depending on conditions.  So, what sorts of filters could I make out of these?

Grabbing a pair of these transformers, a calculator, assorted capacitors and some small trimmer potentiometers and cracking open the "Filter Design Handbook" by A.B. Williams, I fired up the soldering iron, tacked some parts together and "swept" the response by turning the knob on an audio generator and looked at the result on an oscilloscope.

Comment:

With the demise of Radio Shack, these exact transformers are difficult to find.  While suppliers like Mouser and Digi-Key still offer similar transformers, their properties have not (yet) been analyzed to determine their appropriateness for the construction of the filters described. 

Simulation versus real-world components:

Figure 2: 
2-inductor bandpass filter using Radio Shack transformers.  C102 consists
of three paralleled 0.1uF plastic capacitors.
Click on the diagram for a larger version.

Now here's a classic example where the simulation can radically depart from reality in real-world components - in particular, the rather loose definition of what constitutes an inductor when one takes into account the fairly high resistive losses, skin-effect losses, core losses, the self-capacitance of the windings and the losses/effects associated with the unused windings.  The response of the filter wasn't particularly close to what was predicted - particularly in terms of the effect that source and load impedance had on the "shape" of the filter's passband - so I ended up tweaking the values of the capacitors and source/load resistances to get the best-possible response to obtain the goal of a reasonable 300-2400 Hz filter.

Figure 3:
The measured response of the filter depicted in Figure 1.  The
frequency range on the horizontal axis is logarithmic, going from
100 Hz on the left to 10 kHz on the right.
Click on the image for a larger version.

Once I was satisfied with the response obtained manually seeping with the audio generator and looking at the scope I was ready to do some fine-tuning.  A laptop running the "Audacity" program was used to generate several minutes of white noise which was played back in a loop while on another computer I ran the "Spectrum Lab" program.  The signal was routed through the filter and the capacitor and the source and terminating resistors (and, as necessary, the capacitors) were adjusted to get the desired response.

The result is the filter depicted in Figure 2 with its measured response over the range of 100 Hz to 10 kHz being shown in Figure 3.

In short, the results were thus, relative to the response at about 1 kHz:

• -1dB:  Below 380 Hz and above 1600 Hz
• -3dB:  Below 286 Hz and above 2100 Hz
• -10dB:  Below 194 Hz and above 3600 Hz
• -20dB:  Below 109 Hz and above 6200 Hz

Remember that the above aren't insertion loss numbers since the minimum loss of the filter of this sort is on the order of 6dB to 20dB owing to the losses imposed by the source/load resistors that are used to set the proper impedance and the inductors themselves.  Of all of the common types of reactive components (namely coils and capacitors), inductors are typically most subject to real-world losses and often depart most radically from "perfect" or ideal devices.

When it comes to describing the performance of a bandpass filter such as this, the "curve" describing the response is relative to the middle of the desired response and in this case, "0dB" is a relative number in the middle of the passband response.  So, if this filter's minimum insertion loss is 10dB, we'll see that much loss at, say, 1kHz and a total of 20dB of loss at the "-10dB" points.

While not particularly "sharp", this filter is really quite effective considering its simplicity.  In passing wideband ("flat") audio through this filter from a broadcast radio and listening to it via headphones it sounds quite good - not particularly "ringy" or harsh - and will go a long way toward removing the energy that a listener using an SSB receiver won't hear anyway!

With this simple filter it's possible to adjust the response somewhat by tweaking the source/load termination without doing much more than "tilting" the response.  For example, if one wanted to boost the low end a bit, the terminating resistance (R102) can be reduced (to as low as 500 ohms or so) with the penalty of increasing the filter's overall loss.  Adjusting the source impedance (R101) can have a similar effect, but reducing its resistance will have a much more deleterious result on the desired response than adjustment of the terminating resistor.

When constructing this filter I was curious about how sensitive the "inductors" were to each other's magnetic fields.  While monitoring the passband response via the computer, I moved them next to each other in different orientations to see if there was an effect, but there was none that was apparent.  What I did notice that if the transformers were within a foot (25cm) or so my soldering iron, I could hear hum and buzz from the magnetic fields of its power transformer indicating that it would be a good idea to keep this filter at a respectable distance from such things!

Comments on parts:

While gathering some extra transformers for this project I spoke with the manager at a local Radio Shack and he pointed out that they are normally permitted to stock only TWO transformers at a time - although it should be possible to pre-order more via the store or online (link).  Since they might only have one of these transfomers onhand (and some of the "smaller" store locations may not carry them at all!) it's worth checking with a local store (or on their web site) before chasing around to different stores to get them.  At my local stores these transformers are not in the component drawers, but are usually hanging on a hook near the power transformers.

As noted above I've measured the properties of several of these Radio Shack 8 ohm-1k audio transformers from different vintages (one of them was likely over a decade old) and have found them to be quite consistent.  It is safe to say that if a different audio transformer is used - even if its rated impedances are the same - one should not count on it having substantially the same, important properties - specifically the inductance and the particular losses that make it work properly as a filter with the values shown in Figure 2.  In other words, unless exactly the same Radio Shack transformer is used, you can count on having to re-do the entire filter!

It is worth reiterating that the source and load impedance of a filter like this is very important in order for it to work reasonably well and this is accomplished with resistors R101 and R102.  What this means is that you can't simply build this circuit and insert it into a signal path unless the output impedance of the audio source to drive the filter is nearly zero (such as that of an op-amp) and the load impedance is extremely high (such as that of a unity-gain follower.)

Again, such filters will exhibit 6-20dB of insertion loss - much of that due to the source and load impedance matching resistors - but also due to the losses of the transformers themselves.  Below, a few simple circuits will be shown that may be used to provide adequate source and load impedances as well as provide gain to overcome the filter losses.

The types of capacitors are also very important, namely that they should NOT be ceramic capacitors.  While the precise value of the capacitor isn't extremely critical (there should be no problem using capacitors with +/-10% tolerances) ceramic capacitors in the range used in the circuit above (0.1uF, 0.47uF) are likely going to be "X7R", "Z5U" or similar types that have both rather loose tolerances and the tendency to change value - often radically - with temperature:  For many of these capacitors, lowering the temperature to around freezing will cause their value to drop by 20% to 80% or so and if this filter is built using those capacitors it may work properly only at "room temperature" (around 68F or 20C)!

Fortunately, almost any capacitor other than ceramic will probably use some sort of plastic as a dielectric and will have a much better, more stable capacitance with respect to temperature:  Suitable types include metal film, polyester and Mylar to name but a few. (From what I can determine, Radio Shack does not stock suitable capacitors!)

A better filter:

What about adding more stages of filtering?

Figure 4: 
5 inductor bandpass filter providing better high/low cutoff beyond the 300-2400 Hz range.
Click on the diagram for a larger version.

I added a third inductor (e.g. transformer) and tweaked values, but it turns out that I couldn't get quite as good a result as with the filter depicted in Figure 2 so I visited several local Radio Shacks, picked up two more of these transformers and then tried one with 4 inductors:  It was a bit better than the one above, but not by very much, so I constructed one using five inductors (transformers) obtained significantly better performance.  The diagram of this filter is shown in Figure 4.

The circuit in Figure 4 is essentially a repetition of the individual sections in Figure 2 and by adding these additional elements, better rejection of those unwanted frequencies was obtained as can be seen below:

• <=1dB ripple between 350 Hz and 2400 Hz
• -3dB below 302 Hz and above 2600 Hz
• -10dB below 220 Hz and above 3400 Hz



Figure 5: 
Response of the "5-stage" bandpass filter.  The "grunge"
at the far right is not the filter, but pickup from noise picked
up on the clip leads used in connecting things together!
Click on the image for a larger version.

• -20dB below 169 Hz and above 4700 Hz

Most of the improvement in the filter performance was seen in the higher frequencies, but if you are familiar with filter design you may have noticed something:  Adding these extra sections didn't make as much of an improvement as one might expect!  The reason for this is simple:  We have only one usable inductance value available to us with these transformers - about 498 mH - requiring a compromise of the design and the winding of the transformer really doesn't make a particularly good inductor at that!  Had we a larger number of inductance values from which we could choose, we could get better performance with fewer filter elements.  True, these transformers could be put in series and/or parallel to obtain different inductance values, but this would start to violate the "cheap and easy" principle!

Comment:  As it turns out, the above statement about having "...only one usable inductance value..." isn't quite true - See the note at the bottom of this post.

Driving/loading the filter and adding gain:

As mentioned above, these must be sourced and terminated at the proper impedance for them to work properly:  In general, too-low impedance on the input/output will enhance the bass (low frequencies) while setting it higher will boost the highs slightly and any departure from optimal impedance in either direction will increase "ripple" of the filter - a factor most apparent on the 5-inductor version.

Figure 6: 
Source and load amplifiers for the above filter sections.
Click on the diagram for a larger version.

Either circuit in Figure 6 will work and which one you use depends entirely in your preference as both are capable of providing enough gain to overcome filter losses.  In each case the source and load resistor intrinsic to the filter (e.g. R101/R102 for the 2-inductor filter or R201/R202 for the 5-inductor filter) is NOT included in the diagram in Figure 6.

Common to both of these circuits is the 1k resistor and 0.001uF capacitor on the input to remove RF since it's likely that these will be used in a transmitter!

In the op-amp version a resistive divider (R307/R308) is used along with C307 to obtain a clean bias at 1/2 of the supply voltage - a necessity if you are using a single-ended power supply.  U1a is a unity-gain follower used to provide the source impedance for the filter while U1b is a noninverting amplifier that has a high input impedance (as to not affect the loading resistor of the filter itself) and provides approximately 20dB of gain to overcome filter losses:  The gain of this  section may be tweaked as desired  by adjusting the value of R304 or R305.  While a single dual-section LM358 is shown, practically any op amp may be used such as an LM1458 or a pair of '741's.  On the output of the second op amp section a series resistor is included to both maintain stability if there is a capacitor (such as an RF bypass!) as well as to generally improve RFI rejection. 

In the transistor version an emitter-follower is used to drive the filter and at the opposite end, another emitter-follower is used to minimally-load the filter beyond that done by the terminating resistor and to make up for the filter loss the final transistor, Q403, is configured as a simple amplifier, and to increase the gain one may decrease the value of R410 or increase the value of R409:  Note that the Q403 stage is intended to drive a load that is fairly high impedance (1k or more) and may not perform well otherwise.  Finally, R411/C407 are used to keep audio that might appear on the supply due to the current drawn by the transmitter's finals from appearing on the supply rail of this filter driver and prevent feedback (such as "motorboating") at this low-level audio stage!

One may actually cut down on the parts count slightly by designing an op-amp input that terminates at the intended filter-load impedance or one can choose the values of R405/R406 to provide the same voltage ratio and an equivalent resistance of the filter load resistor or one could even design the Q401 stage to source the filter properly:  For the source of diagram simplicity and the purposes of illustration - and at the cost of just a few resistors - this was not done. 

If one has a homebrew transmitter it should be possible to integrate one of the above filters directly into the design.  For example, it should be possible to configure the balanced modulator (such as an NE602) to directly present the necessary load impedance and in which case, one might simply move the gain stage (e.g. the microphone preamp) to the input side of the passband filter to overcome its losses.

Conclusion:

I determined that it is practical to make audio bandpass filters with reasonable performance out of a few readily-available audio transformers.  Using such a filter can result in an audio passband that is well-suited for voice communications via an HF transmitter while removing the high and low frequency components that while nice, aren't strictly necessary for good intelligibility.

I'm certain that with a bit more characterization of these devices as inductors and subsequent  tweaks in the design one could improve the designs above as well as make a variety of other passive filters for CW operation and audio notching.  It may be possible to make use of the other windings of the transformers to derive clever configurations that effectively make more use out of the transformers than simple inductors.

Additional comments:

After writing the above I checked the inductance and "Q" of one-half of the the secondary winding (center-tap to one end) when wired in series with the 8 ohm winding, both "in phase" (boost) and "out of phase" (buck) and noted that between these two configurations, there was nearly a 2:1 change in overall inductance with minimal effect on the "Q".  Taking advantage of this, I re-worked the above filters to see if there was any significant performance improvement.

While there was a very slight improvement in performance, it was rather minimal and hardly worth the effort.  What I was hoping was that the "3-inductor" version would perform significantly better than the 2-inductor version so-configured, but as with the above design, this was not the case.  The "5-inductor" version - with the "buck and boost" winding configurations - did work somewhat better and perhaps I'll do an update based on this in the future.

[End]

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