Replacing the filters in the Drake SP75 Speech Processor

A few months ago I fired up my old Drake TR-7, getting on the air with some friends on 40 meters after not having used the radio for a while.

Everything seemed to work fine, at least until I switched the audio through the matching Drake SP75 Speech Processor as some of those on frequency were having a little bit of trouble hearing me on the noisy band and I started getting the following helpful comments:

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

"Ughh!"

"That sounds terrible!"

"Your 'lows' are completely missing!"

"Your audio sounds 'restricted'"

"Turn it off!"

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

Later, I used some available test equipment (computers and software) to see what had changed, setting up the following:

• Using another computer (a netbook) I ran the Audacity  (link) program, a free, open-source audio editor.  Using that program, I generated 5-10 minutes of white noise and set it to play back that white noise as a loop.

• The generated white noise from that computer was fed into the "Tape" input of the speech processor with the audio level set just high enough to properly drive it.

• The audio output from the SP75 was fed into another computer running the Spectran (link) program - also free - which does audio analysis.  Another program that would work, but is more difficult to use with a much steeper learning curve, is "Spectrum Lab" (link).

The result of this was that I was able to compare the "flat" white noise input to the speech processor by the netbook with the audio spectra coming out of the SP75.  Ideally, the SP75 would not appreciably "color" the audio - that is, the the frequency response of the SP75 should be pretty "flat", not rolling off either the lows or the highs, at least in the frequency range used for speech (e.g. 100-300 Hz to 2700-3000 Hz or so).  Unfortunately, I can't seem to find the screen capture of that spectral plot or else I'd include it here.

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

How the SP75 works:

 

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

This is a combination AF/RF speech processor and it works by first routing the input audio through an XR2216 audio compressor chip.  Then, the audio is double-sideband modulated at around 459 kHz, filtered to produce a lower sideband signal using a pair of 455 kHz ceramic filters, RF clipped, filtered by another ceramic filter, and then demodulated back to audio.

By applying the clipping at RF, the resulting audio is devoid of harmonic distortion since the harmonics would occur at around 900 kHz instead of within the audio range itself.  The only real casualty being that there is now intermodulation distortion, but since the human voice largely contains sounds consisting of noise bursts (consonants) or voiced sounds with harmonics (vowels) the introduction of intermodulation distortion on speech, alone, has relatively little effect on the perceived quality and intelligibility if the clipping process is not taken to extreme.

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

What had gone wrong:

Because of the loss of low frequency audio, I figured that one (or more) of three things had happened:

• One or more electrolytic capacitors in the audio path had dried out and decreased in value causing the loss of low frequency response.

• The BFO, nominally at 459 kHz, had gone off-frequency and caused the audio passband through the filters to shift.

• One or more of the 455 kHz ceramic filters had gone bad.

With the white noise applied to the input and still using the Spectran program to check the audio spectra I went though several of the audio test points noted in the manual, applied them to the audio input of the computer running Spectran and observed that at least to the balanced modulator, the audio was completely flat, ruling out the likelihood that a capacitor had gone bad.

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

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

Identifying the ceramic filters:

In looking at the filters themselves they were clearly made by Murata, marked with "CFW455" followed by what looked like an "I6" printed in white ink while the schematic diagram simply called out a part number of "CFW455I".  In doing a bit of research on the web, I determined that the Murata "CFW455I" had the following specifications:

• Center frequency:  455 kHz
• Input/Output Impedance:  2 kohms
• -6dB bandwidth:  +/- 2 kHz
• Stop bandwidth:  +/- 7 kHz (at -50dB)
• This device was a 6 pole filter

What was also apparent was that this particular filter was no longer being manufactured, so I started looking around for a replacement.

Figure 2:
The new filter (left) and the old filter (right). 
 All but one of the leads lines up with the original.
(When taking the picture, I cleverly rotated one of the filters
by 180 degrees, so keep that in mind!)
Click on the image for a larger version.

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

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

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

Installing the replacement parts:

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

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

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

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

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

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

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

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

 

Figure 5:
The removed and re-routed trace using a short
piece of #30 "wire wrap" wire.
Click on the image for a larger version. 

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

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

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

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

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

Getting the SP75 back into working order:

 

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

What had happened?

From what I can tell, one of two things might have changed:

• These new filters (CFWLB455KJFA-B0) are slightly narrower than the original CFW455I ceramic filters used by Drake.  In this scenario, the BFO and the edge of the audio passband would have been "moved" entirely outside the filter.  I have no way to know this for certain as the only filters I have for comparison are the now-30-year-old original filters.

• The more likely scenario:  The original Drake filters were marked "CFW455I6" - a designation that doesn't seem to be correlate with anything in a catalog that I could find.  Perhaps the "6" indicates a center frequency of "456" kHz?  If this is the case, that would imply that the original filters were specially-selected for the higher center frequency and, perhaps, had matched bandwidths.  Based on what I was seeing, having the passband of the filter shifted up 1 kHz to 456 kHz would have put it in about the right place.

In either case, the original 459 kHz BFO frequency would not be suitable for the new filters, so how to change the BFO frequency?  The BFO frequency was originally set with a quartz crystal - a rather expensive component to have custom made - but there are two easy alternatives:

• Slightly reworking the oscillator to use L/C components such as a 455 kHz IF "can" (transformer) as frequency-determining elements.  This would, at the very least, involve adding a series DC-blocking capacitor were this route taken. A bit of care would need to be taken to assure that this arrangement was temperature stable to within a few hundred Hz over the expected frequency range.

• Using an inexpensive 455 kHz ceramic resonator - also available from Mouser.

I chose the latter since I had several of those, already on hand, plus they had the double advantage of being quite "pull-able" over the range of several kHz and they are fairly frequency-stable, likely to move only a few hundred Hz, at most, over the temperature range that one might experience.  As for the original 459 kHz crystal:  I have wrapped it in paper and plastic tape and secured it inside the SP75 case in case I need it for some reason in the future.

Figure 8:
A Murata 455 kHz ceramic resonator used in lieu of the original 459 kHz crystal and a 180 pF capacitor in parallel with the
frequency trimming capacitor - both components being mounted on the bottom of the board.  As noted in the text, the value
of this fixed capacitor was determined experimentally using the methods described in the text.
Click on the image for a larger version.

Simply dropping the 455 kHz resonator in place of the 459 kHz quartz crystal yielded a tuning range of about 460-463 kHz (the frequency range will vary depending on the nature of the resonator) so I had to "pad" C44, the 7-62 pF tuning capacitor with a 180pF capacitor - the value having been experimentally determined - to get it into the correct frequency range for the filters.  What I ended up needing for my filters was a BFO frequency of 458.7 kHz - easily within the tuning range of the 455 kHz ceramic resonator, but your specific BFO frequency may vary, depending on the ceramic filters that you end up with.

Comment: 

There's no real reason why LSB (lower sideband) must be used when picking the BFO frequency as these filters are symmetrical:  USB would have worked if the BFO frequency had ended up in the right (or wrong) place.

If you use an L/C network for setting the frequency, pick the frequency that gives the best results using the methods described below.  It so-happens, however, that ceramic resonators are easier to move up in frequency than down as this requires just series capacitance, so using a "high side" BFO and LSB is just easier in this case!

How to determine the correct BFO frequency for your filters:

To determine the correct BFO frequency I used the same method that I'd used to determine that the original filters had gone bad in the first place, that is:

• Insert a white noise source - at just high enough audio level to drive the SP75, but low enough to avoid any overload or clipping - into the input of the processor.  The correct level is that which is just high enough to make the front panel "audio" light turn on solidly, and then just a little more.

• Using a program like Spectran to observe the audio spectra, note the "flatness" of the audio output taken from the speech processor and fed into a computer.

By varying the BFO frequency one can see the effects of the filter's bandpass:  Too low a BFO frequency and the upper edge of the filter starts to cut off the low audio frequencies (the processor generates lower sideband, remember!) and too high, the intermodulation products of the clipping can start to affect audio quality if both sidebands are recovered and demodulated.

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

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

Consider the result below:

Figure 9:
The "sweep" of the audio output, using the "Spectran" program, of the SP75 after replacing the filtersusing
a white noise source on the input and sampling the audio on the output.  This is about as good as
one can expect using inexpensive ceramic filters like this, but listening  on the output with an audio
amplifier, it sounds quite good.
This "sweep" was produced with the Spectran program analyzing a white noise input
and averaging over 96 samples to help "smooth" out the result.  Even so, the "roughness" is still evident due to
the fact that we are, in fact, measuring the spectral energy of white noise as our original signal source!  Not shown in this
"sweep", the actual frequency response of the SP75 extended to nearly 5 kHz with gradual rolloff above 3 kHz.
Click on the image for a larger version.

Figure 9 shows a pretty flat audio output - within a few dB - from the SP75.  As noted in the caption, above, listening to an audio source inputted through the processor via an external amplifier, it does sound pretty "flat" to the ear.  As can be seen, there is a bit of roll-off below 1200 Hz and this is entirely due to the ceramic filters themselves, but this could, in theory, be corrected with a simple R/C network:  The roll-off below 100-200 Hz seems to be intentional by the designers of the SP75 and occurs mostly in the output stage, although a bit of it is, in fact, from the "edge" of the ceramic filters' response.

Wrapping it up:

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

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

Note: 

It is worth paying very close attention to the SP75 manual in setting up the input and output levels for the SP75 for the microphone that you plan to use. Unfortunately, if you use several different microphones with the SP75 and they each have wildly different output levels, things get very complicated as only the microphone on which the SP75 was originally set up will be driven properly unless you have outboard attenuators on the microphones to assure that they all have about the same levels!

If the input level is too high, there may be too much audio compression in the XR2216 stage while if too low, the efficacy of the processor itself is reduced.

Also, the output level control should be set so that the audio level is the same when the processor is switched out (e.g. bypassed) and in with clipping set to 0 dB.

[End]

This page stolen from ka7oei.blogspot.com

A 100 watt "Helical" resonator bandpass/notch filters to increase isolation of 20 meter stations during Field Day

Follow-up:

For another article with more measurement and analysis of these filters, see the 31 May, 2019 article - Revisiting the 20 meter "helical resonator" band-pass/notch filters- linked here.

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

Despite separating the antennas as far as is practical with the available geography - about 300 feet (approx. 100 meters) - and arranging the Yagis north-south of each other so that they are pointed generally broadside each other (east/west) for U.S. coverage, we occasionally encounter a bit of interference between the SSB and CW stations on the same band.  This isn't too surprising because, at times, the transmit frequency of one station is just 100-150 kHz away from the receive station of the other and with stations so close together - in terms of frequency and proximity - they are bound to "interfere" with each other, at least occasionally!

Unfortunately, "normal" filtering schemes that one might initially consider don't really apply here.  The methods and means of filtering that one might see applied at HF stations include:

• Tuning stubs using pieces of coaxial cable - typically in 1/4 and 1/2 wavelength segments.  These are typically used to "notch out" signals from, say, 20 meter operations that are bothering a 15 meter station.  This can happen because many radios filter the band(s) below the rather poorly on receive and/or a 15 meter transmitter will often generate low-level noise on 15 meters and all bands below it which can often be problematically radiated by multi-band antennas such as Yagis.  The "noise problem" is mentioned in more detail, below.

• Commercially-available bandpass filters (e.g. "Dunestar" ).  This filter would be used to mitigate the same sorts of problems as the notch/stub coaxes mentioned above, but in smaller, more convenient packages.

• Tunable L/C traps using transmitter variables and standard inductors.  Using the sort of components that one might use for high-power antenna tuners, it is possible to construct L/C filters through which one can transmit that could be used to tune notches to eliminated "problem" signals.  Unfortunately, these are somewhat complicated to use and rather large, so they are rarely seen.

The reason why the above really aren't useful (at least for transmitting) is that they simply don't have a high enough effective "Q".  All that can really do is keep one band (say, 20 meters) out of another band (say, 15 meters) - That is, you couldn't expect it to affect, say, a signal at around 14.050 MHz and have minimal effect at 14.150 or vice-versa since it is impractical to make tuning that sharp using the methods above.  What's worse, if you do build a narrow enough filter to remove a station that is, say, 100 kHz away from you, it has fairly high insertion loss:  This may work for receive if you can separate the two signal paths in your radio, but you certainly can't transmit through it.

Figure 1:
The two 20 meter "Helical" resonators.  The one on the left is a "notch" only
filter used on the SSB station to remove the CW transmitter's energy and the
energy of the SSB transmitter from the CW passband while the one on the right
is a passband filter used with the CW station, both to keep the SSB station's
energy out of its receiver and to reduce low-level noise from the CW
transmitter in of the SSB station's passband.  They are capable of handling
at least 100 watts of RF and the loss caused by the use of
either one of them is on the order of 1dB or less when the coupling is set
for 10-15dB (about 2 S-units) of attenuation at the frequencies to
be rejected.
Click on the image for a larger version.

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

A similar noise problem can also exist in a receiver, seemingly created out of nothing even if the transmitter is completely "clean" - a response to various noise sources in the receiver chain in the presence of strong signals.

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

Or is it?

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

Figure 2:
The coil wound on the glass jar along with the
coupling probe.  This particular filter was lined
with copper foil, but it turned out not to make any
difference, so I didn't bother lining the next one!
Click on the image for a larger version.

I went to one of the big-box home improvement stores - Lowes-Depot, I think - and got several one-gallon, metal paint cans.  Noting that their very thin metal construction made them a bit flimsy I decided that they would require a bit of reinforcement to make them mechanically stable but the main requirements - being cheap and 100% metal - were satisfied.  Technically, they were just a bit undersized for a true 20 meter helical resonator, but I figured that if this didn't work, it would be a relatively inexpensive failure.

A friend of mine gave me a chunk of #6 solid copper wire, but copper tubing of a similar outside diameter would have worked just as well, would have been lighter weight and probably have been easier to work with:  Aluminum wire would have worked reasonably well, too, although a larger size (say 4 AWG) would be preferred to overcome the losses compared to those of copper - but it would be a bit more of a challenge to make low-loss, reliable electrical connections.

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

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

Having initially wound the wire over a smaller form caused it to fit fairly tightly over the larger, glass form, so it "took" well to staying in place while I carefully separated the turns equally and secured their positions.  Note:  After the RTV had cured, the tape that had been used to temporarily hold the turns in place was removed.

Figure 3:
 The mounting of the SO-239 connector on a piece of aluminum
plate for a more stable platform.  This is riveted to the side of the
paint can, but it could have been screwed in place.  This
particular connector was attached at the seam of the can to
take advantage of the thicker metal there.
Click on the image for a larger version.

Winding more turns than I expected that I would need, I set the coil aside for a day or two to allow the RTV to cure and constructed a capacitive coupling probe from a copper plate that was bent to a radius that (more or less) matched that of the coil - See Figures 2 and 6.  This plate was actually made from the shield of a scrap of "Heliax" (tm) cable that was cut length-wise and then pounded flat.  Between the relatively thick copper of the shield material and the "work hardening" from manipulating this copper, it was pretty stiff - especially after bending it to the radius.  Copper flashing could have also been used, the edges rolled away from the coil form to increase rigidity.

For a connection to coaxial cable I cut a piece of 3/16" (4.8mm) thick aluminum plate that was about 2 times the length of an SO-239 connector and about 1.5 times its width, formed it using a hammer and piece of heavy pipe to the same radius as the paint can, mounted the SO-239 connector to it and then pop-riveted the assembly over a hole that I'd made in the side of the paint can that aligned with the top of the coil.  See Figure 3.

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

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

Placing and centering the coil in the paint can, the bottom end was grounded via a hole that I'd drilled in the side of the can for that purpose.  I also placed the coupling probe about 3/8" (9-10mm) from the top of the coil as a starting point for determining resonance and coupling, knowing that this was likely to change.  Using an antenna analyzer I swept the frequency up and down and saw a very prominent deflection that I believed to be the self resonance of the coil at around 12 MHz in my case - and placing my hand near the top of the coil and causing the frequency to shift downwards due to the added capacitance confirmed this.  Taking a fraction of a turn off at a time, I soon moved the frequency up to around 14.5-15 MHz.

Figure 4:
A tuning disk of double-sided, glass-epoxy circuit board
material that was cut using a hole saw and soldered
to the threaded rod.  This disk must be as flat/parallel to
the lid as possible and remain so during rotation
for smooth, even tuning.
Click on the image for a larger version.

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

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

Onto some threaded rod (brass rod is easiest to work with) I soldered a disk of glass-epoxy circuit board material - also cut with a hole saw - to form a capacitive plate that would go up and down to allow tuning, making this disk as close to a right angle to the rod as possible (e.g. parallel to the lid) so that when rotated, the disk maintained an even distance to the top of the coil as it went up and down.  If the disk isn't perfectly "flat", the frequency will be seen to "wobble" up and down while adjusting the tuning.  This same sort of thing can happen if the disk isn't perfectly round and the hole isn't centered, but cutting it with a standard hole saw reduces those problems.

The metal from which this disk is cut isn't really important as it could be also be aluminum or even steel:  It just needs to be stiff, flat and round.  If it cannot be soldered to the threaded rod - which may be the case if steel/stainless threaded rod was used -  then the disk may be electrically "connected" using 1/4"-20 bolts and some lock washers as the protrusion of the bolt through the center should have minimal effect except for the fact that the bolt on the "top" side will prevent the disk from being adjusted as close to the lid of the paint can and slightly reduce the tuning range.  Note:  In Figure 4 I used double-sided circuit board material.  If single-sided circuit board material is used, place the copper side down, toward the coil.

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

I set the disk as high as it would go (against the lid), installed the lid and then re-checked the frequency and as expected found that it was again low (around 13 MHz) due to the capacitance of the metal lid itself as well as the disk and then did more trimming of the coil, a process that required repeatedly taking the lid off, trimming the coil, and putting it back on again to check.

Eventually, I got to the point where I could tune through the 20 meter amateur band and was ready to do some initial testing.

Using an HF transmitter set to just a few watts and a VSWR bridge and combination power meter on one side and a 50 ohm dummy load and another power meter on the other side, I tuned through resonance and noted that I had both a high amount of insertion loss and a high VSWR at resonance.

Figure 5:
The top of the can, reinforced with a disk of thin paneling/
plywood that had been glued to the top with RTV to make the
thin metal top of the paint can much more solid.  Soldered
to the lid - but not visible - is a brass 1/4-20 nut through which
the tuning rod is threaded.  There is a flat and split ("lock") washer
used to set the tension and the piece of copper wire seen in
the picture above is used to prevent the jam nut from
spinning as the knob is turned.
Click on the image for a larger version.

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

Notes: 

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

• Avoid the temptation to increase the coupling too much to reduce losses much below 1dB or so.  If you do this, the resonator will be over-coupled and its filtering effects in the SSB portion of the band will be reduced.   If the transceiver has a built-in antenna tuner to help sort out the mismatch that may be caused by the insertion of one of these filters - particularly as you move away from the center frequency - by all means, use it!

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

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


A "Notch" filter:

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

For adjusting the coupling of the notch filter, the easiest way is this:

1. Connect, using a UHF "Tee" connector", to a radio and a 50 ohm dummy load with a VSWR meter connected between the radio and the Tee connector.
2. Set the notch at 14.050 MHz (or the desired 20 meter CW frequency) using an antenna analyzer.
3. Tune the radio to 14.150 and connect it inline with the notch filter.
4. Transmit into the dummy load with the notch inline and note the VSWR.
5. If the VSWR is lower than about 1.3-1.5:1, move the coupling probe closer to the coil, noting that you'll have to re-tune the coil - possibly removing some wire.  The idea here is to increase the coupling - something that will raise the VSWR a bit - the target being around 1.3: to 1.5:1.
6. Go back to step 2 as necessary.

Note that a VSWR of 1.3-1.5:1 will not adversely affect operation - particularly if your radio has a built-in tuner - but it does indicate that the notch is just starting to have an effect.  If you have the means to do so (e.g. tracking generator and spectrum analyzer or signal generator and high-sensitivity power sensor) you can verify that at the notch frequency.

From experimentation, it has been determined that at about 100 kHz away from the notch, when the notch filter itself causes a VSWR of about 1.5:1 to occur in an (otherwise "flat" 1:1 match) it will notch out the energy at the notch's center frequency.


Note: 

As can be see in Figures 2 and 6, I lined one paint can with copper foil.  The other filter, I left alone, with its original gray protective paint inside the pain can:  There was no difference in apparent "Q" or performance between the copper and steel!

This result was not unexpected as the current is rather widely distributed along the inside of the "cavity" and the ohmic losses there are of less importance than those of the coil itself.  Any future duplications of these filters would not use copper foil lined cans.

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

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

Figure 6:
A closer view of the capacitive probe and its connection to
the SO-239 connector.  This arrangement is somewhat
flimsy, mechanically, and should be re-thought.
This particular filter was lined with copper foil to see if
performance was improved over the plated, painted interior
of the original paint can:  No improvement was noted.
Click on the image for a larger version.

• The Bandpass filter is placed on the CW station, in series with the transceiver.  With its usable passband of +/- 15 kHz or less, it covers a reasonable chunk of the 20 meter CW passband, but it is easily retuned by a operator who simply turns the knob on top while watching the reflected power and will attenuate energy from the SSB station at 14.150, the amount of attenuation increasing as one moved up the SSB portion of the band.  This passband response will reduce not only the signal level from the SSB station to minimize the possibility of receiver overload, but will also attenuate any broadband noise that the transmitter might produce that could degrade the 20 meter SSB operation.  Most CW operation occurs within a 10-15 kHz of a central "spot" - particularly if one "runs" a frequency - so frequent retuning is usually not required.

• The "notch" filter is connected "across" the SSB station's transceiver coax with a UHF "Tee" connector, but is tuned for the center of the CW passband where such operations will take place.  At SSB frequencies, it has practically no effect whatsoever, but at the CW frequencies will be attenuated, minimizing both overload by the CW station and also notching out broadband noise that might be produced by the SSB transmitter itself that might degrade reception at the CW frequencies.  As long as the SSB operator doesn't transmit anywhere below 14.125 (which should never happen!) the VSWR will be practically unaffected.

Does it work?


Yes, actually.  While we haven't needed it every year (we don't know why we can get away without it some years even though we have been using the same rigs and antennas for years now...) when placed into service, it does completely remove - or very much knock down - inter-station interference between 20 meter operations while easily handling the 100 watt output of the radios that we are using.

Figure 7:
The bottom of the paint can, reinforced with a piece of thin
plywood/paneling to keep the glass coil form from wobbling
about.  This piece of wood is attached using RTV adhesive
and allowed to cure for several days before handling.
Click on the image for a larger version.

Notes on these filters:

• Paint cans are quite flimsy.  As you can see from the pictures, several measures had to be taken to "beef up" the paint cans with pieces of paneling to overcome their inherent mechanical instability.  Without these steps, tuning  could be radically affected by just touching the top of the filter, the weight of the coax hanging on the a connector, or just bumping it while handling it!  Even after doing all of these things they are still quite fragile and likely to be de-tuned (or broken!) if dropped!

• They are a pain to tune the first time - and a bit of a pain to set up at each Field Day!  After the initial tune-up, you need to have on-hand an antenna analyzer to know where the notch is and a reflectometer (e.g. VSWR meter) - or better yet, a VNA - is imperative for the CW station to know when the bandpass filter is centered on the operating frequency.

• There's no practical reason why similar filters couldn't be constructed for the 10 and 15 meter bands - or other frequencies in between. For lower bands (e.g. 40, 80) you would need much larger cans:  Think "bucket" sized!

* * *

Note:

For follow-up article with more measurement and analysis of these filters, see the 31 May, 2019 article - "Revisiting the 20 meter "helical resonator" band-pass/notch filters" - linked here.

Sweeps of these filters and analysis:

Figure 8:
A frequency sweep of the notch filter, tuned to the CW segment of 20 meters.
The measured notch depth was on the order of 10 dB.  This filter is designed to
be placed in the feedline of the 20 meter SSB station to reduce energy from
the 20 meter CW station.
Click on the image for a slightly larger version.

The Notch filter

After several years of having been used, I finally got around to "sweeping" them using a spectrum analyzer.  Not having a tracking generator on-hand, I used a broadband noise source, setting the resolution bandwidth to 3 kHz to provide the needed frequency resolution and the video bandwidth to 10 Hz to "smooth" the trace.  The result of sweeping the notch filter may be seen in Figure 8.

Admittedly, the depth of this notch is as great as I'd hoped and while I was at it, I did a bit of experimentation - moving the capacitive coupling probe closer and farther away from the coil.  As it turned out, moving the probe closer to the coil widened the notch and reduced its depth even more while moving the probe away narrowed the notch and also reduced its depth.  This result implies that the notch depth is, at least in part, limited by the unloaded "Q" of the coil within:  You may recall that this effect was briefly mentioned above when I originally wound the coil with 12 AWG wire.

How do I improve the performance?  There are several options:

• Use a larger coil.  As noted above, 1 gallon paint cans are a bit small for 20 meters, so a "full sized" coil would probably have a better unloaded "Q", but something larger than a paint can would be required.

• Use larger conductor for the coil.  The 6 AWG wire is 0.162" (4.1mm) in diameter and being wire, it is solid - and heavy.  Because of the "skin effect", RF energy flows only on the outside of the conductor, so tubing may be used, instead with the added advantage of being lighter and a bit easier to handle.  Obvious alternatives are "1/4" inch tubing (actual O.D. of 0.375", 9.5mm) and "3/8" inch tubing (actual O.D. of 0.5", 12.2mm) which would increase the surface area by a factor of and 2.3 3, respectively, significantly reducing the RF resistance - but it would probably be difficult to fit a "larger" coil within a 1 gallon paint can.

• Reduce the loss of the coil former.  As noted, the coil is supported by a glass jar with the windings held in place with blobs of RTV.  While glass is a pretty good choice for this task, a "skeletal" coil former made of polycarbonate or other low-loss plastic would offer slightly lower loss, be much lighter in weight and allow the coil to be trimmed/lengthened somewhat more easily.  Even if it has only a small effect as compared to that of using a larger conductor, it would be worth doing owing to the savings in weight and being designed to be specifically suited for the task at hand.

• Use more than one pass/notch element.  Cascading notch and/or band-pass resonators will improve off-frequency rejection and allow better control of the shape of the pass/reject band.

Increasing the size of both the container and the conductor would and the selection of appropriate stubs, etc., I'm certain, make a very significant improvement in the depth - probably permitting 20 dB or more at the center frequency while also making this notch a bit narrower.

One thing that somewhat improved the notch (by 3-4 dB) was using a stub of less than 1/4 wave (electrical) length piece of coaxial cable between the signal source and the notch cavity:  The lower impedance of the notch is transformed to a higher impedance at the "radio" end by this stub section, slightly deepening the notch.

While the notch depth is "only" around 10dB this still represents a significant decrease in the energy from another transmitter.  If one takes the oft-quoted advice that states that 1dB increase in signal represents 3dB in undesired mixing products at face value, things like intermodulation distortion in the receiver can be easily reduced by 20 dB or more.

Figure 9:
A frequency sweep of the band-pass filter, tuned to the
center of the 20 meter CW band.  This filter is designed
to be placed inline with the CW station to reject energy from
the 20 meter SSB station - and other stations on other
bands.
Click on the image for a slightly larger version.

The bandpass filter:

Because it is identical, the bandpass filter's response is also a bit worse than I was hoping - and for the same reasons as the notch filter.  In doing experimentation, moving the probes closer expectedly increased the bandwidth, but moving them farther away did not narrow the bandwidth much and significantly increased the insertion loss - yet another indication that the unloaded "Q" of the coil is a bit poorer than desired.

As can be seen from Figure 9, the attenuation at a frequency 220 kHz removed (about 14.250 MHz) is only about 6 dB, or 1 S-unit while the insertion loss at the center frequency was just under 1 dB.  To increase the "Q", the same sort of things mentioned above could be done to improve the coil and this would further increase the off-frequency attenuation while maintaining the same amount of loss - but at the expense of commensurately reduced pass-bandwidth.  As noted above, in the case of the CW station, it is very easy to retune the filter as one moves around in frequency (e.g. watching the reflected power while transmitting and adjusting the tuning) or one can, to a limited extend, rely on the antenna tuner to take some of the resulting mismatch into account without causing much additional loss.

Figure 9 reveals a few other things as well:  It functions well to remove energy from stations operating on other bands.  For example, the filter will attenuate 30 and 40 meter signals by more than 60dB while knocking down 17 and 15 meter signals by more than 40dB.  Another thing to note is that the passband sweep is somewhat asymmetrical:  The attenuation at a given frequency is somewhat greater below the center frequency than above.  To a degree, this is due to fact that the percentage difference in frequencies above the center frequency as represented on the plot is lower than those below the plot, but it is also typical of capacitive coupling.  If inductive coupling were used, this response would be somewhat more symmetrical - but experimentally adjusting the amount of coupling in such a filter is decidedly more difficult to do!

* * *

Comments on the rigs that UARC uses for Field Day:

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

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

Almost all modern, solid state rigs from all manufacturers tend to put out a noise spectra that blankets all HF frequencies ON and BELOW the band on which they are being operated.  What this means, is that if you are operating on 20 meters using a multi-band antenna such as a Windom or Yagi, when that radio keys up on 20 meters, those operating on 20 meters and lower and lower bands will experience an increase in the noise floor - even if there is no modulation!

Note also that this noise could also appear above the current band, depending on the low-pass filter.  For example, it could be present on 17 meters when operating on 20 meters if those two bands use the same low-pass filter, but this depends on the radio's design.

In the case of the TS-450s with the built-in antenna tuners, we have found that simply enabling the antenna tuners - even when operating into a 50 ohm load and the tuner isn't really needed - will attenuate this noise by 15-40 dB on all other bands by causing the tuner to function as a low-Q bandpass filter for both receive and transmit and for this reason there is now a label on all of our TS-450s admonishing the operators to always keep the antenna tuner inline!  After all, no-one really notices the extra 0.25-0.5 dB or so (measured!) loss that the tuner causes at or near 50 ohms!

As noted above, other ways to eliminate transmitter-related noise and QRM on other bands include:

• The use of 1/4 and 1/2 wave notch/stub coaxial filters.  These are band-specific and notch ("suck out") the frequencies for which they are tuned will leaving other frequencies unaffected... more or less.  One must make sure, however, that when changing bands, that the notch is removed and (if needed) replaced with the appropriate notch for another band to prevent a different interference problem or, above all else, you must make sure that you do not transmit on the band for which the notch you currently have installed is tuned!

• The use of commercial, high-power bandpass filters such as "Dunestar" (tm).  These are fairly broad, bandpass filters that cover (more or less) an entire amateur band.  These do a decent job of knocking down out-of-band energy on both receive and transmit, but they can be quite expensive to implement - particularly if you have a set of these at each station!  As with the notch filters, they must be changed when you change bands!

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


Certain radios are banned from our field day site!

The one radio that is explicitly banned from our Field Day site is the Icom IC-706 and its relatives of the same/similar vintage.

When this radio first came out, one was set up as the 20 meter SSB operating positions and within a few minutes of Field Day having started we had to take it off the air because it made operation of the 20 meter CW station - as well as the 40 meter SSB station - completely impossible.  Additionally, its receiver was completely demolished by the other transmitters' on the air as well, no matter what band they seemed to be on.  At the time we didn't have available band-pass filters, which would have probably helped the situation.

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


The realities of Field Day:

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

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

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

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

[End]

This page stolen from ka7oei.blogspot.com

How USB car power adapters can ruin 2 meter mobile reception!

For an update on this article, see the posting about a "Quiet" vehicle USB power supply - LINK.

For decades now, the ubiquitous power adapter found in vehicles has been the "Cigarette Lighter Plug" - a 12 volt receptacle found in pretty much every vehicle made for more than more than half a century.

Now, enter the USB connector.

Not necessarily for data connectivity, these provide 5 volts at up to 3 amps and are used to power/charge about anything from a cell phone, smart phone, tablet, game device, E-reader - even modern Handie-Talkie - to name but a small number of possibilities!  If your vehicle is fairly new, it will already be equipped with a couple of these, but if not, you'll probably have to get something like the one shown in Figure 1, below.

Figure 1:  A typical USB power adapter, available practically everywhere -
and if you have one in your car, it's probably clobbering
YOUR 2 meter reception!
Click on the image for a slightly larger version.

These things seem to be everywhere, and they are pretty much all alike:  They are small and fit pretty much entirely inside the cigarette lighter plug receptacle itself.

While the two USB outlets have different current ratings printed on them, they are actually connected in parallel - but on the two center pins (the "data" pins of the USB connector) there are different resistances that indicate to the device connected to it how much current they can pull:  Not all devices actually pay attention to these "programming resistances" but some (particularly some Apple [tm] devices) do - although plugging into the "wrong" outlet will (supposedly) result in just lower charging rate.

What's wrong with these USB power devices?

Functionally, these devices - at least the ones at which I have looked - seem to "work" just fine:  They output a "reasonably" clean 5 volt source of charging voltage (perhaps 100 millivolts of ripple at 3 amps - nothing that these devices can't deal with, and cleaner than many "wall" chargers) so what's wrong with them?

Nothing - unless you want to listen to the radio.

I got one of these to charge my cell phone and power the GPS receiver in my car to replace the "Y" cigarette lighter adapter that I'd been using in my car - a rather awkward affair that was always in the way of the gearshift and something that kept un-plugging itself due to its swinging around under its own weight, and into it was plugged the original Garmin power adapter and an OEM Motorola car charger - both of which were "reasonably" clean - causing a little bit of QRM only in the weakest-signal areas.

Immediately upon plugging in this new device, pictured in Figure 1, I noticed that something was amiss:

2 meter reception practically disappeared!

70cm reception wasn't as badly affected - but I did notice an impact there as well - and I even noticed that broadcast FM reception was adversely affected when I went up some of the local canyons, all of this clearly evident when I unplugged the adapter and suddenly, everything was "normal" again.

Attempts at modification:

The first thing I did was attempt to modify this adapter to reduce its emission of "grunge" which took the form of a white noise "hiss" across the spectrum - at least at VHF and above - and I did the following:

• The addition of a choke (10 uH) on the DC input of the switching regulator.

• The addition of an SMD monolithic capacitor on the input of the switching regulator.

This and the above removed all traces of noise on the DC input as measured using an oscilloscope,  but a quick check back in the car revealed that this made no difference in the amount of QRM being generated.

I should have known better:  Removing the USB cables then revealed that the "grunge" was being radiated almost entirely on the DC output rather than the DC input, so I went back to the the workbench and did more modification:

Figure 2:
Modifications made to the bottom of the board to make it "quieter" from
and RF standpoint.  Some of the SMD caps can be seen, as can the
RFI filter with integrated beads.
Click on the image for a slightly larger version.

• I added SMD monolithic capacitors to better-bypass the DC output

• I also noticed that the "grounding" of the DC input, that of the main switching circuit and of the DC output were not solidly connected to each other - that is, they were connected through some rather narrow traces that could offer some potentially high impedances.  I bolstered these using pieces of copper strap and wire to reduce these possibly high-impedance paths.

• Routed the DC output through an integrated RFI filter consisting of two ferrite beads and a ceramic capacitor - the red/orange thing that can be seen in Figure 2, above.

Figure 3:
Modifications made to the top of the board.  In the upper-
right corner can be seen the added RF choke while
some of the added grounding can be seen along
the bottom edge.  The cardboard was added to prevent
the added grounding and components from
shorting out when the unit was reassembled.
Click on the image for a slightly larger version.

Doing the modifications did help a bit - but the unit still caused a considerable amount of degradation to 2 meter reception.

How bad was it?

Making the modifications depicted in Figure 2 and Figure 3 reduced the RFI by about 15-20dB which meant that I could now hear strong, local repeaters around town OK, but more distant repeaters with the weaker signal were still significantly degraded.

After modifications, how bad was the degradation on 2 meters?

To answer that question I connected a communications test set to the mobile rig via an "Iso-Tee" and ran a SINAD test with the GPS receiver and a cell phone connected.

The result?

18 dB of degradation on 2 meters.

Still pretty bad - And this was after modification.

Prior to modification, the degradation was on the order of 30-35dB on 2 meters!  I didn't make any measurements on 70cm or quantify the degradation on the FM broadcast band.

If I disconnected the USB cable to the GPS receiver, the degradation dropped to about 12 dB, which made sense since that particular cable ran up onto the dashboard, while the phone sat on the console, down between the passenger and driver seats.

While I was at it, I tried adding some snap-on ferrites to the USB cables to on the output of the adapter, but I could find little actual improvement - but that wasn't too surprising as these devices (the ferrites) are generally better at keeping RFI from getting into devices than getting out of them.

By re-routing USB cables, I could cause the RFI level to vary, but I couldn't get it much better than the 12 dB value mentioned above, but I could make it much worse - well into the 20's.

What to do?

Figure 4: 
 Another typical 5 volt car-type power adapter.  I have several that look
like this on the outside, but on the inside, who knows what you'll find?  One of
them was a well-constructed, well-filtered switching regulator while another
switcher was utter garbage - while yet another one consisted of nothing other
than a 78L05 regulator and a couple of electrolytic capacitors:  While it was
"quiet" from an RF standpoint, but capable of no more than 100 mA
of current, at most - barely enough to charge a phone, slowly, and
not nearly enough to power up the GPS receiver.

At the moment, I don't have any suggestion as to what brand of car USB power converter is "clean" in terms of RFI, but it would suggest that one should avoid those like the one picture in Figure 1.

In looking around, I've also tested a number of other units - some of them being much larger, such as the one pictured in Figure 4, below.  I have several of these - all of them look pretty much alike on the outside but inside, none of them look alike.  One of them came with an old cell phone as is quite well constructed with what looks like proper filtering on the input and output - but with a rating of only 500 mA, it wouldn't be able to power anything but the GPS receiver, alone - if that - let alone several devices at once.

What I finally did was to construct my very own, custom, car USB power adapter using a pair of 3 amp switching converters, placed inside a die-cast aluminum box with extensive input and output L/C filtering:  This unit has proven to be absolutely clean in terms of RFI and you can read about that device here - LINK.

In the meantime, if you have noticed that your 2 meter reception seems to have gone to hell, try unplugging your USB power adapter for a few seconds and see if that has anything to do with it...

How quiet are the USB power converters built into recent-vintage vehicles?

I have no idea, but most people report that they do not seem to cause obvious interference.


Links to other articles about power supply noise reduction found at ka7oei.blogspot.com:

• Containing RF noise from a "pure" sine wave UPS.   Even when it is not operating your sine wave UPS may be producing a lot of HF radio interference!
• Completely containing switching power supply RFI - link.  Sometimes it can be difficult to quiet a switching power supply, so it may be necessary to put it in a box with strong filtering on all of the conductors that enter/leave.
• Minimizing VHF (and HF) RFI from electronic ballasts and fluorescent tubes - link.  Electronic light ballasts, like many switching power supplies, operate in the LF frequency range so "cleaning them up" at VLF/LF/MF frequencies can be a challenge.
• Quieting high current switching power supplies used in the shack - link.  This page describes techniques that can be used to reduce the amount of RF energy produced by switching power supplies that you may be using to power your radios.  Again, higher-inductance chokes may be required at VLF/LF/MF frequencies.
• Reducing switching supply racket - link.  This describes techniques that can be used to beef up the filtering for switching supplies in general.

[End]

This page stolen from ka7oei.blogspot.com