Category: Mythbuster

Adding Automatic Gain Control to the Termination Insensitive Amplifier

Earlier this month Paul VK3HN had a very interesting blog post about adding Automatic Gain Control to Termination Insensitive Amplifiers (TIAs).

Termination Insensitivity is especially important in bidirectional rigs. The shape of the crystal filter bandpass response is very dependent on the impedances presented at both ends of the filter. In bidirectional rigs you are changing the signal path direction through the filter when you go from transmit to receive. If the amplifiers at either end of the filter have impedances that vary depending on what is on the input or output of either stage, you will have great difficulty keeping the bandpass identical as you move from transmit to receive. Termination Insensitive Amplifiers let you do just that — they stay at one fixed input or output impedance (usually 50 ohms) independent of what is attached to the other end of the amplifier circuit. This greatly simplifies impedance matching at the ends of the crystal filter.

When I started building BITX rigs, I asked Farhan about the impedance matching problem. He advised me to use TIAs on both ends of the filter and pointed me to a great 2009 article by Wes Hayward and Bob Kopski. Using the information from that article, I built my DIGI-TIA transceiver, and I have used TIAs in almost all of the rigs I have built since that project.

In his August 2021 blog post, Paul wanted to add Automatic Gain Control to the TIAs. He came up with a way to do this, but we worried that his circuit would have an impact on the impedance of the amplifiers.

Yesterday, Wes Hayward W7ZOI posted on his web site a TIA circuit that lets us do it all: Termination Insensitivity with Automatic Gain Control:


I now find myself tempted to rebuild one of the TIA stages in my Mythbuster transceiver, adding the AGC circuitry from Wes’s design.

Thanks to Paul VK3HN for the blog post on this subject. And thanks to Wes Hayward for the TIA AGC design. It is a real privilege to have direct input from Wes on questions like this.

SolderSmoke Podcast #232 — Mythbuster, Pete’s Tube CW Rig, Pete’s DC RX and Simple SSB Rig, NanoVNA and TinySA, Very FB Mailbag

SolderSmoke Podcast #232 is available — Crank it in Robert!

Featuring a guitar intro by Pete “Bluesman” Juliano, playing his own composition: “Juliano Blues.”

Upcoming GQRP convention and the N6QW rig
Frank Jones and the FMLA — Possible Victory?
IBEW Stickers: NASA, Johns Hopkins APL….
Cycle 25 Lookin Better Today: SFI 93 SN 47
Pete’s Bench:
Toobular! A Tube Transmitter
SR-160
Simple SSB rigs around the world!
KI7NSS’s Pacific 40
Bill’s Bench
The Mythbuster and the Struggle Against the Urban Legend
W2EWL’s Cheap and Easy SSB
W4IMP’s IMP. Articles in ER by Jim Musgrove K5BZH and Jim Hanlon W8KGI
The Spirit of Homebrew SSB. From Electric Radio K5BZH December 1991
Reduced Front End Gain on the DIGITIA
Back on 17! HP3SS sells HBR receiver to Joe Walsh
Maybe another Moxon?
SHAMELESS COMMERCE DIVISION
Test Gear
NanoVNA — Alan W2AEW helped solve mystery of why NanoVNA not providing accurate readout of circuit impedance. Over driving. Need attenuator.
TinySA — Limited Resolution Bandwidth. But you can listen with it! See video on blog.
MAILBAG
— Google Feedburner to end e-mails from the blog 🙁
— Paul VK3HN — TIA AGC? Farhan and Paul looking into options
— Ciprian’s Romanian Mighty Mite
— Dino KL0S SolderSmoke GIF and graphical presentation on sideband inversion
— Allison KB1GMX helped me on 24 volts to IRF 510 issue.
— Dave K8WPE Wabi Sabi and Martha Stewart. And thanks for parts! 40673s!
— Steve N8NM building a 17 meter rig with 22.1184 crystals in a SuperVXO and a 4 MHz filter.
— Dean KK4DAS restoring an old Zenith. One hand behind your back OM.
— Pete Eaton debating SSB or DSB for 17. Go DSB Pete!
— Richard KN7FSZ a FB HBer. Asked about my solid-stating of Galaxy V VFO.
— Walter KA4KXX on benefits of no-tune BP filters like Farhan’s FB.
— Jack 5B4APL on Time Crystals and Homebrewing in the 4th dimension. FB OM!
— Moses K8TIY listens to the podcast with his young son Robert. Crank it in Robert!
— Farhan and the SBitx on Hack-A-Day
— Also Tom’s receiver from junked satellite rig on Hack-A-Day
— Todd K7TFC sent in beautiful message about the spirit of homebrewing. On the blog.
— Grayson KJ7UM was on Ham Radio Workbench with George Zaf
— AAron K5ATG running a uBitx with a homebrew tuner and antenna. Hope I can work him
— Heard Mike WA3O last night on 40 DIGITIA. Water cooled amplifier

W2EWL’s “Cheap and Easy SSB” Rig — And The LSB/USB Convention Myth

In March 1956 Tony Vitale published in QST an article about a “Cheap and Easy” SSB transmitter that he had built around the VFO in an ARC-5 Command Set transmitter. Vitale added a 9 MHz crystal-controlled oscillator, and around this built a simple phasing generator that produced SSB at 9 MHz. He then made excellent use of the ARC-5’s stable 5 – 5.5 MHz VFO. His rig covered both 75 meters and 20 meters. Here is the article:

http://nebula.wsimg.com/2b13ac174f7f2710ca2460f8cf7d6b8b?AccessKeyId=D18ED10DA019A4588B7B&disposition=0&alloworigin=1

Because it used the 9 and 5 frequency scheme, over the years many, many hams have come to think that Vitale’s rig is the source of the current “LSB below 10 MHz, USB above 10 MHz.” This is wrong. An example of this error popped up on YouTube just this week (the video is otherwise excellent):

First, Vitale’s rig had a phasing SSB generator. All you would need to switch from USB to LSB was a simple switch. And indeed Vitale’s rig had such a switch. Pictures of other Cheap and Easy transmitters all show an SSB selection switch. So with a flip of the switch you could have been on either USB or LSB on both 75 and 20. With this rig, you didn’t even need sideband inversion to get you to 75 LSB and 20 USB.

Second, even if hams somehow became so frugal that they wanted to save the expense of the switch, leaving the switch out (as suggested above) would NOT yield the desired “75 LSB 20 USB” that the urban legend claims that W2EWL. As we have been pointing out, a 9 MHz SSB generator paired with a 5 MHz VFO (as in the Vitale rig) will NOT — through sideband inversion — yield LSB on one band but USB on the other.

W2EWL’s rig could not have been the source of the LSB/USB convention. I still don’t know where the convention came from. I am still looking for the source.

But leaving the LSB/USB convention issue aside, Tony Vitale’s rig is an excellent example of early SSB homebrewing, and of a very clever use of war surplus material. In the January 1992 issue of Electric Radio magazine, Jim Musgrove K5BZH writes of his conversations with Vitale about the Cheap and Easy SSB. Tony told Jim that this rig came about because the Central Electronics exciters required an external VFO — they recommended a modified BC458. B&W had recently come out with a phase shift network. Vitale went ahead and built the whole rig inside a BC458 box. FB Tony!

In the December 1991 Electric Radio, Jim K5BZH reports that Tony was recruited into the ranks of SSBers when he watched a demonstration of SSB by Bob Ehrlich W2NJR in November 1950. Tony very quickly started churning out SSB rigs. His daughter Trish Taglairino recounted that when her father had “done something great again” there would be a parade of hams to the basement shack. About 30 guys showed up when Tony put his first SSB rig on the air — they sent out for beer.

Thanks to Jim for preserving so much SSB history.

Mythbuster Video #16 — GLOWING NUMERALS! In Juliano Blue!

I added two San Jian frequency counters to the front panel. In addition to making the rig a lot easier to operate, they add a classy touch of Juliano Blue to the project.

I got my counters here: https://www.ebay.com/itm/224223678132

There is a limitation of some of the the San Jian counters: https://soldersmoke.blogspot.com/2021/02/a-problem-with-san-jian-plj6-led-counter.html

But this limitation didn’t cause any problem with this rig: In this case I just plugged in the IF frequency of 5.2397 MHz. I connected the input to my VFO running around 9 MHz. For 20 meter signals, I select the “up” option; the San JIan counter just adds the IF frequency to the VFO Frequency. For example 9 + 5.2397 = 14.2397 MHz . For 75, I select the “down” option. Here the San Jian just subtracts the IF frequency from the VFO frequency — for example 9 – 5.2397 = 3.7603 MHz.

The band select switch operates relays that select the proper band-pass and low-pass filters. This switch also alternatively turns on either the 20 meter San Jian or the 75 meter San Jian.

Mythbuster Video #15 The Mythbuster Signal As Seen in the NA5B WebSDR

Mehmet NA5B has an excellent WedSDR receiver in Washington D.C., about 9 miles east of me. I often use it to check my signal quality. I think this video shows that the 10 pole crystal filter is working and is producing a signal with very sharp drop-off outside the 2.7 kHz passband. You should focus your attention to the passband (yellow vertical lines) near 3895 kHz. That’s me.

Once, when I was describing my 40 meter DIGI-TIA to an SDR guy, he seemed surprised that I was using a — gasp — crystal filter. “Your skirts must be atrocious!” he said. My HDR sensibilities were deeply offended.

I had hoped that the 10 pole crystal filter would produce skirts so nearly vertical as to make my signal indistinguishable (in the waterfalls) from the SDR signals. At least at this low signal level, it appears to be working.

Mythbuster Video #14 — ON THE AIR!

On August 5 I put the Mythbuster on the air, making QSOs on both 75 and 20 meters. This video is from the following day — I was on 75 meters. My first QSO with this rig was on 20 with S57DX. That was a good omen. TRGHS

I had no feedback or spur problems with the BITX40 module amplifier circuit. And I didn’t release any IRF510 smoke. There is, of course, a lot more output on 75 than on 20. That’s just the nature of the IRF510. I get about 4 watts out on 20 and about 9 watts out on 75 (with a 12 volt supply). At Pete’s suggestion I might replace the IRF510 with a Mitsubishi RD06HHF1.

I had a couple of minor problems getting the transmitter going. I will describe these in a future video.


Mythbuster Video #13 — RF Power Amplifier, and Relay Switching Plan

In this episode we enter into the most fraught part of the construction project: the production of RF power. This is where amplifiers stubbornly turn into oscillators, and where components release magic smoke, or at least burn the fingers of hopeful builders.

I kind of ran out of room when I built the low-pass filters. But, thinking ahead, I wanted to have them on a separate board. And it is good that they ended up in the far corner of the rig.

Just going from one band to two bands adds to the complexity of the rig. I had to add two relays, one to switch the low pass filters, the other to switch the bandpass filters. I ended up with 5 DPDT relays in this transceiver. It was very helpful to have a plan and a diagram for the relays and all the switching.

It looks like each of the three RF amplifier stages provides about 15 db of gain — about what I need to get to the 5 watt level.

Video: Introduction to the TinySA Spectrum Analyzer

The TinySA has some very cool capabilities, and this short intro video provides a good sense of what it can do.

I am learning how to use the TinySA so that I can check the output of my Mythbuster transceiver (I now have the first portion of the transmitter working.) I tried to use the TinySA to check the carrier and opposite sideband suppression on my new Mythbuster transceiver, but I think the max Resolution Bandwidth (3 kHz) is too high for me to do this. Please let me know if I am missing something. That would have been a very useful capability.

The rest of the videos are here:

It also functions as a signal generator that also provides AM and FM modulated signals. You can also have a waterfall on the spectrum display. Very nice.
I have not yet figured out how to listen to the signals. This is one of Erik’s videos — it looks like you have to solder in a connection for audio out.
Thank you Erik Kaashoek.

QST Repeatedly Got Sideband Inversion Wrong

It kind of pains me to do this. These articles are from a long time ago, and the author is an esteemed Silent Key, but the myth about the origins of the USB/LSB convention is still out there, and as a homebrewer of SSB gear I feel obligated to point out these examples of the error that that myth is based on.

Last Friday, Pete WB9FLW and I were talking about homebrewing SSB rigs. I recommended a series of QST articles by Doug DeMaw. “Beginner’s Bench: The Principles and Building of SSB Gear” started in QST in September 1985. There were at least five parts — it continued until January 1986. (Links to the series appear below.) I hadn’t looked at these articles in years, but when I did, a big mistake jumped right out at me: In the first installment, on page 19, Doug makes the same mistake that he made in his Design Notebook:

“Now comes the conversion section of our SSB generator. We must move (heterodyne) the 9-MHz SSB signal to 3.75-4.0 MHz. Our balanced mixer works just as it does in a receiver. That is, we inject the mixer with two frequencies (9 MHz and 5 MHz) to produce a sum or a difference output frequency (9 – 5 = 4 MHz, or 9 +5 = 14 MHz) If we are to generate 75 meter SSB energy, we must chose the difference frequency. We could build an 20-meter SSB transmitter by selecting the sum of the mixer frequencies. The RF amplifiers and filter (FL2) that follow would then have to be designed for 14-MHz operation. In fact, many early two-band homemade SSB transmitters were built for for 75 and 20 meters in order to use this convenient frequency arrangement. The use of upper sideband on 20 meters and lower sideband on 75 meters may be the result of this frequency arrangement (the sidebands become inverted when switching from the difference to the sum frequency.) ”

Those last two sentences are incorrect. They repeat the “Myth,” or the “Urban Legend” about the origins of the LSB/USB convention. Contrary to what many hams now believe, with 9 MHz filter and a 5.2 MHz BFO it takes more than just switching from sum frequency to difference frequency to invert one of the sidebands.

There are two conditions needed for sideband inversion to take place:

1) You have to be taking the difference product (DeMaw got that right)

2) The unmodulated (VFO or LO) signal must be larger than the modulated signal. (DeMaw and the ARRL obviously missed that part. Repeatedly.)

This is another way of stating the simple, accurate and useful Hallas Rule: Sideband inversion only occurs when you are subtracting the signal with modulation FROM the signal without modulation.

For DeMaw’s claim to be correct, one of the SSB signals going into the balanced mixer would have to invert, and the other would have to not invert. Let’s see if that happens: He has the sideband signal being generated at 9 MHz and the VFO running around 5 MHz.

9 – 5 = 4 But we are not subtracting the modulated signal FROM the unmodulated signal. SO NO INVERSION

9 + 5 = 14 We are not subtracting at all. SO NO INVERSION.

Doug’s convenient frequency scheme WOULD work if he’d just switch the frequencies of the filter and the VFO. With a sideband generator on 5.2 MHz and a VFO around 9 MHz you do get the happy 75 LSB, 20 USB arrangement without the need to switch the carrier oscillator/BFO frequency. That is what happened in the Swan 240, and that is what I have in my Mythbuster rig. I am listening to both 75 LSB and 20 USB without changing the carrier oscillator/BFO frequency. My filter/BFO/product detector is set up for USB. With this arrangement the 75 meter LSB signals DO invert, and the 20 meter USB meter signals do not, so both are able to make use of my USB BFO/product detector without shifting the BFO frequency.

This error shows up again in DeMaw’s the May 1989 QST article “A Four Stage 75-meter SSB Superhet” (reprinted in the ARRL’s QRP Classics book). Here he writes:

“Should you want to cover both the 75- and 20-meter bands you can build a 20-meter version of FL-1 and band switch the two filters. As with the 75 meter only version, an IF of 9.0 MHz (Y1) is required. With this arrangement the 20 meter band will tune backwards from the 75 meter band, but upper- and lower-sideband reception will occur, as required, without changing the BFO frequency (Y2). This two band scheme with a 5-MHz VFO is an old one!” NOTE: FL1 is the bandpass filter, not the IF filter.

Doug’s mistakes in this area may simply be due to the fact that he was more of a CW guy. And this is something that is quite easy to confuse: 9 and 5 will get you to 75 and 20, but you have to make sure the VFO is at 9 if you want to make use of sideband inversion and avoid having to shift the BFO/ carrier oscillator. I’ve made this mistake myself:
https://soldersmoke.blogspot.com/2012/05/usblsb-urban-legend-debunked.html

In October 1993 I wrote to DeMaw about his Four Stage 75 meter SSB Superhet. I think I was looking for details on how to put it on 20 meters. As I recall, Doug wrote back telling me to just pick 20 meter values for the input bandpass filter. Had I done so, I would have discovered that — for the reasons cited above — this just wouldn’t have worked on 20. His BFO and filter were set up to receive LSB signals. That’s fine for the incoming 75 meter LSB signals. But on 20 — contrary to DeMaw’s thinking — there would be NO sideband inversion. I’d be trying to listen to 20 meter USB signals with a receiver set up for 20 meter LSB.

Did anyone else notice these errors. Were there ever errata notices in QST on this?

This is a reminder that you should take all technical articles and schematics with a grain of salt. Many contain errors. We are all human, and this is a complicated subject with lots of details.


https://pe2bz.philpem.me.uk/Comm/-%20ARRL/Build-Service-Design/Receivers/PrincipleOfBuildingSSBgear/8509017.pdf




The Unicorn! A 75 LSB /20 USB Receiver (That Can’t Work)

Don’t get me wrong — I’m a huge fan of Doug DeMaw. His books and articles are a treasure trove for ham radio homebrewers. Also, Doug was an honest guy who admitted in the preface to his QRP book that at times he did not fully understand the circuits he was building; that kind of honesty is rare, and is very helpful to amateurs who struggle to understand the circuits we work on.

But everyone makes mistakes, and Doug made one in his “W1FB Design Notebook.” I present it here not as a “gotcha” effort to nitpick or sharpshoot a giant of homebrew radio, but because this error illustrates well the depth of the 75 LSB/20 USB myth, where it comes from, and how important it is to really understand sideband inversion. Here is the mistake:

That’s just wrong. A receiver built like this will not allow you to listen to 75 LSB and 20 USB “without changing the BFO frequency.” (Am I the first one to spot this error? Didn’t anyone build this thing, only to discover that it, uh, doesn’t work?)

Here’s a little drawing that I think illustrates why the mythical scheme will not work:

All confusion about sideband inversion could be avoided with the simple application of what I think we should call “The Hallas Rule”:

“Sideband reversal occurs in mixing only if the signal with the modulation is subtracted from the signal that isn’t modulated.”

Be careful here: I think some arithmetic carelessness is responsible for much of the myth. Taking the difference frequency is not enough to produce sideband inversion. Read the Hallas Rule carefully: For sideband inversion to occur, the signal with the modulation must be subtracted FROM the signal without the modulation.
———————————————
About the Swan 240’s SSB generation scheme:

I first stumbled on this problem when building my first SSB transmitters in the Azores. I was using a VXO, and a filter pulled out of a Swan 240 (5.173 MHz). I started with VXO crystals at around 12.94 MHz. The rig worked, but I couldn’t pull the VXO crystals very far. So I switched to crystals at around 23.3 MHz (you can pull higher frequency crystals farther). But look what happened: My Carrier Oscillator frequency had been set up to receive USB signals on 17 Meters. With the 12.94 MHz rocks, that worked fine: 18.150-12.977 = NO INVERSION. But it all changed when I went to the 23 MHz VXO rocks: 23.323-18.150 = INVERSION! This had me scratching my head a while. I had to draw myself little spectrum pictures (like the one above) before I realized what had happened. To get it to work — to get it to produce USB on 17 meters — I had to move the Carrier Oscillator to the other side of the passband. Good thing that Swan 240 came with TWO BFO crystals (5.1768 MHz and 5.1735 MHz). I just had to change the crystal.

For 75 and 20 meters, the Swan 240 uses the correct 5.173 MHz filter with a 9 MHz VFO to get the happy situation of 75 meter LSB and 20 Meter USB WITHOUT changing the BFO/Carrier Oscillator frequency. This is the Mythbuster scheme. Unlike Doug’s receiver, it works. The scheme also works in the Swan 240 on 40 meters because for 40 the Swan rig has the VFO running from 12.073 MHz to 12.513 MHz. Here too, no change in the BFO/Carrier Oscillator frequency is needed. But the Swan recommended a modification that would allow operation on 20 LSB and 75/40 USB! It used a BFO/Carrier Oscillator crystal of 5.1765 MHz and a switch mounted on the front panel. Luckily, my junker Swan (acquired from HI8P in the Dominican Republic) had the second crystal — mine was 5.1768 MHz. It was that crystal that allowed me to get my Azorean SSB transmitter to work using the 23.9 MHz VXO rocks.

Mythbuster Videos 8 and 9 — The Old Military Radio Net plus “Zero Beat and The Vertical Skirts”

I like to listen to the Old Military Radio Net on Saturday mornings. This week I was listening with the Mythbuster receiver. The AM carriers provided a good opportunity to observe the effects of the steep skirts of the 10 pole crystal filter. We start at zero beat, with the BFO exactly on the carrier frequency. If I turn the VFO dial in one direction, I in effect move the passband in a way that puts the carrier in the passband. And it is no longer zero beat with the BFO, so we hear the heterodyne (the beat!). But if I turn the VFO dial in the other direction, the carrier is now outside the passband. Even though the BFO would produce a tone, we don’t hear a tone, because those steep filter skirts are keeping the carrier out. We do continue to hear some of the sideband frequencies, because they remain in the passband. The very sharp drop-off of the carrier tone is a good indication that the steep skirts of the crystal filter are doing the job.

“Zero Beat and the Vertical Skirts” Sounds like the name of a Punk Rock band, doesn’t it? Anyway in this video I explain what happened in Mythbuster Video #8 (above). I explain why we can hear the Old Military Radio Net carriers when I tune the VFO in one direction, but not in the other.

Mythbuster Video #7: Bandswitch, Reverse Polarity Protection, CW with Clarifier Offset

I have the speaker mounted on the front of the board. I kind of like it like that. I now have a bandswitch, and reverse polarity protection (no more living dangerously for me). That Yaesu VFO clarifier circuit might prove useful should I decide to give this rig CW capability. I once again find myself thinking that I might never put this in a metal box. Frank Jones had the right idea.

Mythbuster Video #6 — On to 20 Meters (But With Bandpass Filter Woes). Please help solve the mystery!

Here’s how I started with the Elsie program. Note that to get a 50 ohm match on both ends it needs an impractically low value for the coils (.064 uH).
But Elsie lets you specify the coil value. So I then I went with 1 uH. But with this value you don’t get 50 ohms at either end. You need a matching network. Elsie provides this too!
I asked Elsie to match my BP filter to 50 ohms. It provided several options to do this — I went with a simple capacitive impedance divider. But alas, I was now bumping up against the 7 limit of the free version of Elsie so I had to reduce the number of LC elements from 4 to 3. Bummer.
With 3 LC tuned circuits and matched to 50 ohms the plot looks OK. But I would have preferred 4 LC circuits.
The rftools website created a BP filter for me with 4 LC elements, and matched to 50 ohms. Very useful. https://rf-tools.com/lc-filter/
But here’s my problem: With both the filter designed by Elsie and the one designed by rftools, I found that the filter passband was too low. It was in the 12 – 13 MHz range. I found that by removing 3 turns from the 1 uH coils I could move the passband up to the desired range. But why the discrepancy? I was measuring the coils and the caps with an AADE meter. I was testing the passband both with a NanoVNA and with a combination of an HP8640B sig gen and a Rigol oscilloscope (with the filter terminated into a 50 ohm resistor). Any suggestions on why these filters should have passbands lower than predicted would be appreciated.

Mythbuster Video #4 — First Signals, 75 meter Bandpass Filter, Yaesu VFO output

This receiver required almost no coaxing or tweaking, probably because I had been so careful about testing and measuring each of the stages.

I have been pleasantly surprised at how well the receiver works without an RF amplifier ahead of the first mixer. But I need to know how much AF gain I have in order to understand how/why the entire receiver works so well. I think I have about 35 db of gain (combined) through the two TIAs and the crystal filter. That would mean that all of the remaining gain is provided by the AF amplifiers (with some loss in the product detector). I haven’t really measured the gain of the AF preamp/LM386 combo, and I had some trouble measuring the input impedance of the pre-amp with the NanoVNA.

The 75 meter LC filter to the left of the VFO is actually a bandpass filter, not the lowpass filter. And what I call “the mixer” to the right of the VFO is really the Product Detector/BFO.

For the 75 meter bandpass filter, I used the ELSIE program.
75 meter Bandpass Filter designed in Elsie. 10 turns on a T50-2 toroid yield .46uH.
Here’s the plot from Elsie on the 75 meter BP filter.
Alan W2AEW asked for a picture of the VFO output.
On this shot I had the probe between theVFO and the
outboard booster amp that I built to bring it to 7dbm.