Congratulations to Kacy KI7NNS for a really nice rig.
Tom also sent me Firmware sourcecode that may be reusable for STM8 users, and the FPGA design file (for Quartus users). If anyone has a GitHub or similar site that can host these files, please let me know and we will send them to you.
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.
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.
It has been repeated so often and for so long that many of us have come to believe it. I myself believed it for a while. Like many myths, it has a ring of truth to it. And it is a simple, convenient explanation for a complex question:
Why do ham single sideband operators use LSB below 10 MHz, but USB above 10 MHz?
Here is the standard (but WRONG) answer:
In the early days of SSB, hams discovered that with a 9 MHz SSB generator and a VFO running around 5.2 MHz, they could easily reach both 75 meters and 20 meters (True). And because of sideband inversion, a 9 MHz LSB signal would emerge from the mixer as an LSB signal (True), while the 20 meter signal would emerge -- because of sideband inversion -- as a USB signal (FALSE!) That sideband inversion for the 20 meter signal explains, they claim, the LSB/USB convention we use to this day.
Why this explanation is wrong:
There is a very simple rule to determine if sideband inversion is taking place: If you are subtracting the signal with the modulation FROM the signal without the modulation (the LO or VFO) you will have sideband inversion. If not, you will NOT have sideband inversion.
So, you just have to ask yourself: For either 20 or 75 are we SUBTRACTNG the Modulated signal (9 MHz) from the unmodulated signal (5.2 MHz)?
For 75 meters we have: 9 MHz - 5.2 MHz = 3.8 MHz NO. We are not subtracting the modulated signal from the unmodulated signal. There will NOT be sideband inversion.
For 20 meters we have 9 MHz + 5.2 MHz = 14.2 MHz. NO. No subtraction here. No sideband inversion.
So it is just arithmetically impossible for there to be the kind of happy, easy, and convenient USB/LSB situation described so persistently by the myth.
---------------------------------
We discussed this several times on the podcast and in the blog:
https://soldersmoke.blogspot.com/2015/05/sideband-inversion.html
https://soldersmoke.blogspot.com/2012/05/usblsb-urban-legend-debunked.html
This myth shows up all over the place:
We see the myth here:
http://n4trb.com/AmateurRadio/Why%20The%20Sideband%20Convention%20-%20formatted.pdf
Here the web site owner warns that this is "highly controversial." Really? Arithmetic?
The myth is very old. Here is a clip from a 1966 issue of "73" magazine:
https://worldradiohistory.com/
Finally, to my disappointment, I found the myth being circulated by the ARRL, in the 2002 ARRL Handbook page 12.3:
SolderSmoke Podcast #230 is available for download
http://soldersmoke.com/soldersmoke230.mp3
Really nice work. BLU is Spanish for SSB. (Juliano BLU?)
But I think Guillermo needs to build ONE MORE BOARD! A VFO or a VXO. Analog. To finish the job. Guillermo tells me this is in the works -- he selected an IF of 11.0592 MHz specifically so that he can use a variable oscillator built around a 4 MHz ceramic resonator.
Complete schematic and PC board patterns on his site:
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