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Saturday, July 31, 2021
Thursday, July 29, 2021
Video: Introduction to the TinySA Spectrum Analyzer
Monday, July 26, 2021
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.
Thursday, July 22, 2021
The Unicorn! A 75 LSB /20 USB Receiver (That Can't Work)
Tuesday, July 20, 2021
Mythbuster Videos 8 and 9 -- The Old Military Radio Net plus "Zero Beat and The Vertical Skirts"
Saturday, July 17, 2021
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.
Friday, July 16, 2021
Mythbuster Video #6 -- On to 20 Meters (But With Bandpass Filter Woes). Please help solve the mystery!
Thursday, July 15, 2021
Wednesday, July 14, 2021
Mythbuster Video #4 -- First Signals, 75 meter Bandpass Filter, Yaesu VFO output
Tuesday, July 13, 2021
Mythbuster Video #3 -- Using the VFO from a Yaesu FT-101
Monday, July 12, 2021
Mythbuster Video #2 -- 10 Pole Crystal Filter
Sunday, July 11, 2021
A Video Series on the Mythbuster 75/20 Rig -- Video #1
Wednesday, June 30, 2021
10 Pole Crystal Filter Passband as Seen in Antuino and NanoVNA
Saturday, June 26, 2021
SolderSmoke Podcast #231 -- Travel, SST, Mythbusting, Filters, TIAS, NanoVNAs, DC RX in SPRAT, Drake A Line, Spillsbury, STICKERS! Mailbag
Thursday, June 17, 2021
The Stubborn Myth about USB and LSB
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.
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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: