Most commercial built ham SSB radios made during the 1960' often touted as being cost effective (read cheap), provided coverage of the ham bands with only one choice of sideband. The SWAN 240 gave you LSB on 80 and 40 and USB on 20. The Hallicrafters SR-160 and even the National NCX-3 and NCX-200 had the same deal. Today that is viewed as a shortcoming.
It was the uptown radios that gave you a choice of sidebands. Even the venerable Bitx40 had only one sideband choice. I found a way to install USB/LSB on the Bitx40 by switching the LO from 5 MHz to 19 MHz for USB and that did the trick. Obviously with FT-8 you would like USB on 40M.
I had never thought of the way to do the USB/LSB process by using a fixed Mixer oscillator frequency to switch sidebands. This was not apparent to me until I saw the block diagram of the B&W 6100 produced during that time period.
This is really an interesting approach and although two blocks are labeled Balanced Modulator #1 and Balanced Modulator #2, the second one is a Mixer stage.
What happens is the standard Balanced Modulator (#1) with Carrier Oscillator at 3.2 MHz and an Audio signal from the Mic Amp, produces an output that is fed to an Upper Sideband Filter. Thus, all SSB signals start as Upper Sideband.
From here the USB is fed to the Mixer (BM#2) and combined with the fixed crystal oscillator signal operating at either 12.4 MHz or 6 MHz. This is where the magic happens.
In the case where the Mixer oscillator frequency is at 12.4 MHz a subtractive mix is 12.4 - 3.2 = 9.2 MHz and the sideband is inverted. The signal now passes to a 9.2 MHz IF stage and is LSB.
In the case where the Mixer LO is at 6 MHz then the mix is additive so 3.2 + 6 = 9.2 and the sideband is not inverted thus we have a 9.2 MHz USB signals passing through to the IF stage (ALC).
Two more frequency translations occur with the VFO signal and a frequency translator operating in the 40 to 60 MHz range. The ultimate result was all band coverage (80-10M) with selectable sideband. This was leading edge technology, with vacuum tubes in the 1960's.
I don't think many ham shacks had a B&W 6100 and in time B&W folded their tent.
Most Crystal Filters for commercial radios are not USB only filters. However, I know that in the Drake Lines and in some of the oddball Swan gear they have separate USB and LSB Crystal Filters. Some of the homebrew filter topologies favor LSB and so this might be a trick to use with those filters.
Things I never knew until today. But let us not pass too quickly over what was just presented. Looking at it today, there were some mighty smart designers here in the USA during that time. (Proudly I state that B&W was located in Bristol, PA.)
There is a reason why at that time so much leading-edge stuff originated here in the USA, and it is pure and simple. Our hobby had a strong technical grounding unlike today's 45 Minute Extra Class License awardees. Homegrown in the USA technical innovations in our hobby has been surrendered to the offshore manufacturers. You need only look at the top of Rob Sherwood's receiver test data list, and the radios are from Yaesu and Icom. A tip of the cap to the ARRL. You own it!
Them that know, now don't care if you know.
73's
Pete N6QW
