You may know about this discontinued kit:
Well, in this design, one starts with a 28.322MHz crystal and divides it by two several time by means of flip-flops they could have used a 74LS93 instead and spare one IC). The FB40 reaches the frequencies 28.322 (obviously), 14.161, 7.0805 and 3.54025. All frequencies are in rather busy regions, good for QRP, not so good for QRSS.
Why not doing the same trick with the color burst harmonics? Starting with a 14.31818MHz oscillator, resulting frequencies are 7.15909 and 3.579454. One could also consider to double the frequency, which will get us to 28.63636.
But, because of all this dividing and doubling, FSK is somewhat out of the game. But, in contrast to the original FB40, the available frequencies could be amplified simultaneously, making some three band QRSS operation thinkable: three PAs, three low-passes. From a propagation standpoint, that would be a nice daytime night-time experiment, displaying the opening and closing of the respective bands.
There are cheap crystals and oscillators for 3.579545 and 14.31818, none for 40m however. The 80m and 20m frequencies have been shown suitable in earlier tests.
There is a point in the FB40 which I don't like, the oscillator is keyed. This make the design less suitable for QRSS. Also would I go for one low-pass filter for each band.
In the case I am going to build the 3 band qrss mept, that's what I am going to through together:
- 20m: oscillator 14.31818MHz, spectral filter by means of a crystal, buffer (keyed) and amplifier (74HC240), low-pass
- 40m: divide the oscillator frequency by two (7474 or 7493), spectral filtering by Ls and Cs, (keyed) buffer and amplifier (74HC240), low-pass
- 80m: divide the 40m frequency by two (7474) or use the appropriate division of a counter (7493), spectral filter with a crystal, (keyed) buffer and amplifier (74HC240), low pass