VFO frequencies

[David, K3TUE]

In looking at VFO's I have come up with some questions.

It seems that some transmitters utilize a single VFO frequency range and mix in a second frequency to make the desired frequency.

It seems that other transmitters, however (eg. Johnson Viking I), use a few different frequencies which they either use directly, or multiply 2 or 3 times or perhaps they are just filtering for the second or third harmonic.
Since transmitters like a Johnson Viking I seem to have a continuous-tune final I suspect that all the band switch does is determine how the VFO is modified for use.

If they are multiplying, then I imagine the band switch is selecting the appropriate multiplication factor of the VFO based on how the matching VFO is designed (eg. 80m is either directly generated by the VFO or multiplied by 2 from the same range as that used for 160m).

If they are filtering for harmonics it's probably as above, just with selected LC filters for desired harmonics.

Which is it?  Or does it depend on the radio?

[w3jn]

You're right about how both work.

And which scheme is which depends upon the radio.  Multiband SSB rigs are almost always heterodyne (ie single range VFO mixed with a low level SSB IF signal, and mixed again for the final band).  This is because you can't multiply a SSB signal without wrecking it (doublers generally run Class C, a class A or B amplifier won't have enough harmonics to efficiently multiply).

Most xmitters with a class C final use a VFO of one or two ranges then multiply up, because you're not mudulating the signal until the final.  You can distort the signal 9 ways to sunday before it hits the final and it won't matter.

[W2JBL]

   this is also done for reasons of stability and freedom from weird nuetralization problems- if you run the VFO at the operating frequency you can run more risk of RF getting back into the VFO. this can lead to FM'ing of it's output, or the whole thing just taking off by itself and losing VFO control. this is a set of design standards that were pretty much set in stone by the late 30's. if you look at the rigs like the ART-13 and 32V2 their oscillators run at least one octave below the operating frequency on all bands. the result is a very stable transmitter. on the opposite end of the equation take for example the ARC-5 which is a Hartley directly driving the PA at the frequency of operation. as the load on the oscillator varies under modulation the frequency pulls, resulting in FM. in general practice if you run the VFO at the operating freq you need to have at least one stage isolating the VFO from the PA. in most hammy type rigs by the time you get to 10 meters you are multiplying the VFO by four to get there. this leads to another set of stability problems- drift and pulling get multiplied with each added stage of multiplication. in practice the Viking II and many other rigs run the VFO on 160, multiplying to to 75 or 40, and run on 40 for 20 through 10. the result is a reasonable comprimise that works well even with crude VFO's like the Johnson 122 or Heath VF-1.     

[k4kyv]

I have never had a problem running the VFO directly on freq on 160.  A weakness with rigs like the Johnson Ranger, Valiant, I, II and DX-100 is that the VFO does run straight through on 40m.  Some of these rigs are hopelessly unstable on that band.

There is no problem with harmonics in class-C stages because the tank circuit Q is sufficient to suppress harmonics to a safe level.  A little more care is required at the final output tank circuits to make sure the signal is harmonic-free.  A separate antenna tuner usually takes care of any harmonic problems.