[HBR] 6U8 as mixer in heterodyne VFO

[Walt Hutchens]

Ian said:

> A VFO configuration that interests me for receivers and/or transmitters is to
> mix a stable low-frequency oscillator with a higher-frequency crystal
> oscillator take the difference, and filter. For example, 2.5 - 3MHz mixed with
> a 10MHz xtal to get 7.0 - 7.5MHz.

That's certainly the way to get a solid VFO on the bands above 80M.   It
also offers a very good way to key a TX without having either backwave or
VFO drift after standby periods:  Key the xtal oscillator and bias
subsequent stages so they'll be cut off with no drive -- or use a sequential
keying system.   

The configuration I'd think about is a push-pull VFO driving a dual triode
mixer in the cathodes with a push pull crystal oscillator driving the grids.
Tie the plates of the mixer together as mixer output is single ended,
amplify from there.

I have a receiver that uses that oscillator/mixer setup -- the input to the
grids is the signal from the antenna -- and it works very well.

The advantage of that scheme is that both oscillators are rejected in the
mixer output -- that is, they cancel out in the plate circuit, meaning less
chance they (or a harmonic ...) will appear in the TX output.

I would definitely fiddle with the VFO frequency as you would still not want
a harmonic to appear in any ham band on which you'll use the VFO.

Walt KJ4KV