"exray" wrote in message
...
In terms of old transmitters from the 20s/30s...In a crystal oscillator I
understand the concept of setting the oscillator output tank to favor the
harmonic from the crystal. (Stop me if I'm wrong already...)

But in a doubling amplifier stage am I counting on having enough harmonic
content at the input or am I creating the harmonic with the non-linearity
of the amplifier?

TIA
-Bill WX4A

Hi Bill.

Remember that single ended frequency multiplier stages are usually operated
in Class-C where the nonlinear operation of the stage produces the
harmonics. In a Class-C stage, the grid (of the tube since we are talking
about vintage transmitters) is biased such that the plate current only flows
in short pulses. The narrower the pulse width, the greater the harmonic
generation of the stage. If you look in the old RCA Transmitting Tube
Manual, there is a design procedure where the "conduction angle" of the tube
is chosen for proper harmonic generation.

Frequency doubling is unique in that two Class-B stages may be used in a
push-push arrangement. Here the grids are driven in push-pull while the
plates are connected together in parallel. The resulting waveform will
essentially be the equivalent of full-wave rectification of the input
signal. Without going into Fourier series, the resultant waveform only
contains even harmonics of the input signal while the fundamental driving
frequency is cancelled out.

Fortunately I was already a ham operator when my high school math class
taught Fourier series *. I immediately saw the practical value of this
mathematical concept and it made good sense to me. Your question is a good
one and reading some of the tutorials on Fourier series (do a Google search)
will be very useful to your understanding of harmonic generation and
intermodulation distortion. I hope that my simple explanation will start
you in your own exploration.

73, Barry L. Ornitz WA4VZQ

* More years ago that I care to admit! :-)