On Fri, 21 Nov 2008 22:30:24 -0400, exray
wrote:

This is a really dumb question but it dawned on me that I did not know
the correct answer.

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...)

In an overtone oscillator, the resonator actually forces the crystal
to mechanically resonate at 1/3, 1/5, 1/7, 1/9 etc. of the crystal
width. Due to the end effects, the frequency is *not* _exactly_ 3, 5,
7, 9 etc. times the fundamental frequency, but quite close. In
principle, the oscillator is producing a single frequency, the
(harmonic) mechanical resonance frequency of the crystal.

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?

The non-linearity of the stage will produce the harmonics, which are
_exact_ integer multipliers of the input frequency. Symmetrically
clipping stages generate strong odd harmonics, while asymmetric
stages create strong even harmonics. The following stages need to
filter out the desired harmonics.

So if you need exactly 30.000... MHz, you either have to use a
10.000... MHz fundamental crystal oscillator followed by a tripler
(and filtering stage) or order an _overtone_ crystal for exactly
30.000... MHz.

Running a nominally 10.000... MHz fundamental mode crystal in an
overtone oscillator tuned at 30 MHz will not produce exactly 30.000...
MHz but something quite closely, due to the end effect.

Paul OH3LWR