On Fri, 21 Nov 2008, exray wrote:

Date: Fri, 21 Nov 2008 22:30:24 -0400
From: exray
Newsgroups: rec.radio.amateur.homebrew
Subject: Doubling

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...)

I think this is correct, but the books say that tuning the output of the
oscillator can "pull" the frequency of the oscillating crystal. I have
sometimes seen this.

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?

Despite what at least one other person responding to this said, I can rest
assure you that if you run a doubler/multiplier stage even in a linear
mode, AND if you tune the output of that stage to the multiple harmonic,
you will definitely get output at that harmonic frequency which is
stronger than the input drive voltage. In the last few years I have built
many tube stages and observed the harmonic voltage output on a wideband
oscilloscope. As a matter of fact if you ever get a wideband scope and
look at the locked output waveform as you tune through the both the
fundamental and the harmonic frequency you will be very surprised at what
you will see. All of the descriptions in all of the handbooks I have read
(a few) explain this from a theoretical perspective but don't bother to
actually show, with photographs of actual scope traces, how this works.
It would just take an extra page or two and would make people think about
what they are doing.

All amplifiers have some non-linearity, the question is what effect this
has on you meeting "purity" of emissions requirements. The more important
question is whether you are getting the gain/drive that you want from a
given stage of amplification. Reducing unwanted spurious emissions might
require more tuned circuits or measurement using a receive with an S-meter
and operated many wavelengths from your antenna. Most "appliance
operators" just buy a commercial rig and don't worry about anything;
homebrewers might not worry either if their signals go through a tuned
circuit, an antenna tuner, and an antenna for a narrow frequency range.

If you really want to blow your mind, then hook an oscilloscope to the
output of a mixer with two low harmonic content input sine waves to be
mixed. The raw output will look like hell on a scope. The only way to see
the mixed (say, difference) frequency will be to go through at least a
couple of tuned circuits that are tuned for the wanted sine wave
frequency.

I've done this stuff. There are a couple of other minor matters that are
not quite correct in our ham handbooks, too.


TIA
-Bill WX4A