On Sun, 14 Dec 2008, NoSPAM wrote:

Date: Sun, 14 Dec 2008 23:27:11 -0500
From: NoSPAM
Newsgroups: rec.radio.amateur.homebrew
Followup-To: rec.radio.amateur.homebrew
Subject: Doubling

"Bill Janssen" wrote in message
...
NoSPAM wrote:
"Telstar Electronics"
wrote in message
...
On Nov 22, 8:43 pm, Stray Dog
wrote:
? 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.

Huh? No way... you MUST have non-linearities to make a doubler.
Actually you do not need any nonlinearity to make a doubler (quadrupler,
etc.).
Assume you have two Class B (or AB) stages that are driven in push-pull.
The outputs are connected in parallel. And to make things even more
linear, let each stage have a resistive load. Each stage will produce a
linearly amplified (but inverted) version of the input signal FOR THE
POSITIVE HALF of the driving waveform only. Being driven 180 degrees out
of phase with the input signal, the second stage will produce a linearly
amplified but (again inverted) version of the input signal FOR THE
NEGATIVE HALF of the driving waveform. Both outputs will have a DC offset
of the plate (collector, drain) voltage.
Class B or even Class AB in the circuit you described are non-linear. Try
that circuit
with Class A biasing.

Bill K7NOM


All that is really required is that the active devices have a different gain
with positive input signals than with negative input signals. This is easily
achieved with Class B and Class AB stages. As long as both stages are
identical the fundamental and odd order harmonics will cancel. You are
correct that with two Class A stages where the gain is identical for either
polarity of input, the output signal will perfectly cancel. To make the
method work here, you could synchronously switch the input signal between two
perfectly linear stages. My point was that a full-wave rectified signal
contains only even order harmonics.

In the real world, as Stray Dog pointed out, ALL amplifier stages are
nonlinear to some degree. The reason that Class AB and B amplifiers are
considered linear RF amplifiers is that the tuned circuit on the output
supplies supplies the "missing half" of the waveform. Without the tuned
circuit, harmonics of the 2nd, 4th, 6th, etc. order as well as the
fundamental are present. Odd order harmonics are only found if the gain is
nonlinear for positive input signals. The tuned output stage passes the
fundamental and suppresses the harmonics.

Thanks for pointing this out, Bill.

73, Barry WA4VZQ


I'll just add a footnote. When I actually built a few "buffer" amplifiers
(tube jobs, 12BY7s, 6AG7s, etc), and for the hell of it, hooked up my
scope (an old Tektronix solid state scope with one microsecond/div
timebase, max) and actually looked at the sine wave (it looked 'nice' by
the way) and then tuned the air variable through both the fundamental or
the second harmonic (and I'm talking about 2-3 mHz signal source), I was
amazed to be able to easily see the extra "peaks" come out of the
"valleys" of the fundamental and I'm running these tubes at zero bias, low
plate voltage, too. Look in the tube manuals for any class C tube and they
talk about -50 to -70 v, grid negative wrt cathode. Class B and below talk
about negative bias much lower but still pretty negative.

Like I said, I was surprised. This _should_ be discussed in the ARRL
handbooks (maybe it is, but I couldn't find it [maybe I didn't look hard
enough?]) and it would be worth 1-2 pages to show everyone what these
signals have in them.

Here is another goodie (true story). R-390 local oscillator (runs 2.4 to
3.4 mHz, single 6BA6 tube). Had it set to about 3 mHz and looking at that
"nice" (I have no harmonic meter to measure distortion) sine wave on the
scope, and I "loaded down" the oscillator output lead with a tuned circuit
and tuned that circuit to about 6 mHz. Guess what? Got double the number
of peaks on the scope, just as with the linear amplifier. All calculate
out on peaks vs time base divisions. So? Does anyone want to suggest that
having the output LC circuit of an LC free-running oscillator tuned to
double the frequency of the LC circuit is making it "oscillate" on its
second overtone? ;-)

Yeah, I checked resonant frequencies with a GDO on all this stuff, too.
I'm not making any of this up.

For the record, I also have an old Knight Kit RF oscillator (100Kc to 400
mHz on 3rd harmonic) and put that into my scope and the waveform looks
like crap (but you can pick up the signal on a SW receiver set to where
the scale matches the frequency of the oscillator). And, the
shape of the crap changes from one end of the band to
the other. Also have an old HP audio oscillator (high quality stuff) and
it puts out a _very_ 'nice' sine wave no matter where in the range you set
the dial (one Hz to 200 kHz).

73 all,