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...)

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

"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! :-)

NoSPAM wrote:


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.

Thanks Barry, I get it. I was becoming distracted by some of the old
1930s articles touting the tritet osc for its ability to create more
harmonic output (true) for directly driving following stages.

I suppose thats just another way of reaching the same goal.

(btw, haven't heard from you in years - I recall your great input over
on r.a.r. +p. No, it hasn't changed )

-Bill

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

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

On Nov 21, 8:24*pm, "NoSPAM" wrote:
"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! *:-)

Wow, there's a name I haven't seen for a while. I must be frequenting
the wrong groups. I was just thinking about you a couple days ago.
Hi Barry!

More about what Barry wrote: in the limit as the conduction angle
goes to zero and you generate a very narrow pulse of current, the
harmonics end up all the same amplitude. That's for an impulse of
zero width. Unfortunately, given limited amplitude of the current in
that very narrow pulse, the total energy becomes small. As you widen
the pulse, you'll see that the "comb" of harmonics no longer has
constant amplitude, but the amplitudes as you go up the "comb" (higher
in frequency)drop, and there will be a frequency at which they go to
zero, and then increase again (and go to zero again, and increase
again). The magnitudes follow a sin(x)/x shape, for perfectly
rectangular pulses. This becomes interesting for a the design of
frequency multiplier stages: if for example you want to get x4 out of
a stage you better NOT run it at a conduction angle that results in
nulling of the fourth harmonic! I think I was bit by this a time or
two in my youth when I didn't understand this. (I'm working on
something right now where I want that comb of harmonics to be all very
nearly equal amplitude up to about 100MHz, and that tells me how
narrow the pulse must be.)

Cheers,
Tom

On Nov 21, 9:30*pm, 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...)

You're right on the trail. Most oscillator circuits are operating deep
in class C. The exceptions are called "marginal, doesn't always start"
oscillators :-).

Some oscillators make the crystal operate on an overtone. An overtone
is often very very close to a harmonic. In this case the LC tank
chooses the overtone where gain is going to be greater than one.
Overtones close to odd harmonics are usually much more active in the
crystal.

Other oscillators make the crystal operate on the fundamental, and the
output picks off the harmonic. This is where the electron coupled
oscillator shines.

It's possible to have the crystal operate on the overtone, and then
electron-couple to pick a harmonic of the overtone. You see this in
some 40's/50's/60's era VHF projects.

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?

Mostly creating. It doesn't hurt if there's some harmonic content at
the input. Again, for efficiency most of the power stages will be in
class C already, and if they need to multiply in a non-power stage
they'll set it up to make a lot of harmonics.

Individual stages are sometimes configured in push-pull to favor odd
harmonics over even ones, or are biased to be favorable for the
desired harmonic.

Tim.

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...)

You're wrong already, kinda.

Most of the schematics that I've seen from back then have the crystal
oscillating at it's fundamental. If the energy extracted from the
oscillator is at twice the crystal frequency it's because of harmonics
generated in the tube.

I don't have a lot of reference material to look at, but I don't think
that using a crystal's overtones to generate RF really picked up until
the 50's (it was probably done during WW-II, but I only see it put forth
as a common method starting with my '50's ARRL handbooks).

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?

You're creating the harmonic with the nonlinearity of the amplifier. A
class C stage (which is pretty much assumed for CW transmitters) is very
rich in harmonics, and the harder you drive it the higher the harmonics
go. So it's pretty easy to get one to generate considerable energy at a
harmonic frequency, which you then pick out with your tank circuit.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

Tim Wescott wrote:
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...)

You're wrong already, kinda.

Most of the schematics that I've seen from back then have the crystal
oscillating at it's fundamental. If the energy extracted from the
oscillator is at twice the crystal frequency it's because of harmonics
generated in the tube.

Ok, I'll buy that. One description I read of the tritet osc described
it as being an oscillator with inherent class c amplification, hence the
plate circuit being tuned to the desired 'harmonic' and the crystal is
indeed operating at its fundamental freq.

-Bill

On Nov 27, 1:30*am, Tim Wescott wrote:
I don't have a lot of reference material to look at, but I don't think
that using a crystal's overtones to generate RF really picked up until
the 50's (it was probably done during WW-II, but I only see it put forth
as a common method starting with my '50's ARRL handbooks).

In 1930's QST's it's not too uncommon to see neophytes warned that
crystals will often oscillate on something other than their marked
frequency. They didn't call this overtone operation, though.

BC-604's (WWII era) start with a ridiculously low crystal (400ish kHz)
frequency and multiply up but I think the reason for this is more to
do with FM deviation than anything else. ("Armstrong method"?) For
many decades, broadcast FM stations similarly started with low crystal
frequencies and multiplied up.

Tim.