"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

On Dec 14, 8:22*pm, "NoSPAM" wrote:
Actually you do not need any nonlinearity to make a doubler (quadrupler, etc.).

Nonsense... please read the definition section at
http://minicircuits.com/pages/pdfs/doub9-2.pdf

"Telstar Electronics" wrote in message
...
Nonsense... please read the definition section at
http://minicircuits.com/pages/pdfs/doub9-2.pdf

You are grasping at straws trying to defend your beliefs. Their definition
only applies to their products. You should have read the entire article.
If you consider the single discontinuity of an ideal diode at the origin as
nonlinear, then you are correct. In practice, two Class AB or B
amplifiers, operated over their most linear region (each individually
producing harmonics and intermodulation over 40 dB down from the
fundamental), can be used in a push-push configuration to produce even
order harmonics. I believe "Stray Dog" and I both have shown that single
ended Class A amplifiers DO produce harmonics.

The Mini Circuits "doubler" is, I believe, essentially a full wave
rectifier using Schottky diodes. If the diodes were ideal, i.e. had no
highly non-linear region at low voltages, there would be no fundamental
output or odd order harmonics. From my earlier discussion of full wave
rectification, perfect diodes would produce the fourth harmonic 14 dB lower
than the second harmonic, and the sixth harmonic slightly over 7 dB down
from the fourth. The Minicircuits device produces at its output the
fundamental and odd order harmonics in addition to the desired even order
harmonics. It also requires a drive level of between 0 and 20 dBm. Too
low a drive and the doubling action disappears; too high a drive and the
amplitude of the higher harmonics increases (until the device burns out).

Some further research into the push-push doubler reveals that two sharp
cutoff pentodes would do a better job than triodes for this application.
Also junction field effect transistors follow square law characteristics
over a fairly wide range making them ideal in frequency doubler operation
too. It is also possible to nearly achieve ideal diode behavior with the
use of very high gain amplifiers with feedback through the diode. See the
following Intersil ap-note for details:
http://www.intersil.com/data/an/an1114.pdf.

My post of the graphs has not appeared on the "alt.binaries.ham-radio"
Usenet newsgroup, even on my nntp server which still insists that the
newsgroup exists. I'll try again using "alt.binaries.radio.misc" this
time. My thanks go to "Stray Dog" for his efforts in also experimenting
with a single ended 6C4 triode.

73, Dr. Barry L. Ornitz WA4VZQ

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.

Tim Shoppa wrote:

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.

They /might/ be suggesting that frequency can change with loading. I find
that some of the reference crystals I use are quite some way off their marked
frequency when given capacitive loading that differs from that recommended by
the manufacturers!

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.

Some of the broadcast transmitters I worked on 25 years ago used this method
for FM, and were /really/ difficult to line up! They also included complex
circuitry for the required "pre-distortion" of the audio to compensate for
the non-linear deviation you got out of a crystal oscillators. Some
manufacturers tried to overcome the distortion issue by using phase
modulation and the "right" audio curves, but these required even more stages
of multiplication!

One of my earliest jobs as a broadcast transmitter engineer was to develop a
PLL to replace the horrible multiplier chains in some of these transmitters.
I used (normally) either half or quarter frequency generation, and used the
last one or two multiplier stages. The CMOS PLL circuitry could be prone to
bizarre effects with high field strengths, so they were built in sealed
diecast boxes, and the lower frequency generation meant that the high power
output stages were unlikely to couple back into the oscillator!

Bob