I read these pages and couldn't find a reference to the f/2 discussion.
It seemed to discuss injecting energy into a resonant cavity using a charge pump
running at twice the cavity frequengy and also injecting it at other frequencies
to obtain mixing products.
Second, Steve's comment about "any thing (even noise) getting into the amplifier
is an invalid argument in that any ligitmate signal that enters the amplifier is
a mixing product and we already discussed that. If it were noise ....well noise
is random and so the harmonics and mixing products would be noise and randon as
well.
---
Ron
jeff millar wrote:
> Here's two web pages that discusses using variable capacitance to produce
> gain at f/2 as part of a larger discussion of varactor diodes and parametric
> amplifiers.. These pages specifically discusses why f/2 works.
>
> http://www.tpub.com/neets/book11/45k.htm
> http://www.tpub.com/neets/book11/45l.htm
>
> I know this doesn't directly address the question of oscillation or signals
> at f/2, but apparently that technique lived and died before the internet so
> only exists in libraries. Steve Thompson's email of 5/16 described the
> mechanism well:
>
> In linear circuits, I expect it's not possible, but few circuits are
> without
> non-linear aspects that become significant under some conditions. As
> best I
> remember the article I read, the argument goes that if anything (even
> noise,
> maybe) causes f/2 to appear in a circuit, then IMD between f/2 and f
> produces
> 3f/2, then both f and 2f with 3f/2 produce f/2 and so on, so it can
> become
> stable and self sustaining if the circumstances are right. The same
> analysis
> can apply to f/3 etc. It's very common to find it when doing load pull
> tests
> on solid state PAs.
>
> Steve
>
> I'm still looking for the proper references. Does anyone have a textbook on
> varactor multiplier theory and design?
>
> In general parametric amplifier theory uses an RF "pump" to dump energy into
> a non-linear device, which then can look like a negative resistance
> terminating a transmissionn line. And negative resistance terminations have
> the characteristic of reflecting more power than incident. Given gain and
> a circuit, you can generate an oscillator...and that oscillator can
> injection lock to the pump...thus transforming input energy to subharmonics.
>
> jeff, wa1hco
>
> ----- Original Message -----
> From: "Tom Rauch" <w8ji@contesting.com>
> To: "'Amps'" <amps@contesting.com>; "David Robbins" <k1ttt@arrl.net>;
> "Michael Tope" <W4EF@dellroy.com>
> Sent: Saturday, May 18, 2002 6:45 AM
> Subject: Re: [Amps] al-1500 parasitic?? or some other problem?
>
> > > Tom, I thought Dave said that the 21 MHz spurious signal tracked his
> > > fundamental and held a 3F/2 relationship with it all the way across
> > > the 20 meter band. I don't see how a the 14 MHz fundamental beating
> > > with a fixed frequency oscillation could do this (Dave was the spur
> > > tracking really exactly 3*F/2 WRT to the 14 MHz signal??).
> >
> > A possible clue would be to change frequencies and watch the spurious
> > frequency change.
> >
> > > Can a free running oscillator injection-lock to a pump signal near its
> > > 2nd or 3rd harmonic? If so, perhaps that would explain it? Another
> >
> > Locks at half the frequency are nearly impossible, because the
> > positive peak of the oscillating system would have to lock on both
> > positive and negative excursions of the main signal. True sub-
> > harmonics in non-linear devices are totally impossible.
> >
> > One thing that works is to understand the complexity of and the
> > special devices with special characteristics required to **
> > intentionally** make the same problem. Considering how we would do it
> > intentionally, we can guess at how likely it is to occur in an
> > accidental circuit.
> >
> > I remember in a local electronics class the teacher heard my 160-
> > meter signal on a broadcast radio at about 900kHz when I was on about
> > 1800kHz. he used it to demonstrate "sub-harmonics" to the class, and
> > the question even appeared on an exam.
> >
> > When I caught wind of it, I had him do a simple test and beat an
> > oscillator against my alleged "sub-harmonic" while tuning the radio
> > dial. The pitch of the sub-harmonic changed when he tuned the radio,
> > which it never would have done if it was a sub-harmonic.
> >
> > I explained to him it was a receiver deficiency, aggravated by the
> > 455 IF and local oscillator choice. The local oscillator was at 1355
> > when the receiver was on 900kHz. (455+900=1355) My signal was at
> > 1810. 1810-1355=455
> >
> > What worked out to be a subharmonic really was a IF image.
> >
> > The point of all this is we sometimes miss simple in-your-face
> > frequency relationships. With so many oscillators in the multiple
> > conversion receivers and transmitters, and other signals in the
> > outside world, a lot of stuff can be going on.
> >
> > A virtually impossible scenario is a f/2 unintentional divider
> > remixing, because sub-harmonics are virtually impossible if not
> > totally impossible to intentionally generate, let alone have by
> > accident.
> >
> > On the other hand accidental mixers are simple and common, easy to
> > build, and can occur many places in the outside world...but they
> > require the correct accidental frequencies.
> >
> >
> >
> >
> >
> >
> > > possibility would be some poorly shielded RF device with a frequency
> > > divider that is capturing on Dave's nearby TX signal (or one of its
> > > harmonics). The frequency divider could produce the F/2 component
> > > which in turn could beat with the 14 MHz signal and to produce the
> > > 3F/2 component. The fact that the spurious signal is bidirectional in
> > > its response to Dave's antenna suggests to me that its due to a
> > > harmonic of his signal that is created either in or before the 20
> > > meter antenna (I would expect a Yagi to look bidirectional at its
> > > harmonics, whereas it would be unidirectional at its operating
> > > frequency).
> > >
> > > Interesting problem.
> > >
> > > 73 de Mike, W4EF...........................
> > >
> > > ----- Original Message -----
> > > From: "Tom Rauch" <w8ji@contesting.com>
> > > To: "'Amps'" <amps@contesting.com>; "David Robbins" <k1ttt@arrl.net>
> > > Sent: Friday, May 17, 2002 6:45 PM Subject: RE: [Amps] al-1500
> > > parasitic?? or some other problem?
> > >
> > >
> > > >
> > > >
> > > > > Well, I can now say for sure that it is not the amp. I have found
> > > > > that by feeding only the top 6 ele 20m beam and pointing it either
> > > > > due east or west I can generate the f/2 and 3f/2 products with as
> > > > > little as 45 watts. The beam heading is not what I was expecting
> > > > > as it puts both the 40m and 15m towers off the ends of the 20m
> > > > > elements. The low power needed does really narrow it down to
> > > > > something related to that top antenna or something very close to
> > > > > it... especially since I can not generate them with 1500w to a 4
> > > > > ele 20m beam 45' below it.
> > > > >
> > > > > I can also say that its not the guy wires as the 17 year old rusty
> > > > > ones were replaced yesterday.
> > > >
> > > > Hi Dave,
> > > >
> > > > I'm positive you are wasting time looking for non-linear joints
> > > > producing a F/2 or 3F/2 product. Sub-harmonics are impossible to
> > > > produce by virtue of a non-linear system. . Look for a mixing
> > > > product with another local oscillator or signal of some sort, or a
> > > > spurious oscillation in a device with gain.
> > > > 73, Tom W8JI
> > > > W8JI@contesting.com
> > > >
> > > > _______________________________________________
> > > > Amps mailing list
> > > > Amps@contesting.com
> > > > http://lists.contesting.com/mailman/listinfo/amps
> > > >
> > >
> > >
> > >
> > > _______________________________________________
> > > Amps mailing list
> > > Amps@contesting.com
> > > http://lists.contesting.com/mailman/listinfo/amps
> > >
> >
> > 73, Tom W8JI
> > W8JI@contesting.com
> >
> > _______________________________________________
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> > http://lists.contesting.com/mailman/listinfo/amps
>
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