Hi Jeff.
Thanks for you mail.
I think the topic is drifting where one fellow had a problem with a spur on his
amp at 1.5X the fundamental.
The discussion the came to suggestions that the AMP was creating a sub-harmonic
by itself....in other words , the amp was the problem.
Other suggestions were that the exciter had a spur and the amp was re-inforcing
it or mixing or tripling it.
Then we drifted to sub-harmonics and my statement was that any non-linearities
would be on the fundamental waveform and therefore would be higher in frequency
and sub is not possible in that it would be a non-linear variation from one
cycle to the next to be lower in frequency. To be an odd fraction of the
fundamental would mean that the non-linearity effects one cycle of the rf
fundamental but not the next!
In your example , you are introducing two signals and mixing and the statement
didn't realy mean to cover the mixing operation even though an amplifier can and
is used as a mixer. If you have a mixer and provide only one signal , the mixing
products do not appear.
So I think we are talking about one specific part of the discussion that is
theoretical in nature. A non-linear amplifier does not choose which rf cycle it
it will amplify linearly or non-linearly.......can you agree on this point?
All of you examples include some type of mixing operation and the center of the
discussion ruled out mixing.
The noise issue suggests that the amp is conditionally stable.
---
Ron
jeff millar wrote:
> The reference to gain at f/2 is there...near the top of the second
> reference...
>
> For proper operation, the capacitance must be decreased
> when the input voltage is maximum and increased when the
> input voltage is minimum. In other words, the pump signal
> frequency must be exactly double the frequency of the input signal.
>
> The statement makes the point that a circuit with non-linear reactance can
> have gain at f/2. The the author goes on to generalize
>
> A parametric amplifier that is not phase-sensitive, referred to as
> a NONDEGENERATIVE PARAMETRIC AMPLIFIER,
> uses a pump circuit with a frequency higher than twice the input signal.
> The higher-frequency pump signal mixes with the input signal and
> produces additional frequencies that represent both the sum and
> difference of the input signal and pump frequencies.
>
> This statement makes the point that a circuit with non-linear reactance can
> have gain at any frequencies lower than the pump frequency.
>
> Steve's point about noise helps to understand how the subharmonic gets
> started. quoting from his email...
>
> 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.
>
> This describes how any oscillation gets started. In other words, a circuit
> with gain starts out with zero signal and then a bit of noise occurs that
> gets amplified and filtered and feeds back....leading to a build up of self
> sustaining oscillation. In familiar oscillators, a filter ensures that gain
> and oscillation occurs at a single frequency.
>
> It's valid to call a parametric frequency divider a sub-harmonic generator
> because the frequency of oscillation isn't controlled in by a filter, but by
> the mixing of oscillation, harmonics of the oscillation and the pump
> frequency.
>
> Michael Tope provided a reference
> (http://smirc.stanford.edu/papers/JSSC99JUN-hamid.pdf) that shows a
> subharmonic generater using a mixer and gain. That circuit looks a lot more
> like a classic oscillator than a parametric amplifier, but the same
> principle applies.
>
> Also recently on this list, Peter Chadwick provided a reference to a book
> that covers the subject
>
> Parametric dividers can be made with a varactor: see
> Manassewitsch, Frequency Synthesisers, Theory and Design,
> second edn., p376 et seq.:
> One would expect a power dependency in the effect.
> Carrier storage devices can also be used to make a divider -
> see Manassewitsch again.
>
> jeff, wa1hco
>
> ----- Original Message -----
> From: "Ron" <w8ron@stratos.net>
> To: "jeff millar" <wa1hco@adelphia.net>
> Cc: <W8JI@contesting.com>; "'Amps'" <amps@contesting.com>
> Sent: Monday, May 20, 2002 10:43 PM
> Subject: Re: [Amps] Non-linear subharmonics was: Nonsense, AL-1500 parasitic
>
> > 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
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