It's possible to generate a 1.5x multiplier with a nonlinear C or L. I once
designed a 144 to 226 multplier with a varactor diode and some tuned
circuits. The circuit contained an idler tuned circuit at 1/2 input, which
mixed with the input to generate 1.5x. Some literature exists for non
integer varactor multiplers...but they never became popular, too touchy and
unstable.
Another way weird things can happen with "pumped" nonlinear C (or L); they
can look like a negative resistance. Parametric amplifiers use that
technique. Feed RF power into a nonlinear resonant circuit, usually a
microwave cavity with varactor, but possibly an HF tank with overloaded
ferrites...then feed the signal of interest into the resonant circuit, which
reflects back with power gain (a line terminated with negative resistance
reflects more power that applied). If you have nonlinearities in the
magnetics, it can create parametric amplification/oscillation...possibly
injection locked to the pump frequency.
Synchronous motors and generators suffer from these effects, it helped cause
the big east coast blackout in the 60's. 60 Hz is the pump frequency, the
motor magnetics have nonlinearities, and the parasitic oscillation occured
as cyclic acceleration and deceleration of the generator speeds...until all
the generators tripped off line.
The fact that this occurs at high power and with varies with SWR indicates
possible nonlinear effects in either C or L.
Do you use ferrites in the tank, maybe they're saturating?
Do you have a questionable quality ceramic C in the tank, they're
piezioelectric and can act nonlinear?
Could the 7 MHz network act like an idler when set to 14 MHz, anything
unconventional about the band switching?
jeff, wa1hco
----- Original Message -----
From: "Gary Schafer" <garyschafer@attbi.com>
To: "David Robbins" <k1ttt@arrl.net>
Cc: "'Amps'" <amps@contesting.com>
Sent: Tuesday, May 14, 2002 5:46 PM
Subject: Re: [Amps] al-1500 parasitic?? or some other problem?
>
>
> David Robbins wrote:
>
> > Ok, thanks for all the testing suggestions and comments. Below I will
> > try to answer all the questions and suggestions so far. I also find it
> > hard to believe it is produced by the amp, but am running out of other
> > causes.
> >
> > I have tracked the spur from 21000 to 21450 (tx from 14000 to 14300) it
> > is always exactly 1.5 times the tx frequency, the frequency of it is not
> > affected by the amp tuning controls. The onset point does vary with
> > load, near 14000 the swr is about 2:1 and the spur doesn't start till
> > about 1kw output, at 14300 the swr is 1.2:1 and the spur is there at
> > about 200w output from the amp but is much more sensitive to the amp
> > tune control.
> >
> > I have generated it with and without the input bandpass filter and the
> > output stubs in all possible combinations with 2 different radios.
> >
> > I unfortunately don't have an adequate dummy load right now, but even at
> > almost 1:1 swr at the best frequency for the stack the spur is still
> > there as loud and predictable as ever. (note the stack is 300' from the
> > shack, with the shack off the side of the beams, and there is no other
> > sign of rf feedback in any equipment, all grounds have been checked)
> >
> > I have swapped the ft-1000mp for a ts-870, there was no change in the
> > spur at all.
> >
> > The amp does not have a qsk board.
> >
> > There is nothing in the rx antenna jack on the ft-1000mp or the ts-870.
> >
> > I can't believe it is a design flaw as the amp functioned normally for
> > several years with no problems. And this is so repeatable now that I
> > can't believe we didn't notice it or that someone else hasn't also seen
> > it in multi station contest operations.
> >
> > David Robbins K1TTT
>
> ---------------------------------
> Dave,
>
> What are you listening to the spur on? Could it be generated in the
> receiver? When you listen to it is it a clean carrier or is there any
other
> modulation on it?
> Try turning off the computer? Any other electrical items that might have a
> 1.5 mhz oscillator in them? Local radio station at 1500kc?
>
> 73
> Gary K4FMX
>
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