Soeren, OZ1FTU, made this observation:
>I tuned up on 20m, and was I surprised. The AL1500 produced signals up to
>350MHz!!! No wonder the neighbor had complained about TVI. Parasitics or
>not, I don't know. The broad band spectrum only exists when the amp. is
>excited. Keying the bias on/off can't produce any unwanted behavior.
>And yes, I checked the TS-680 for harmonic suppression, and the signals
>was definitely not coming from the driver, though it wasn't the best I've
>seen.
I don't want to poo-poo parasitics, but there are some fundamental
engineering points that you should be sure of before coming to that
conclusion:
Be careful about overloading the spectrum analyzer. Most of them will
immediately produce all sorts of spurs on the display if driven above 0dBM
or whatever the peak limit of the instrument is. It only takes one carrier
or harmonic to do this, and make a real mess on the screen. If the TS680
seemed fairly clean, but the second measurement (the AL1500) seemed dirty,
be sure that the levels from your sampling device are near the same, so
that you don't have a different condition (overload that is) for the S/A.
When looking at harmonics of radio transmitters for FCC type acceptance or
type notification, one uses a notch filter on the fundamental, to knock it
down 30 dB or so, without affecting the harmonic levels. This gains dynamic
range for the spectrum analysis, otherwise the fundamental would overload
it and create even fictitous levels of both harmonic and nonharmonic
components.
Were these "signals to 350 MHZ" harmonically related? or were they
intermod? or parasites? A look at their individual frequencies would help
detemine which. Harmonics would be a different thing entirely, as far as
treatment goes.
Finally, keying or pulsing an RF signal creates a broadband spectrum of
(sinx/x like?) sidebands around the fundamental and the harmonics. It is
one of the problems with pulsed RF systems, trying to determine what a
carrier looks like (stability, parasites) with a spectrum analyzer. We see
it all the time with our RF plant. I don't know the rep rate or duty factor
of your keyer, but something to keep in mind. Maybe others who use fast
pulsers care to comment. The experience I am talking about is with 60-100
Hz rep rates and millisecond pulses or less. What is the spectrum like from
these electronic pulsers that hams use?
John
K5PRO
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