On Tuesday 14 May 2002 10:41, Tom Rauch wrote:
> > What I am seeing is this... with the radio set for 14024.5 I tune the
> > amp and all appears normal. Except when going above about 800w a
> > strong spur suddenly appears on 21036.9, almost exactly 1.5 times the
> > transmitter frequency. The spur can be moved by tuning the 20m radio.
> > It is not moved by adjusting the tune or load controls on the amp.
> > With the amp tuned to 1500w output on 20m the spur can be turned on
> > and off with fairly small adjustments of the amp tune control (5-10
> > degrees max), it does not change over fairly large adjustments of the
> > amp load control. The onset point of the spur is not stable,
> > sometimes it doesn't start until the amp is up to almost full power,
> > sometimes when turning the tune control the start and stop points are
> > not the same. But once started the spur is very stable and its
> > frequency is not affected by the tune or load controls, not what I
> > would expect from a parasitic.
>
> What do you have plugged into the receive antenna jack on the MP,
> anything? I've had problems with transceivers oscillating when using
> separate receive antennas because of poor external antenna port
> isolation. Those problems are very power level sensitive.
>
> It's also somewhat common for solid-state PAs are unstable running
> into narrow filters. Did you try removing the filter between the amp
> and the radio?
Frequency division (producing products on 1/2f ans 3/2f etc) is not unusual
in solid state PAs, especially using bipolars. It's often self sustaining
once started rather than fleeting or transient. If the frequency doesn't
change with the amp tuning, I can't see it's a parasitic in the amp. Look for
rf feedback paths.
Steve
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