Mark,
After thinking about this some more I'm starting to
wonder if it is more of a IMD type of issue. I'm
thinking that the secondary weak signal is in reality
the DSP stage "demodulating" a spur generated by an
earlier stage. Again the "sliding window" design
aspect of the last (DSP) IF is probably a factor in
what you are seeing as well.
I did some playing around with a VERY strong signal
and did indeed note the 2 responses above and below
the desired signal +/- ~5.5KHz. If you use the PBT
control a on these secondary "signals" you may reach a
point were it's tone will suddenly jump, I attribute
that behavior to LO1 making one of its 2.5KHz
frequency steps. oddly enough though I only saw that
with one of the "spurs".
What I also noticed with this really strong signal
being feed to the Pegasus that it will also appear at
12 and 24 KHz away (both +/-), again I would
characterize these as IMD responses. I won't make any
guesses what the signal level was that I fed to the
Pegasus in order to see these responses because I have
no accurate way to measure it.
73,
Duane
N9DG
--- Duane Grotophorst <n9dg@yahoo.com> wrote:
> Mark,
>
> What I think you are seeing is an image response of
> the 12KHz (nominal) 3rd IF. As you may recall there
> is
> a 467KHz LO3 signal being mixed with the 455KHz 2nd
> IF
> signal to arrive at the final 12KHz (+/- 1250Hz) 3rd
> IF.
>
> I think you would have also found another at 1.8125
> if
> that frequency was otherwise quiet. If you can
> inject
> a locally generated strong signal into the RX does
> the
> offset of these "images" vary between 5.375 and
> 6.625
> KHz away by chance depending on the actual frequency
> of the main (desired) signal? If so then I would
> believe that there is also an interaction between
> LO1?s 2.5Khz tuning step size and LO4 - which is
> mathematically manipulated (tuned) inside the DSP
> algorithms.
>
> 73,
> Duane
> N9DG
>
> --- Mark Erbaugh <mark@microenh.com> wrote:
> > Tonight, I was tuning around 160 with my Pegasus.
> I
> > heard W1AW very loud, clear and steady on 1.818,
> but
> > I also heard them with a weak a fluttery (I could
> > see the audio frequency wavering on my waterfall
> > display) at 1.8235. I tried to copy them with
> > another radio, but no luck, so I am assuming that
> > this was a spur inside the Pegasus. Any idea what
> > causes it?
> >
> > 73,
> > Mark
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