Scott,
In addition to the audio intermod with dense CW signals, I noticed a
very weak spur either side of a very strong carrier which can apparently
be changed by adjusting the SHIFT or WIDTH control. I may have said
this before so sorry if I'm redundant.. For example, a 40 over 9
carrier at 1560 khz will see a weak spur about 9.5 khz (I think) above
the carrier (when on LSB). That spur tunes "faster" than a normal
signal, and can be heard either side of its null beat note. Then, it's
frequency can also be changed by turning the shift or width controls
(about 100 hz steps). Just a wild hunch: Could both this spur and the
CW intermod problem all be related to nonlinearity in the mixer
arrangement that implements these shift and width functions? (I believe
they shift filter passbands by mixing in the IF somehow.)
Also, I read the QST review of the Icom 775DSP. Interestingly, they
made the same comment about that radio, i.e. what appeared to be audio
intermod with CW signals. But there, they thought it was due to poor
filtering of the AGC, causing the AGC line to reverse modulate early IF
stages. It's not the same radio, but sounds suspiciously like a similar
problem. (The FT-1000 should have undergone some changes in the AGC
circuit to fix an overshoot problem also mentioned by QST in it's FT1000
review.)
So here are some more theories to think about. I would be very
interested in hearing what you learn from the Yaesu guy on this one. I
may try to contact him myself as well.
Jim N4BE
sellington wrote:
>
> >This intermod is apparently happening in the IF Unit, not
> >in the front end. I can eliminate this intermod by cranking back the RF
> >Gain control, which increases AGC voltage to the IF Unit.
>
> That's possible, but I'm not sure it explains how the strength of the
> spur varies linearly with the input signal strength. Perhaps the
> AGC maintains a constant signal level at the stage generating the spur,
> but I'd have to think about it. I still think the most likely cause is
> some leakage of an oscillator harmonic into a mixer somewhere.
>
> Still waiting for K7JA to return my call.
>
> 73
>
> Scott K9MA
> sellington@ssec.wisc.edu
>
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