Doesn't NAR change the filter bandwidth and then the controller adjust
the BFO frequency and the main oscillator frequencies to keep the offset
from BFO to IF filter the same and the main oscillator to compensate to
keep the received zero beat frequency the same?
In a word, no.
The Omni VI has only five discreet selectable BFO frequencies and they
are selected depending on mode CW TX / LSB (TX or RX), or CW RX / USB
(TX or RX) / FSK TX space, FSK TX mark. They are not dependent on
either 9 MHz or 6.3 MHz filter selection.
If the BFO were more finely controllable, there would be no need to
produce both the 219 and 221 filters, which are both 250 Hz BW 6 pole
filters. The only difference between them is their center frequencies.
The fact that switching the NAR 9MHz filter in and out changes the
birdie frequency in Steves Omni suggests to me that that part of the IF
chain is also part of the feedback loop generating the spurious
oscillation. Or as someone else has suggested, the spur is created in
part of the digital circuitry and changes due to the added current used
by the NAR LED.
I'm sure glad mine does not have that problem. I hope Steve can find an
easy way to make it go away in his rig.
DE N6KB
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