Hi Dave,
My Omni VI is not a plus version, and has no F1 or F2 keys. I don't
think that the + version does either, and I am guessing that you really
meant to say N-1 or N-2. The rest of my response is based on that
assumption.
The 6.3 MHz IF, which is the IF that can be shifted in frequency
using the PBT (Pass Band Tuning), is after the 9 MHz IF. If you are
using a narrow filter in both the 6.3 MHz IF and the 9 MHz IF, and you
have the PBT adjusted so the the pass bands do not coincide, there will
not be much signal getting through. Try adjusting the PBT control for
maximum signal strength or noise level.
Once you have adjusted the PBT for maximum signal or noise while
using one of the narrow 9 MHz IF filters, there will still be a small
change in overall noise level as you switch between using one of the
narrow 9 MHz filters and not.
The filters you have in the narrow 9 MHz filter slots may also not
have identical bandpass center frequencies. In that case the setting of
the PBT control needed for maximum throughput, through both the 9 MHz IF
and the 6.3 MHz IF, will not be identical for both N-1 and N-2.
There is also the issue of the jumper settings used to compensate
for the filter loss. The jumpers only give you a couple of filter loss
compensation options. Yet there are a lot more than just two values of
loss for the many available filters. For any particular 9 MHz IF filter
there is always a "best" jumper setting to use, but seldom a "perfect"
setting.
DE N6KB
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