An Inrad roofing filter mod. installed in the OMNI VI/VI+ renders the NB
function inoperative.
The reason for this is that the noise pulse is stretched in width by more than
2000 times as it
passes through the front end circuits and the narrow (either 600 Hz or 2.4 kHz)
roofing filter. For
example, a 0.5 µS noise pulse appearing at the antenna input is stretched to
several mS as it passes
through a 500 Hz wide filter inserted in front of the OEM 15 kHz roofing
filter. Since the signal
pickoff for the NB function remains after the OEM roofing filter, the NB
circuitry sees this
lengthened noise pulse, which it is not designed to process.
My own roofing filter (home brew 4-pole, 500 Hz BW, centered @ 9.000500 MHz) is
selectable from the
front panel using the PROC button. This facilitates selection of the Inrad #753
(400 Hz BW) or TT
#221 (250 Hz BW) filters with the N1/N2 buttons independent of the roofing
filter and permits use of
the NB (if needed) by deselecting the PROC.
Since I do not operate SSB, this is not a problem (for me) with the narrow
filter. If one were to
install the Inrad 2.4 kHz roofing filter mod, the Processor function would not
be disturbed with the
PROC selected, however the 2.4 kHz roofing filter would not provide optimum
performance in the CW
mode, and the NB function remains inoperative with the roofing filter mod.
selected.
BTW: The FM receive mode is also inoperative with the roofing filter mod.
because of the narrow
bandwidth preceding the FM receiver section; FSK mode is inoperative because
of the roofing
filter(s) incompatible offset(s).
Dave, K1FK
Fort Kent, ME
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