I think 2.4 is a little wide and I often use the pass band tuning to
roll off highs in my Corsair II then emphasize their roll off with a DSP
audio filter or a passive speaker filter rolling off at 1800 or 2100 Hz.
The main problem with setting up for a wide filter is that the same
crystal is warped for carrier frequencies for USB, LSB, and CW. Just
three capacitors used in various combinations. The manual says after
adjusting them once for the normal filter, they need to be checked
because they do interact. So instead of setting frequencies of 9.00300,
9.000750, and 9.000000 you need something like 9.003200, 9.000550 and
8.999800 with the 2.8 Khz filter if it has the same center frequency.
That's beginning to get to the limits of warping a simple 9 MHz crystal.
For these adjustments see page 3-21 of the Corsair II manual. Then the
frequency counter will be in error 200 Hz on either sideband, in the
opposite direction. That's if the new wider filter has the same center
frequency and both sides move. I see on page 3-29 that the counter for
the digital display PRESUMES the VFO is either 9.000 or 9.003 and
there's no provisions for changing it. Changing the hardware logic may
not be a viable alternative but one could try it.
The filters are the same for the Omni and the Corsair II.
I do find some VFO harmonics leaking out to the adjacent TV and to my
VHF gear when the VFO is driving the oscillator mixer, not when its
driving the main mixer direct. There are some birdies in the receiver
from VFO harmonics finding crystal harmonics. The RF selectivity does
occasionally allow nearby 75 meter signals to leak through on other
bands. I'd like to see more shielding and more RF tuned circuits. A
metal case would be a start, leaving out the plastic end caps.
It should be practical with the original filter to set the BFO for 440
Hz offset. Just set it for 9.000440 instead of 9.000750 and hope the
interaction and range of the other capacitors allows maintaining the USB
and LSB frequencies correctly.
73, Jerry, K0CQ
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Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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