Barry Gross wrote:
>pure conjecture on my part - only one of the two oscillators (either
>USB or LSB) should effect cw, and the effect would probably be to
>shift the cw offset/sidetone, but I don't honestly know. I no longer
>own the Omni VI, and don't even have access to the owner's manual.
>
>
I do own an Omni VI and a manual for it, so I can answer this question.
The Omni VI has a single BFO. Only one oscillator circuit for the BFO.
The frequency it operates on is controlled by SELECTING one of two
crystals and one of three trimmer capacitors. There are a couple of
other trimmers that are always in the circuit. I emphasis the word
"select" because there is no variable control of the BFO frequencies
from outside the oscillator circuit. What the crystals and the trimmer
capacitors are set to work at is what you get, with no variable external
control. The rest of the radio has no control over it other than just to
select one of the five possible frequencies. The rest of the radio also
has no "knowledge" of what frequency trhe BFO is actually generating.
The control and logic circuitry that controls the VFO and bandswitching
oscillators "assumes" that the BFO is running at the correct specified
frequency for the particular mode the radio is in. Therefore if you
adjust your BFO frequency incorrectly, your operating frequency display
will be incorrect. Also the CW sidetone generation assumes correct BFO
frequency, so if you adjusted the BFO frequency for LSB (which is used
for LSB RX, LSB TX and CW RX) your sidetone would no longer be an
accurate representation of where your CW TX frequency is relative to
other signals in your receive bandpass.
The other oscillators in the Omni VI differ, in that the bandswitching
oscillators have analog control via a DC correction voltage. So the
control system keeps it exactly where it needs to be. The VFO of course
is under control of the logic board. Both the VFO and the bandswitching
oscillators accuracies are dependant on the reference oscillator on the
logic board.
The 15.3 MHz VCXO on the bandpass tuning board is added to and then
later subtracted from the RX IF so its net effect on frequency
calibration is zero.
To sum it up:
If you adust C16 on the BFO board in order to change your LSB TX and RX
BFO frequency relative to the 9 MHz IF filter bandpass, you will make
your frequency display incorrect in LSB TX, LSB RX and CW RX, and the CW
sidetone will no longer be accurate.
If you adjust C12 in order to change your USB TX and RX BFO, you will
only mess up the frequency display for USB TX and USB RX. You would also
have to readjust C13 and C18 if you care about your FSK TX frequencies.
By the way, there are a couple of errors in the BFO schematic in both of
my Omni VI manuals, showing C12 connected across Y1 instead of across
Y2. And D11 should be connecting C16 to Y1 instead of to Y2.
I hope this helps you make the right decision.
DE N6KB
"I before E except after C, and when sounded as A, as in neighbor and
weigh"
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