> Perhaps ALL Omni-VI can be fixed that way. E.g. crank in more
>shift until they are clean. TRY IT!
Yes, technically this works, but if the CW transmit offset is not fixed at
the design frequency of 9,000.400 kHz, the frequency display will not track
accurately. The Omni Six is capable of agile CW offsets in the range of
400 Hz to 990 Hz. Ten-Tec's choice of 9,000.400 coincides with the 400 Hz
lower agile offset. Other offset choices are then determined through
microprocessor control. This is where the Omni V and Omni Six part ways.
The Omni V's offset is fixed at 600Hz. The Omni Six must be capable of
shifting from 400 Hz to 990 Hz. The convenience of this feature is offset
by the problem of having to choose a lower limit fixed offset of 400 Hz;
square on the knee of the 2.4 kHz filter. Additionally, when an offset
OTHER than 400 Hz is chosen, the chirp effect can be magnified even more.
Want proof? Try this: listen to the output of the Omni Six while operating
SPLIT. To start, try an exaggerated offset of 100 kHz or so between VFOs A
and B. Listen on A, transmit on B. Now, listen to the CW note on an
external receiver. To further magnify the effect, try an offset of 200 kHz.
True, one does not generally use offsets so far apart except under unusual
circumstances. This is probably a worst-case scenario. Again, the Omni V
outperforms the Omni Six when using large offsets. Why? The Omni V
introduces more T/R delay to allow the synthesizer to settle. The Omni V is
capable of very large offsets before any chirp is heard that is caused
solely by the synthesizer's settling effect. Operating the Omni V with
large offsets takes some getting used to: it throws off my CW timing because
of the additional T/R delay. The Omni Six is not affected by this, but the
end result is synthesizer-generated chirp from inadequate synthesizer
settling time. Bottom line: It's a trade-off between synthesizer-generated
chirp and additional T/R delay.
To summarize:
-The Omni Six's best CW performance occurs with a 400 Hz CW offset in the
Simplex mode. Synthesizer-generated chirp (not caused by the BFO chirp),
becomes more pronounced at large offsets and splits.
- Changing the CW BFO frequency will move the BFO carrier into a safe
portion of the 2.4 kHz filter passband, but dial accuracy suffers by the
Delta of the resulting offset from 9,000.400 kHz. There's no other
reasonable way to compensate for this without rewriting the control
software.
- Newer generation Omni Sixes may be more susceptible to the BFO-generated
chirp. Ten-Tec redesigned the 9 MHz BFO board somewhere in production. The
concept is great, only the choice of the LSB crystal should be moved from
9,000.000 kHz to 9,000.500 kHz, allowing for a 100 Hz negative CW BFO shift
rather than a 400 Hz positive shift. I made this change, and BFO-generated
chirp is now undetectable.
-Paul, W9AC
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