[TenTec] OMNI-VI problem
Dr. Gerald N. Johnson
geraldj at weather.net
Mon Oct 5 09:18:03 PDT 2009
On Mon, 2009-10-05 at 09:35 -0400, K8mp at aol.com wrote:
> The Omni VI has a 5.0 - 5.5 MHz synthesizer (actually it goes from
> about 4.97 to 5.53 MHz) which is heterodyne converted, mixing with the
> band select crystal oscillator, to become the local oscillator. The
> frequency readout is not based on a frequency count of any of the
> components that produce the LO, but rather is a based what the
> synthesizer is being told to generate by the microprocessor, along with
> some arithmetic which includes the crystal oscillator frequency and the
> BFO frequency.
Actually the synthesizer is up in the 450 to 500 MHz region for better
phase noise and then divided down to 5 to 5.5 MHz where its used alone
for 80 and 20 and mixed with a crystal for the other bands.
Loss of either oscillator will cause a dead receiver.
The 160m crystal is 5.8 MHz. Add 1550 that's 7350, quite a ways from the
9 MHz IF, but twice 5.8 is 11.6 subtract 1550, that's 10.050, still
outside the filter. 5.8 + twice 1550 is 8.900 MHz. Still a supersonic
beat note. Twice a 1600 MHz BC station would mix with the 5.8 MHz
crystal to pass through the IF at 9 MHz.
With the oscillator mixer output tuned to 10.8 MHz there will be more
crystal second harmonic than fundamental with no VFO drive and that will
be at 11.6 MHz. But other possibilities are possible if the BC signal is
I'd concentrate on the synthesizer.
The mixer crystals are not in an oven.
73, Jerry, K0CQ
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