[TenTec] Orion Sub-Main synthesizer offsets

[pfizenmayer]

DE K7HP

>>  I have never encontered a condition with both rx on same
>> freq, not to say it could not happen if both were general coverage . But
>> I
>> think it has to do with mathematical resolution limits in the two
>> synthesizers . They are "phase locked" in that both start from the same
>> TCXO
>> but the schemes are different.
>
DE VE3EA/YN1NT
>
> It's worse than that.
> There are TWO independent frequency references.
>
> The obvious one is the TCXO, and the other one is the codec clock.
> The ways they are combined in the main RX and in the sub RX are quite
> different,
> which causes the observed frequency differences.
>
> There is no practical way to calibrate this out permanently.
>

DE K7HP

It looks to me like the A7-81917-6 MAIN and A7-81971-7 SUB  codec use the
same oscillator at 14.36 mhz with the MAIN codec being the active oscillator
and that feeds a clock signal to the SUB.

Then the DSP clock CLKINA and CLKINB are both driven from a 32 mhz
oscillator in the CPU/DSP logic board A7-81917-1.

The only other clock oscillator I see is the CPU clock on A7-81917-1 and I
would think that would not play into this.

The only thing I have found written on how the sub rx 1st LO is derived says
"The sub receiver employs a standard PLL synthesizer, moving in 2.5-kHz
steps. Fine-tuning to 1 Hz is achieved in DSP software through complex
mixing, described further below"

This was in Doug Smith's writeup on the Orion but in his document I do not
find where he gets back to the "as described below" .

Is anyone aware of a document that I can get to study this some more ????

I have toyed with making an external third LO for the sub rx at approx
464.0625 khz that I could warp a bit to get the two receivers so they don't
beat against each other - at least for a while. But maybe there is another
approach.

I have not set up a way to measure the fractional hz offset , but roughly on 
my Orion it seems the offset is the same on all ham bands.

All the above discussion is referenced to the O565 - The OI 566 LO/mixing 
scheme has been changed in the MAIN - and that is what I have now but I 
think the princples are the same. Of course even after all this , the delays 
thru the two receivers are also different so this complicates things except 
on a continuous signal.

Thanks -- Hank K7HP