[TenTec] Orion Sub-Main synthesizer offsets

[shristov]

DE pfizenmayer <pfizenmayer at qwest.net>:

> 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.

Correct.


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

Correct, but irrelevant.


> 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.

Correct.


Frequency synthesis boils down to following formulas:

  nominal Main RX Freq   =   A * TCXO   +   B * Codec clock
  nominal Sub RX Freq    =   C * TCXO   +   D * Codec clock

When tuned to the same NOMINAL frequency, Main and Sub RX
employ different factors, that is A <> C and B <> D.
This is the root cause of the trouble.

Remember that Sub RX LO1 tunes only on 2.5 kHz steps,
and finer steps are achieved in DSP software.
This causes the difference between A vs. C, and B vs. D.

Suppose TCXO frequency is accurate, but the Codec clock isn't:

  actual Main RX Freq   =   A * TCXO   +   B * (Codec clock + dF)
  actual Sub RX Freq    =   C * TCXO   +   D * (Codec clock + dF)

Actual RX frequencies differ:

  actual Main RX Freq - actual Sub RX Freq  = 
      (A - C) * TCXO   +
      (B - D) * Codec clock  +
      (B - D) * dF

First two terms evaluate to zero (nominal frequencies are identical),
but the third term remains.

Frequency difference is a complex function of the nominal frequency.
It NOT necessarily the same on all bands.
In fact, it varies across the single band!


73,

Sinisa  YT1NT, VE3EA