# [amsat-bb] FO-29 transponder block diagram

Dani EA4GPZ daniel at destevez.net
Mon Oct 30 23:00:37 UTC 2017

```El 30/10/17 a las 23:23, Robert Bruninga escribió:
> Im not following this closely, but the transponder I think is inverting.
> Thus the uplink dopper is subtracted from the downlink dopper whereas the
> beacon on the satellite is impacted by the full dopper of the speed of the
> spacecraft.  So they will not be the same.  Do the math and see if that
> comes out to the 1.26  you observed.

Hi Bob,

I'm not looking for a Doppler effect here. I've already corrected for
that using the TLEs. It's the local oscillator drift in FO-29 that I'm
trying to study.

However, I've done some math that matches my observed 1.26 within 5%
error. The transponder is indeed inverting, with uplink on 145.95MHz and
downlink on 435.85MHz. Assuming that the IF frequency f is below
145.95MHz (a 10.7MHz or 70MHz IF would be typical), there are two
possibilities:

a) The first LO runs at 145.95 - f and the second LO runs at 435.85 + f
b) The first LO runs at 145.95 + f and the second LO runs at 435.85 - f

In both cases, the total frequency shift through the linear transponder
is 145.95 + 435.85. Assuming that both LOs are derived from the same
oscillator (or that otherwise drift similarly), then the drift in the
transponder would be proportional to 145.95 + 435.85, whereas the drift
in the beacon would be proportional to 435.85. Now the quotient

(145.95 + 435.85)/435.85 = 1.33

matches the 1.26 I've measured within 5% error.

So for me this explains completely the effect I'm seeing, but it would
be good to confirm it with the block diagram.

73,

Dani.
```