[amsat-bb] FM birds

[Jim Sanford]

Tracking doppler on 2m and 70cm was hard enough at field day.  I like 
2GHz and up, but do not want to even think about the doppler.

In the KITSAT days, I used an IC-751 as the receive IF.  It has a true 
FM detector, with discriminator centered at 0-volts.  It steps in 1 or 
10 Hz increments, even on FM.  I wrote a simple radio-control program 
which would take data from InstantTrack, calculate frequency from that, 
and set the radio on the expected frequency at the beginning of the 
pass.  Didn't matter if there was a little error, once AOS happened, the 
AFC put the receiver dead-on, and I got good data for the entire pass -- 
hands free.  It was a joy to watch.  And that was a simple digital mode.

The other thing that is exciting about going digital, is the idea of 
CAT-5 (or fiber) to the box at the antenna, not coax.

Digital will enable so much . . . .

73,
Jim
wb4gcs at amsat.org

On 7/27/2014 1:24 AM, Phil Karn wrote:
> On 07/22/2014 07:49 AM, wa4hfn at comcast.net wrote:
>
>> SSB/CW birds are the only way to go and if you build it they will
>> come  .
> Think about what it would be like to tune for Doppler while working SSB
> through a LEO satellite on microwave.
>
> Before you say "computer control", work out the effect of even a tiny
> error in your clock or orbital elements on a high elevation pass and
> remember that SSB has no carrier or pilot tone for closed-loop frequency
> tracking.
>
> Sure, you could have a second receiver track the satellite's beacon. But
> if you're going to all that trouble, why not just have it track a
> single high speed data stream from the satellite, one that's much less
> affected by Doppler in the first place?
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