On 2/16/11 4:58 PM, Joe Leikhim wrote:
Thanks Hal;
Transceivers A and B (C etc) would extract clock and 1 PPS from their
own GPS. The idea is to use 1 PPS (or derivative) to reset periodically.
Yes the propagation delay A-B would limit the hopping/spreading rate
unless some mechanism to correct the offset at the receiver end. (I
don't see this as big problem for the experiment).
The traditional approach for tinkering is to start in sync, and then
slow one down until you have the right delay. After all, you know that
there's a positive time delay between the two stations.
If the distances are short, you can use some sort of
pushbutton/flip-flop scheme to drop one clock pulse at a time from the
generator.
At a 1 Mchip/sec rate, a chip is about 300 meters long.
As others have commented, the usual scheme is to rig up some sort of
tracking loop. In the analog world, a popular scheme is an "early/late"
comparator, since it's easy to get two outputs from your shift register
that are one chip different. You run both of them into a pair of
correlators/mixers, and then take the two outputs and subtract one from
the other to generate your error signal to drive the VCO.
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