Typically older systems had high stability ovenized oscillators
(0.001ppm at VHF or slightly better at uhf). The newer systems are
synthesized and locked via the 1ppm pulse from a GPS. This allows
something like 0.00001ppm while three satellites are in view of the
GPS RX. Most systems are correctly setup with an offset. The offset
is based on typical maximum doppler shift of 60 mph for the frequency
band in use. This is to insure that the doppler shifted site does not
get to that 1/2 hertz type point. The frequency setup is fairly
critical for the old systems and needed alot of rechecking until the
ocsillator settled in. The newer systems can mostly be tweaked
remotely.
The audio delay is also critical. This is a pain to setup and
adjust. The delays on the newest systems are digitally controllable
and so are stable but still fussy to adjust. It ought to be
interesting for some hams to build up a system like this for enhanced
coverage. We all have alot of spare money and time to play! (Right!)
73 Ed K3SWJ
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