[amsat-bb] going digital

Phil Karn karn at ka9q.net
Mon Jul 21 00:50:02 UTC 2014

On 07/20/2014 03:45 AM, John / NS1Z wrote:
> Is there some reason why a digital signal cannot be passed thru an
> analog/linear transponder? What goes in is what comes out. 

I forgot to mention that a FM repeater is not a linear transponder.
Although FM is constant envelope and a FM RF power amplifier can
therefore be made pretty efficient, it will not support a
power-efficient modulation mode like coherent BPSK.

Single channel FM is about the worst possible choice for a multiple
access satellite uplink. Not only is it analog and noncoherent, but
because it's noncoherent it has a capture effect. For a signal to come
through at all, it must capture the channel over all noise and
interference. This also severely limits the power improvement that can
be attained with forward error correction; if the demodulator is below
threshold, coding can't help you.

Depending on the demodulator design the capture ratio is somewhere
around 10 dB or slightly less, which means that the capturing signal
must be at least 10 times as strong as *all* of the interferers
combined. This makes it more or less useless unless there's only one
user (in which case it's no longer multiple access) or the users are
highly disciplined (which is hardly the case in the amateur service).


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