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). --Phil _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
