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Re: Geosynchronous Satellites

on 12/26/00 9:03 AM, Hamish Moffatt at hamish@cloud.net.au wrote:

> What are "digital AM", "digital FM", "analog FM"? What do these
> terms mean, and can you give an example? You've completely lost me
> here.


When I speak of analog signals, I mean a signal that is a voice modulated
signal.  So SSB, AM or FM voice would all be analog signals.  So analog FM
is standard FM voice or other audio (music as is the case for broadcasters).

Digital forms of modulation are things such as BPSK, PSK, MSK, FSK, AFSK,
GMSK, etc.  In it you are applying a data stream to the carrier wave and
modulating that carrier wave according to the pattern.  There are many, many
additional forms of modulation that haven't even made it into ham radio: QAM
(Quadrature Amplitude Modulation - A digital AM signal), pi/4 DQPSK, etc.
In the cell phone world TDMA, GSM and CDMA all use digital forms of
modulation.  In ham radio, RUDAK on AO-40 would be used for digital
communication via the satellite.

For nearly all forms of digital modulation you need some degree of linearity
in the amplification.  I think the only one you can really get away with
using a truly saturated amplifier would be GMSK which forms the basis for
GSM.  But I'm not an expert on digital modulations and am unsure how
non-linear an amplifier could be used.  The key is that if you want to use
non-linear amplification, you must have an RF envelope of constant

For voice/audio (analog) modulated AM or SSB signals you need linear
amplification.  For CW or analog FM, you do NOT need linear amplification.

Did that help?



Jon Ogden
NA9D (ex: KE9NA)



"A life lived in fear is a life half lived."

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