Satgen366  Bandpass and Bandplan   by GM4IHJ            30th March 96

For newcomers to Digital satellites there are a number of puzzles,
which do not get a mention in most Radio Amateur literature. Foremost
amongst these is the speed at which digital messages can be sent. The
fastest satellite at 9600 bits per second is only one third as fast as
the speed of a digital telephone input to Internet at 28,000bps over
old fashioned copper wire . Why are internet modems using old
fashioned connections , able to operate so much faster than our best
satellite links ? Every one knows that quartz fibre links are the way
to go for higher speeds up into the Mb/sec range, but even where old
copper wire technology is still around in rural Scotland , we can get
by on Internet at 28,000 but can only get along at 9600 via satellite.

The problem lies not in the connecting medium, but in the terminal
equipment. If your telephone exchange is very old you may be limited
to 2400 bps, and in the satellite case it is the transmitter and
receiver at either end which similarly reduces the speed at which the
link can be used, not the link itself.

For years we have used satellite equipment operating analogue single
sideband voice with a bandwidth of about 2.25 kHz. As a rough rule of
thumb, to recover a signal at a distant terminal you need to be able
to sample it at approximately 1.5 times the signal audio modulation
fundamental frequency . At 1200 bps this means a bandwidth of about
1800 Hz plus a margin for tuning error ( essential with a signal
affected by doppler shift). So the SSB equipement works reasonably at
1200 with its transmitter and receiver bandwidth of about 2.25 Khz.
But if you try sending at 2400 bps , you need a minimum bandwidth of
3.6 Khz approx which is not available in SSB equipment.

At 9600bps as used in the digital Uosats and Koreasats you need a
bandwidth of approximately 9.6 * 1.5 = 14,400 Hz minimum. Which might
at first sight seem well within the capabilities of amateur radio
Frequency modulated equipment. But there is a feature present in all
FM equipment where by the transmitter signal has its high frequencies
enhanced and the receiver de emphasises the received signal to
compensate for this, So as supplied up until recently most Amateur FM
equipment did not have linear wideband characteristics.

It was therefore necessary for radio amateurs wishing to use 9600 bps
links, to modify their equipment, bypassing the transmitter emphasis
circuits by taking the digital signal to a point near to the
transmitter varactor, and, then bypass the de emphasis circuits in the
receiver taking the digital output directly off the FM discriminator.

Recently some manufacturers of amateur radio equipment have introduced
"digital wideband " inputs and outputs on FM equipment. But it appears
that not all are doing this in a standard manner. So while a Tx made
by X may link to a receiver made by X at 9600 bps, it may not link
very well to a receiver made by Y ( which works only to a transmitter
made by Y). This latter point is important whether you plan using 9600
bps FM via satellites or via terrestrial links.