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Re: 9600 Baud modem schematic

Hi Cliff.

> Thanks Lee, I did not know it was on the AMSAT-bb.  I sent Assi to
> John's home website.  Either way, he will be able to judge that
> John's design does not use a randomizer but compensates for the DC
> offset in a different way.

Actually, it does use a randomizer/scrambler for compatibility with the
current "standard" for 9600 baud FSK communications that dictates its

It is true, however, that the design is such that the scrambler really
isn't necessary for proper operation (between two non-scrambled modems).

This is because the clock extraction circuitry and the DC-coupled FSK
audio response throughout the design essentially eliminate two major
reasons for using a randomizer in the first place.  (The third reason
being to uniformly spread the RF spectrum across the transmit channel
independent of the data (or lack thereof) being sent.)

The other two reasons are rather weak since we are transmitting
AX.25/NRZI data, not RS-232.  AX.25/NRZI provides its own measure of
frequent state transitions to enable fairly easy clock extraction
on the receiving end.  (1200 baud BPSK signals aren't randomized,
and there's no problem with clock extraction).

As far as the elimination of a DC component in the FSK stream is
concerned, a small one still exists even when passing data through
a scrambler/randomizer.

> Lee McLamb wrote:
> > Assi,
> > 
> > John Magliacane's 9600 modem design is available on the amsat web
> > site.  
> > 
> > http://www.amsat.org/amsat/articles/kd2bd/9k6modem/9k6modem.html
> > 
> > 73,
> > Lee-KU4OS


73, de John, KD2BD

Visit John on the Web at:

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