[GreenKeys] Vintage RTTY operating question

[Jim Haynes]

On Sat, 29 Dec 2018, Nick England wrote:

> Please forgive the newbie ignorance.....
> What is the best way to listen for RTTY in a high noise environment?

The answer is, "it depends..."

There are two ways to demodulate the FSK signal:  one is to treat it as FM
and use a limiter-discriminator, and the other (more popular now) is to
treat it as a frequency-diversity pair of on-off keyed tones.

For limiter-discriminator you want to restrict bandwidth ahead of the
limiter.  Then the discriminator needs to be linear over a considerably
wider bandwidth than the signal, and needs to be followed by a low-pass
filter.  Hardly any amateur demodulator designs work this way; usually
the discriminator peaks are set to the mark and space frequencies.

For the two-tone detector the bandwidth limitation is in the tone filters
themselves.  You want them to be no wider than necessary.

When you bring in AGC things get more complicated.  You'd really like the
AGC to work on the signals you are trying to detect and not on whatever
might be nearby in frequency.