[GreenKeys] ARRL Handbook TU Limiter Circuit wanted

[Jim Haynes]

We have to remember, too, some things about FM limiter theory
that were mentioned in the November 1944 Sprague article in
Electronics, but didn't really take hold in the amateur
community until Don Wiggins wrote about it in RTTY in 1960.

If you have a desired signal, and a weaker interfering signal,
putting that through a limiter will improve the signal/noise
ratio, but what comes out of the limiter is an FSK signal that
has spikes in the instantaneous frequency.  If the discriminator
is linear well beyond the mark and space frequencies the spikes
will average out.  Sprague noted the need for the widely-
linear discriminator, but didn't explain the reason for it.
So all the ham discriminators were built with the discriminator
peaks at the mark and space frequencies, and the spikes fail
to average out.

There was also some (non ham) work by Elie Baghdady at MIT around
1960 with positive feedback around the limiter, the optimum being
that the circuit weakly oscillates in the absence of signal.
I built a circuit like that at the time and I think it improved
FSK reception, but I didn't do any serious testing so I don't know
if it really did.  The neat thing about the Baghdady circuit is
that if you can get it to oscillate near the mark frequency
you get mark-hold for free.

Well then we discovered two-tone operation and the limiter
discriminator method of demodulation went into the dust bin
of history.

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