[RTTY] The Problem with AFSK ...

[Joe Subich, W4TV]

With all the recent talk that AFSK is cleaner than FSK, this morning
on 12 meters I watched a classic example of the problems with using
AFSK and the reason I would rather put up with the "hard" keying of
some FSK implementations rather than supposedly "cleaner" AFSK.

I started tuning the band and noticed what initially sounded like fast
keyboard CW but it had a familiar RTTY cadence.  When I could not make
any sense of the "CW" I turned to the Elecraft P3 to look more closely.
Then I noticed the "CW" signal was appearing and disappearing at the
same time as an RTTY signal which I decoded as a YV4.  I then watched
as the YV4 clicked up and down his waterfall ... as he moved down in
frequency the "CW" signal began to show both tones - when he went up
in frequency the "CW" signal (now obviously OOK space) would disappear
as the harmonics of his AFSK input reached the upper skirt of the IF
filter in his rig.

This behavior is all to typical of AFSK signals where the audio is
applied to the mic input at such a level that it overdrives the fixed
gain mic preamp *or* cases where the soundcard output is run wide open
and the ADC is clipping.  There is another possibility ... devices
that hang a diode on the sound card output to drive a "PTT" circuit
based on the presence of tone may also introduce clipping if the
diode is not carefully isolated either by operating on the "other"
output channel or by using a buffer amplifier.

Yes, a properly filtered AFSK generator may be cleaner at the source
but that does not account for the 3KHz+ wide phase noise (synthesizer)
platform at -20 to - 30dB on many rigs in SSB mode, it does not account
for hum and digital noise at -40 dB on many "inexpensive" commercial
and home made interfaces, it does not account for common mode RFI
(feedback) on many AFSK interfaces, and it does not account for the
discrete "off channel" interference from rigs with carrier leakage,
poor opposite sideband suppression and audio harmonics due to overdrive
or improper set-up.

-- 

73,

    ... Joe, W4TV



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