On Fri, 14 Oct 2005 10:50:03 -0700, Tim Wescott
wrote:


bandpass filter for CW. This is _not_ the kind of thing you could do
with FM.

Indeed Tim. The complications as I see it in predicting the SINAD for
FM when you know the receiver NF is that most narrow band
communications applications of FM are closer to PM because of the
pre-emphasis characteristic.

FM + 6dB/octave preemphasis over the entire modulation passband is PM.
(PM is where the modulation index (dev/fm) is independent of fm (the
modulating frequency)).

However, the receivers in my experience are over de-emphasised (at the
top end) presumably to get better SINAD. In fact, I think
specifications of the de-emphasis curve are commonly stated along the
lines of +1 to -3dB of -6dB demphasis relative to 1Khz over 300 to
3000Hz. This accomodates a over de-emphasis at the high end for little
loss in intelligibility and a dB or so improvement in sensitivity
figures.

Filter / demodulators and CTCSS IM also contribute to distortion
products significantly.

The result of demod distortion, PM with a slope across the passband,
and uncertain high pass filtering to accomodate CTCSS makes prediction
of S/N out from C/N in a bit of a guess in FM comms receivers, not
nearly as accurate as you suggest for linear receivers.

Owen
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