On Wed, 11 Mar 2009 11:55:11 -0700, K7ITM wrote:

On Mar 11, 6:41Â*am, Tim Shoppa wrote:
Class C operation seems very well suited to HF CW operation. At least
if the resulting envelope looks "nice" by ARRL handbook definition of
keyclick reduction, I guess.

But is it good enough for RTTY?

How about for PSK-31? Olivia?

What I've got is an old Eico 720 (Novice 75W-class transmitter biased
for deep class C, for those too young to remember) that I currently use
for CW, but would like to try out for RTTY or maybe PSK-31 by feeding
appropriate waveforms into the "VFO in" jack. My gut feeling is that
RTTY would be OK but don't really understand how class C will mess up
the PSK-31 waveform or introduce intermod that will make the resulting
spectrum broader than necessary.

The Class C stages will clearly fail the usual SSB two-tone testing but
what is a more appropriate test to say that it's good enough - or not -
for PSK-31?

Tim N3QE

PSK-31 is just slow phase-shift keying, no? If the 720 had very narrow
tuned circuits in it, it could be a problem preserving the fidelity of
really fast phase-shift keying, but for the speeds involved, a very tiny
percentage of the bandwidth of the transmitter, there simply shouldn't
be any problem. I suppose there could be an issue if the transmitter
tries to self-oscillate, but if it does, you've got bigger problems than
PSK-31 transmission!

PSK31 uses a raised cosine amplitude weighted amplitude function in order
to contain the occupied bandwidth. Use of a class C amplifier disrupts
the weighting and causes the bandwidth to spread considerably. The
demodulator will still work fairly well but the spectral spreading is
considered un-neighborly!

bart
wb6hqk