On Wed, 11 Mar 2009 21:11:18 -0500, Bart Rowlett wrote:

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

So, PSK-31 is out.

(Unless you want to get fancy and amplitude modulate to preserve the
envelope at the same time that you phase modulate the VFO input to
preserve the phase. There's almost no sane reason to try this, but it'd
be fun if it worked. Kind of a "Look how well my old iron works! (pay no
attention to the DSP behind the curtain).")

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