----- Original Message -----
From: "Rick Karlquist" <richard@karlquist.com>
> It is entirely possible to generate PSK signal that has a constant
> envelope. You simply hold the amplitude constant and rotate the phase 180
> degrees, instead of holding the phase constant and ramping the amplitude
> down to zero, flipping the phase, and ramping the amplitude back up again.
> Unfortunately, the designers of PSK31 chose not to do
> this, either because they thought it couldn't be done, or for other
> reasons.
>
> Rick N6RK
While its entirely possible to do what you describe, Rick,
I think you pay a big bandwidth penalty when you slew
the phase of the carrier while it is on. I am not entirely
familiar with the PSK31 specification, but I suspect that
they are using root-raised cosine symbol shaping to
minimize the occuppied bandwidth of the signal. In
its intended application (e.g. a satellite networks where
all the signals level are about the same) BPSK works
just fine. In this sort of application, if the IMD sidebands
introduced by the system PA are down 20 to 30dB, they
will have no impact on overall system performance
since all of the adjacent signals are about the same
strength. In amateur HF applications, we have much higher
dynamic range to contend with. In this case, the bandwidth
reduction benefits of raised cosine pulse shaping get
lost to some extent by the spectral regrowth from the
AM modulated PSK signal passing through our moderately
non-linear PAs.
I am not sure why more people aren't using MFSK16.
Its reported to handle phase distortion much better and
has a constant envelope as compared to PSK31. Its
probably less bandwidth efficient than an ideal PSK31
signal, but as we have been discussing, real PSK31
signals are plagued with IMD that may make the real
bandwidth tradeoff (e.g. bandwidth at the PA output
vs. bandwidth in the textbook) more favorable for
MFSK16.
BTW, part of the solution to these problems is to
get rid of the soundcard on the modulator side and
do direct modulation of the carrier via DSP in the
exciter (software defined radios). That won't solved
the IMD problems, but it will get rid of hum, noise, and
audio harmonic distortion.
73 de Mike, W4EF...................
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