Does anyone know if continuous phase modulation (CPM) of the RF carrier will
work with PSK31 systems?

In most current embodiments of PSK, a raised cosine shaped pulse is applied
to a balanced modulator which is driven by the RF. This has the effect of
gradually lowering the amplitude of the RF envelope as the point of 180 deg
phase reversal occurs . This minimizes the number of sidebands which would
otherwise be generated if a discontinuous 180 deg carrier phase reversal
were allowed to happen during full carrier level. It also causes the
transmitted signal to have an amplitude envelope which must be preserved,
otherwise spectrum spreading will occur.

The DDS chips used in a lot of new QRP designs commonly have a programmable
phase-offset register which adds a constant phase offset to the RF carrier
generated by the chip in very small phase steps. It seems to me that a very
well-controlled, constant envelope phase modulated signal could be generated
by sequencing the phase offset register through all the values in a
raised-cosine pulse prior at the transitions of each transmitted symbol.
This would result in a very simple, bandwidth-efficient, constant-envelope
PSK31 transmitter implementation in QRP rigs that employ DDS.

Someone suggested to me that the DSP decoder systems used in most PSK31
software require an amplitude envelope in order to derive sync. However, the
DSP decoder probably implements narrow-bandpass filtering prior to the
detecion process, and this narrowband filtering of the constant-envelope PSK
signal would probably restore the envelope anyway, so it should work.

Is there a flaw in this reasoning?

Joe
W3JDR