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

W3JDR wrote:
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?

No flaw in the reasoning, but some ambiguity in the assumptions. This
is something that you'd have to try and see how it works.

On the plus side, the classical method of deriving bit synchronization
in PSK isn't sensitive to the amplitude of the signal; if that is what
was used in your system then you're home free.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Posting from Google? See http://cfaj.freeshell.org/google/

"Applied Control Theory for Embedded Systems" came out in April.
See details at http://www.wescottdesign.com/actfes/actfes.html

Yes, there IS a slight error in the rational.

If you consider the phase constellation, a constant envelope implementation
of PSK will require more bandwidth. The paths from one constellation point
to another are more direct with the classical non-constant envelope
approaches. To keep the envelope constant, the constellation trajectory
must stay on the unit circle (and sometimes even pass thru other possible
symbol states). Since the constellation trajectory must go a larger
distance in the same period of time (gotta get there in one symbol period!),
more bandwidth is required. Things have gotta be able to happen faster in
the I and Q channels.

Dan

On Feb 11, 1:42 pm, "W3JDR" wrote:
Does anyone know if continuous phase modulation (CPM) of the RF carrier will
work with PSK31 systems?

(snip...)

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.

(snip...)

Is there a flaw in this reasoning?

Joe
W3JDR

The phase transitions will use bandwidth too.

We already know the power spectral density of PSK31.
What is the power spectral density of your proposal?
Numbers, please.

Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89mg pied a terre..."
ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte

The phase transitions will use bandwidth too.
What is the power spectral density of your proposal?
Numbers, please.
----------------------------------------

Well of course any phase transitions will use bandwidth, but 1800 0.1 deg
transitions spaced out over 1/2 bit-time will will use less than a single
180 deg instantaneous transition, so it should be bandwidth-conservative. I
don't have the numbers (perhaps some of our more mathematical colleagues can
comment), but I'm sure the result will be a reduced-amplitude set of
low-order sidebands traded off for additional low-amplitude higher order
sidebands. My question wasn't whether the modulation would use less
bandwidth, but whether it would 'work' at the receiving end.

Joe
W3JDR



"laura halliday" wrote in message
oups.com...
On Feb 11, 1:42 pm, "W3JDR" wrote:
Does anyone know if continuous phase modulation (CPM) of the RF carrier
will
work with PSK31 systems?

(snip...)

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.

(snip...)

Is there a flaw in this reasoning?

Joe
W3JDR

The phase transitions will use bandwidth too.

We already know the power spectral density of PSK31.
What is the power spectral density of your proposal?
Numbers, please.

Laura Halliday VE7LDH "Que les nuages soient notre
Grid: CN89mg pied a terre..."
ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte