Dear Andy, LF Group,
I did try some experiments to do this in 2001 - the variable-phase modulator
circuit is still available from G3YXM's web pages:
http://www.wireless.org.uk/moritz.htm
It isn't particularly elegant, but it does do the job - I expect a soundcard
DSP implementation wouldn't be all that hard. I can't remember if I tried it
on the air or not - but certainly I did try some local experiments using a
low level signal summed with off-air LF noise, using the VE2IQ "coherent"
modulator/demodulator software. I seem to remember at the time that there
was little difference in "readability" between the ramped-phase BPSK signal,
and the standard envelope-shaped BPSK waveform. The spectrum was not quite
as clean as the envelope-shaped BPSK, but would have been perfectly
adequate. Perhaps it is time to revisit this technique.
The availability of DSP has meant modulation systems can be almost
arbitrarily complicated with little hardware penalty, but I agree with Andy
that other practical considerations play a great part too. One sees some
elaborate modulation scheme advanced at length, after which the inventor
triumphantly announces it offers an improvement of 0.35dB over cruder
techniques, and you start to wonder if it wouldn't just be easier to make
the antenna 100mm higher! I think the ability to use a non-linear TX with
constant amplitude signal is worth a 3 - 6dB advantage in terms of reduced
power dissipation and/or circuit complexity, e.g. a 100W linear PA probably
uses the same amount of components, heatsinks and DC power as a class D PA
of 300W.
Cheers, Jim Moritz
73 de M0BMU
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Andy
Sent: 26 November 2004 12:06
To: '[email protected]'
Subject: RE: LF: Linear amplifiers and MSK
Importance: High
....
A few years ago I remember making a few experiments using a scheme that has
rarely been used - gradual PSK - where instead of ramping from 0 to 180
degrees
directly the phase moved progressively over, say 10% of the bit period.
Seem
to remember trying it wirh the VE2IQ Coherent software, but not sure if this
was ever over teh air, or just across a bench.
Perhaps it is worth revisiting, with a soundcard implementation of PSK at,
say
1 B/s and a gradual ramp. And why not include a GPS locked timing option
too..
Andy G4JNT
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