Well, I may be wrong, but the relationship between duty cycle and
fundamental harmonic of a rectangular signal should be of the sin(x)
kind ( sin(x)/x should be the relationship between the order of the
harmonic and output voltage at the harmonic frequency);
I tried once to vary the output voltage of my class D PA varying the
drive duty cycle and, to have a straight relation between the DC
level and the output, I fed the PWM comparator with a sinewave instead
of a triangular wave; the relationship between DC input and duty cycle
in this case follow an arcsin(x) law, which compensates the sin(x) between
the duty cycle and the ouput voltage.
The main problem was that, as pointed out by Johan, at low duty cycles
the output waveform was a bit "spikey" and the ouput filter was not able
to restore a pure sine wave, maybe also beacause of some feedthrough of
the gate drive signal.
Claudio, IN3OTD
Andy wrote:
Varying the drive duty cycle to change the RF level does not give the result
expected at first sight. While the DC level does vary proportional to the
duty cycle, the RF carrier, which is revealed via the second term of the
Fourier expansion of a rectangular waveform does not. Instead, there is a
SIN(X) / X component in the waveform which gives a horribly non linear
relationship between duty cycle and RF out. It may be possible to
pre-distort the drive waveform to work over a limited range of amplitudes,
but I hate to think what the lookup table would contain !
The only real way to alter the amplitude is to change the power supply to
the PA stage and a PWM modulator, ie. a switch mode PSU working open loop is
not too complicated a beast to make if it really is necessary to adjust
amplitude.
But is it really worthwhile going to these lengths just to operate PSK31.
There are several other schemes already in use with equally good signalling
capabiliies that work with constant amplitude - ie JASON, Coherent (by
VE2IQ) and a host of externally synchronised coherent PSK schemes as yet
barely tried. No one has looked at MSK yet either. That is constant
anmmplitude, and with a bit of imagination could be generated from a DDS as
well as from the ubiquitous soundcard.
Andy G4JNT
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