Dear Graham, LF Group,
Re: noise generation with class D amplifiers - this is not really a problem
with the amplifier, but has more to do with the way the signal is produced.
If you apply an FSK or PSK signal direct to a class D/E amplifier, there
shouldn't be a problem, since the phase information will be preserved.
However, modes like PSK31 also apply envelope modulation to the basic PSK
signal to reduce the bandwidth of the signal - the signal amplitude is
reduced to zero at the instants phase keying occurs, which eliminates the
"key clicks" that would otherwise happen. Most soundcard signal generation
schemes produce the phase/amplitude modulated signal directly at audio
frequency, which is then translated in frequency and amplified as neccesary.
If such a signal is applied to a switching-mode amplifier, the amplifier
will act as a hard limiter, producing a constant amplitude output from the
varying amplitude input. This creates a problem when the input signal
amplitude is near zero, since the output is then effectively limited noise
at full amplitude, as in an FM IF amplifier. This will result in rather
unpredictable noise sidebands at the TX output. This problem can be solved
by amplitude modulating the DC supply to the amplifier so that the input and
output signal amplitudes track each other, as in my EER design. You could
also get rid of the noise problem by not amplitude modulating the
soundcard-generated signal, but this would of course bring back the key
clicks problem.
A multi-tone signal applied to a switching mode PA would have the same
problem with noise where the input signal amplitude is near zero, plus
intermodulation products between the tones will be produced. As well as
making the transmitted signal "wide", the extra spectral components might
also interfere with the decoder aquiring and demodulating the signal,
depending how these algorithms work.
So really you need to have a way of amplitude modulating a high efficiency
class D/E amplifier if you want to use the more complicated types of
modulation. The designers of "soundcard modes" generally expect them to be
used with an SSB phone TX, which they are 99.9% of the time, so it is
convenient to generate the complete signal at audio. For the newer types of
transmitter, really one would like to have seperate amplitude and phase
modulation signals. This is done in most new solid-state high-efficiency
broadcast transmitters, but amateurs are lagging behind a bit here.
Cheers, Jim Moritz
73 de M0BMU
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