Hi Doug,
I have been poking around on Internet looking for a scholarly article
or two on generating SSB using switching techniques for the RF
generation.
There is a lot of material out there. But not many practical circuits
that we hams could simply copy.
It appears that GaN MOSFETs are the promise for generating the 2 - 30
MHz RF,
Yes, there are some good devices of this type. But they aren't the only
choice, of course.
and I found some hints on how combining two RF streams with amplitude
modulation on one, and phase modulation on the other can produce any
modulation type, but no detailed information on anyone who has
actually done it, or the math involved for that matter.
The principle behind this is that ANY signal, even the addition of many
individual signals, such as might come in from your RX antenna, can be
reduced to phase and amplitude information. Or even just to instant
voltage at a high enough sample rate.
A practical method for transmitting SSB and any other modes (one, or
several simultaneous signals), is to generate a constant amplitude RF
signal with phase modulation on it, and a separate, lower frequency
amplitude signal, then combine them. In a modern world, these two
signals are generated by Software Defined Radio techniques, but for
purposes of easier understanding you can generate them with analog
circuitry from an original low level SSB signal, by passing the signal
through a limiting amplifier (leaving only the phase modulation), and in
parallel passing the signal through an envelope demodulator (simple AM
detector). In any case, the constant amplitude, phase modulated RF
signal is then used to drive a high efficiency power amplifier, for
example one operating in class E, while the amplitude signal is used to
modulate the switching power supply of that amplifier, and thus control
the output amplitude.
This technique has several problems that result in low emitted signal
quality, unless they are fixed through various measures such as
multistage modulation, predistortion, etc. But it can produce any
desired signal, or groups of signals, at high efficiency and high power.
Google for "envelope elimination and restoration", "EER", "HELAPS", and
see the work of Karl Meinzer. What he did flew on phase 3 ham sats, many
years ago. All signals on a satellite transponder were encoded in just
one phase and one amplitude signal.
But power-supply-modulated class E amplifiers aren't the only way to
join phase and amplitude information into the final TX signal. You can,
for example, build a bridge of 4 RF power transistors, take the TX
signal from the bridge output via a bandpass filter (tank circuit), and
modulate each side of the bridge by one separate square wave having
phase modulation. These two square waves are phase-modulated in unison
with the original phase information, and they are phase-modulated
differentially with the suitably predistorted amplitude information. The
result is again a high power RF signal generated at high efficency, that
can contain one or many individual signals of any modulation type, but
this method doesn't require a modulated power supply, and thus does away
with its frequency response limitations, phase delays, etc. Instead it
requires very fast switching transistors, much faster than those for a
class E amplifier.
And if you have really fast transistors, you could build a plain simple
class D amplifier: For this you have to generate an RF signal on a
frequency several times higher than the highest TX frequency. Let's say,
300MHz for a transmitter that has to do HF+6m. This signal is
pulse-width modulated so as to encode the instant voltage of the signal
you want to transmit. This allows you to generate any desired waveform,
and that means the ability to generate any modulation method, and
unlimited numbers of simultaneous signals. This pulse-width-modulated
VHF signal is amplified by very fast switching transistors, at near 100%
eficiency, and the resulting output simply goes through one good low
pass filter. That allows you to transmit on ANY frequency from zero to
the lowpass cutoff frequency, and on any number of frequencies and bands
at the same time, if so desired. No per-band filters are needed! The
catch is the need for extremely fast RF power transistors, and I'm not
aware of any currently available transistors that could reasonably do
this job at the kilowatt level. Instead we would have to resort to
complex techniques of combining a large number of tiny microwave
transistors.
But I'm pretty convinced that this technique has a future, if not a
present. Given some development in semiconductors, the simplicity of
needing just one fixed low pass filter is a winning factor.
This just scratches the surface... There is much more!
Manfred
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http://ludens.cl
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