In concept, it should be possible to generate SSB digitally at any frequency
and any power level. Clearly, it is possible to generate low-level SSB with
a PC sound card, but the output frequency is typically limited by the speed
at which the card can sample, digitize and process the audio. This speed
limitation caps the output at around 15-20KHz with typical sound cards.
The output power delivered to the load is purely a function of how much
voltage the D-A converter can deliver to the load. If, as you suggested, one
took the 8-bit digital signal (pre-D/A converter) amplified each bit in 8
binary-weighted 'lossless' voltage (not power) drivers, and summed the
output in a purely resistive load, then one would have a high-power, high
efficiency SSB exciter.
Here are the problems I see:
1) Speed. The A/D, DSP, and D/A have to be able to process audio samples at
twice the highest RF output frequency (2.5-3.0 times in practicality)
2) Precision: The high power weighted binary output drivers need to produce
very accurate voltages
3) Spectral purity: The output spectrum will have aliases and 'birdies'. The
number and amplitude of these is a function of the number of bits in the
digitizing process, and the accuracy of the high-power D-A at the output.
4) Complexity: Enough said about this.
A more practical way to get a similar result might be to generate low-level
SSB in a high-speed DSP, and amplify the analog result it in a class-B
amplifier whose bias is dynamically adjusted by the DSP processor in
anticipation of the instantaneous RF level. This should produce a
highly-quality signal with good overall power efficiency.
Joe
W3JDR
Check out my project page:
http://mysite.verizon.net/jdrocci/.
"The Eternal Squire" wrote in message
...
Hi,
I'm considering making a class E amplifier for single-sideband output, but
from what I
understand, modulating the plate voltage produces double-sideband. I'd
like
to
avoid that if I can.
I'm thinking of taking regular audio, digitally processing it into
single-sideband using a
soundcard, and then digitizing that to 8-bits.
I'd then create 8 class E amplifiers that put out 1/8w, 1/4w, 1/2w, 1w,
2w,
4w, 8w,
and 16 watt outputs for the same signal; modulating each one as CW in
accordance
with the audio bit.
I'd combine the outputs through a power combiner.
Questions:
1) Would this scheme be possible for creating SSB by parallel switching
of
CW
amplifiers in accordance with a digitized audio signal?
2) Would I need to create assymetrical power combiners to combine the
amplitudes
of the different signals, or could I get away with simply making a
symmetrical
8-way combiner.
Thanks in advance,
The Eternal Squire
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