I find this interesting mostly because it stretches my head a bit on how to generate SSB.
Traditionally, we amplitude modulate and then filter out all but one sideband,
(unless you're old enough to remember the B&W 2Q4).
This results in a roughly 3khz wide band of RF signals, all at different amplitudes.
That's primarily a frequency domain approach.
Guido reports that just nudging around a PLL oscillator without any amplitude variation
results in an intelligible SSB signal. (Though Allison mentions that the lack of
amplitude variations will result in undesired artifacts.) Seems halfway between
a frequency domain approach and a time domain approach.
Weird, but more surprising to me is that a $10 8bit RTL SDR can capture a couple mhz of spectrum.
So could easily capture all of 40 meters during a contest, and from that data resolve all those signals simultaneously.
You could run that backwards, and transmit all of 40 meters using a single 8 bit DAC.
(Don't try this at home!). That's a time domain approach.
The QCX provides a way for Guido to shape the envelope through the key shaping integrator at Q6,
though I can imagine there are better ways to do this. Many of the cheap STM32F ARM processors
provide a suitable 12 bit DAC that could be made use of for this. And the 12 bit ADC's would
be better at listening to the microphone as well.
That the $1 Si5351 is responsive enough to get usable results is quite a surprise, the only real bottleneck
there is the I2C interface. I wonder if we can go faster than 800khz, perhaps by ignoring the ACK
coming back from the Si5351? Erik mentions that Guido's code does not do burst writes, so we could
pick up a factor of 2 or 3 there while holding it at 800khz. In short, this is a very interesting demo
platform, but there's plenty of room for improvement if signal purity is lacking.
If I2C proves too slow for a clean signal, the $14 Si5340 has an SPI interface that would allow
further experimentation. I'm sure there are other PLL devices that could be considered.
The ARM chip plus Si5351 cost a total of about $3, the remainder is a CW transmitter with "key shaping"
driven from the ARM's DAC. The result could be both cheaper and more power efficient than any other scheme
for generating an SSB signal.