FPGA System A modulator

[ppppenguin]

It should be possible to do a digital VSB modulator, even without using sheep. It would need a low pass filter to define the VSB curve. Or a high pass and subtraction if it turns out easier that way. I'd need to have a good think about the calculations. I know in principle how to do SSB so I suppose it could be done by a pair of SSB modulators, one for each sideband. Then add the results and also add a carrier. There might be a simpler mathematical method.

Looks like the Weaver method, previously known as the "3rd method" is the preferred route for doing digital SSB: https://en.wikipedia.org/wiki/Single-sid..._modulator My understanding is that the Weaver method avoids awkward filters (filter method) and the need for quadrature baseband signals (phasing method). Getting I and Q baseband signals is hard enough for audio, looks hideous for video. It would be an all-pass filter giving a 90 degree phase shift over a wide BW. I suspect it would be an awful lot of multipliers in a FIR*.

Incidentally, this is not the same Weaver who wrote the classic books on TV measurement.

*PS: I've just scared myself by finding this paper: https://pdfs.semanticscholar.org/b460/c6...83fc21.pdf

[ppppenguin]

Inevitably, others have explored digital VSB techniques. This paper looks less initmdating than some: https://www.mwrf.com/systems/digital-app...sb-signals

Others are behind paywalls such as: https://ieeexplore.ieee.org/document/815786

[Terry]

I've just scared myself by finding this paper: https://pdfs.semanticscholar.org/b460/c6...83fc21.pdf

Your only scared by it - I'm absolutely terrified by it!

[`FRANK.C]

Hi Jeffrey and Terry
To me RF is a black art that I know little about particularly in analogue. So I hope to keep things as simple as I can get away with. To me the MC44BS373 is a godsend.

I have done a sweep. Two photos below the first is of 40 to 70 MHz in 1 MHz steps and the second is from 1 to 70 MHz again in 1 MHz steps.
I think the first thing I should do is to fit a fully working AD9744 and a transformer for balanced to unbalanced.

Frank

[ppppenguin]

That's not too bad. Over any Band I channel that's fairly flat.

Terry, that article scared me because I've studied most of that stuff. When I was at uni back in the 1970s I would have been on top of that sort of maths and filter theory.

[`FRANK.C]

I have replaced the DAC chip with a fully working one and made a transformer for its output from a ferrite bead.
Photos below of the audio carrier modulated with a sine wave and of its trapezoid.
It looks fairly clean and linear.

Frank

[Murphyv310]

I'd be happy with that result Frank. Looks near enough spot on to me.

[Terry]

It looks fairly clean and linear.

Are you sure, Frank? I would say it looks very clean and linear ...

[ppppenguin]

Looks excellent

[`FRANK.C]

It looks good the logic appears to be keeping up with the high clock frequency.  But there is still a little noise on the carrier. I don't think that it will be a problem for audio but video may be a different mater. I haven't tried dithering yet.  I will combine audio and video next and see how that goes.

photo of unmodulated carrier below

Frank