[sdiy] Square to Sine

[ASSI]

On Tuesday, April 4, 2017 2:26:30 PM CEST rburnett at richieburnett.co.uk wrote:
> I have always wondered about magic sinewaves for something like this.
> For those not familiar they are sinewaves generated digitally using PWM
> techniques but the transitions in the PWM pulse-train are carefully
> chosen using some clever mathematics to force most of the low-order
> harmonics to zero.  This *greatly* reduces how much you have to filter
> the signal in order to get a decent sinewave with nice low THD.

This is the same working principle as any delta-sigma modulator (ternary in 
this case), just with a very low oversampling.  There are quite a few papers 
that probe the fascinating relationship of various pulse trains vs. the 
resulting spectrum in order to determine or eliminate the resulting harmonics, 
you could easily keep yourself busy reading about that for a year at least.  
For audio, the solution of choice is to just oversample the heck out of it, 
chuck all the noise harmonics way up beyond audible and then put an LPF behind 
it.  For power inverters this doesn't quite work since the energy expended for 
each switching cycle will reduce the efficiency and it is generally not 
possible to switch really fast even if that wasn't a concern.  Please heed the 
note about the necessary timing precision (and that actually means down to the 
switching waveform, not just the timing of the control signal), which would be 
even more demanding a requirement if you were to attempt this for audio 
frequencies.  I guess it's still possible to do in an FPGA, but if you also 
want frequency agility (you still can't do "real" modulation since that undoes 
the zeroing of the unwanted harmonics) it gets into rather excessive clocking 
requirements.


Regards,
Achim.
-- 
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