[sdiy] Driving SSI2144 freq control with PWM from a microcontroller

[rburnett at richieburnett.co.uk]

Yes, it is not a magic bullet.

To be honest something like a well-designed elliptic filter can be quite 
effective at suppressing switching noise from a single PWM output if the 
notch in the frequency response is placed at the switching frequency.

And if you've got a couple of PWM output pins available (and can invert 
the logic sense of one of those outputs) then "unipolar PWM" is an 
effective way of achieving ripple cancellation at the quiescent 
operating point.  This is often used in PWM (switching) "Class-D" audio 
amplifiers where it is very effective at reducing ripple current at the 
switching frequency, losses, and radio frequency interference.

I won't go into the details, but you can see the typical waveforms by 
googling for phrases like "bipolar vs unipolar PWM" if interested.  
Unipolar PWM is often implemented using an H-bridge to drive a 
loudspeaker that is tied between the outputs of the two bridge legs, 
where each bridge leg is driven from its own PWM signal.  This generates 
the classic 3-level Unipolar waveform across the speaker.  But a similar 
improvement in ripple can be realised by directly combining two digital 
PWM outputs through resistors into a common filter capacitor.  At 50% 
duty ratio you get complete cancellation, and the degree of cancellation 
deteriorates as you move away from 50% with ripple falling to zero again 
by the time you get out to 0% or 100% duty ratio.

It's even possible to combine more PWM outputs to reduce the ripple 
further and force it to zero at more duty ratios.  Google polyphase or 
multi-phase PWM if interested in the gory details!

-Richie,


On 2022-11-06 13:50, Ashlyn Black wrote:
> Oh gosh I've definitely joined the right mailing list.
> 
> 
> I saw that article a while ago, thought I'd stumbled on a great bit of
> secret sauce and had been planning to use it in future projects.
> 
> However after watching that video, I did a little further
> investigation and found that the outputs of the active ripple
> cancellation topology and a passive two pole topology are not only
> comparable, they are in fact identical (at least in the idealized
> conditions of a simulator.)
> 
> Attached is two screenshots, both with 30kHz, 0% to 50% duty cycle
> transitions. The top graphs are the outputs of the active topology,
> middle is passive topology, bottom is an overlay.
> 
> ...
> 
> Sort of feels like I've been duped, ha ha. At least it never made it
> into production and now I have spare microcontroller pins that would
> have otherwise been used as complementary PWM outputs.
> 
> Thank you so much for the discussion. :3
> 
> - Ashlyn
> 
> On 6/11/22 19:37, René Schmitz wrote:
>> On 06.11.2022 01:03, Ben Gebhardt via Synth-diy wrote:
>>> Interesting discussion!
>>> 
>>> I ran across this EDN article and want to try it out when I get a 
>>> chance.  Seems like a good compromise between and RC and full blown 
>>> active filter.  Has anyone done this before for synths?  I wonder if 
>>> it’s practical for this application.
>>> 
>>> https://www.edn.com/cancel-pwm-dac-ripple-with-analog-subtraction/
>> 
>> 
>> There is a video about this from Prof. Sam Ben-Yaakov: 
>> https://www.youtube.com/watch?v=eJNuHtzOKzY
>> 
>> TL;DW: It turns out it's not adding a real advantage when compared to 
>> a passive 2-pole filter. Which doesn't need the inverter, and uses the 
>> same number of passives.
>> 
>> BTW, check out that channel if you're interested in power electronics, 
>> SMPS and the like.
>> 
>> 
>> Cheers,
>> 
>>  René
>> 
>> 
>> -- synth at schmitzbits.de
>> http://schmitzbits.de
>> 
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