[sdiy] PWM noise in analog ground in Optigan-like system

[Tom Wiltshire]

On 29 Mar 2010, at 17:50, cheater cheater wrote:

>> Could you try an oblique strategy, like change the PWM frequency  
>> to be
>> outside the audio range? If you could get it up to 30KHz, all your  
>> problems
>> might magically disappear. That'd be my (typically hacky) approach!
>
> I think this would still generate sub-harmonics

Please explain. Harmonics above, for sure (switching on/off is a  
pulse/square wave, therefore lots of third, maybe second, harmonic),  
but below? How come?

Any lowpass filtering you do (caps on the rails, ferrite beads,  
whatever) is going to be *more* effective the higher the PWM  
frequency. If that frequency is also outside the range that you can  
hear, any remaining noise won't be perceptible.

Thanks,
Tom