Dave Tweed wrote:
> If the hardware PWM doesn't give you the resolution you need,
> couldn't you use delta-sigma techniques to keep track of the
> accumulated error, and adjust the width of future pulses based on
> that? Perhaps a bit of dithering (pseudorandom noise) could be added
> to help hide any spurious tones generated thereby.

You can dither the PWM duty cycle and get better duty cycle resolution, but
note that this introduces additional frequencies below the main PWM
frequency.  For example, if you are dithering between two duty cycle values
in successive cycles, then you introduce noise components at half the PWM
frequency.  Whether that matters or not depends on the application, but it'=
s
important to keep in mind the extra resolution isn't free.  Of course makin=
g
the PWM frequency half and using a fixed duty cycle makes the power in the
lower PWM frequency much higher.

The problem with delta-sigma and accumulated error techniques in general is
that you can't guarantee a lower frequency bound of the PWM noise part of
the signal, which in turn means it's hard to not have it interpreted as the
real signal.  Again, there are ways to mitigate this, but it has to be kept
in mind.


********************************************************************
Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products
(978) 742-9014.  Gold level PIC consultants since 2000.
--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.