[time-nuts] Software Sawtooth correction prerequisites?

Henk ten Pierick henk at deriesp.demon.nl
Wed Aug 1 08:50:08 EDT 2007

On Aug 1, 2007, at 0:51, Dr Bruce Griffiths wrote:

> Henk
> There is no theory to show that sigma delta modulators of order higher
> than 2 are actually unconditionally stable.

Yes, but with the Gerzon-Craven theory is is possible to predict  
stability of noise shapers. My experience with Gerzon-Craven in a  
number of noise shapers is very good, and upto 11th order (never  
tried more).

> Merely simulating the device is not conclusive proof that the  
> modulator
> will never saturate, albeit infrequently.
> Most implementations include saturation detection circuitry that  
> resets
> the modulator should this occur.

We use feed-forward and graceful degradation. In this way saturated  
noise shapers recover correct. In our application overdriven noise  
shapers can not be avoided at input signal transients.

> If saturation isn't too frequent then for most purposes this is only a
> minor annoyance.
> However cascaded first order modulators as employed in the MASH
> technique are stable in theory and practice.

How to match analog circuits with digital circuits for the mash, that  
is the question. How do you avoid leakage?

> However if one adopts a non linear control theory approach, one can
> actually design high order modulators that both stable in theory  
> and in
> practice.

Unconditionally? Do you have a link for me?


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