[sdiy] Talking of Q

Ian Fritz ijfritz at comcast.net
Sat Jun 18 04:55:24 CEST 2011

At 06:04 AM 6/17/2011, Justin Owen wrote:
>-----Original Message-----
>From: Tom Wiltshire [tom at electricdruid.net]
> >> Can anyone provide some Q range/factor amounts for some real world 
> filter designs?
> >Yeah, I'd like to know about this too.
> >I always pictured it as something that looks like x^2 - a graph curving 
> off upwards exponentially towards y-axis infinity. Maybe that's not how 
> it is...
>That's what I'm seeing from the ideal Q calculations - but I guess I 
>assumed that at some point, in the real world - hardware, power supplies, 
>frequencies(?) etc. would step in and say 'stop'... but even if that point 
>does exist - I don't know where it might be...

A practical limitation is that the Q of a filter usually has a significant 
variation with frequency. This shows up as a high-frequency Q enhancement 
and is caused by opamp nonidealities. This can be compensated for, but only 
up to a point.  The stability also depends on the filter topology.

If your filter will oscillate then it can have infinite Q.  But the more 
important thing to look at is how stable the high-Q state is, especially 
with respect to frequency variations.

A handy factiod to remember is that the the Q can be calculated by looking 
at the ringdown time after pulse excitation.  Q is pi times the number of 
cycles it takes to ring down by a factor of 1/e.


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