Originally Posted by blargg
How could a stable IIR filter go into oscillation? And if it's unstable, I'd think most inputs would drive it into oscillation (but I haven't done much with IIR filters myself).
Well the issue is that a "stable" IIR filter is only stable for a subset of possible input conditions. Now you may design the system in such a way that it will never give the IIR filter an input that will cause it to oscillate, or you can design it so that non-ideal characteristics such as saturation will come in to play before it oscillates. But irrespective of that, it's still a design condition to think about.

Originally Posted by blargg
FIR filters can also have non-linear phase response. A minimum-phase FIR filter has most of the energy at the beginning, just like an IIR filter. I'm assuming this is what you mean by "transport delay".
The first filter will still have linear phase response. It has lower transport delay than the second one, but the transport delay is still constant across all frequencies. Since the phase response p = 2*pi*�*T/�s (� = input frequency, T = tap number with maximum amplitude, �s = sample frequency) for an FIR filter, you can't end up with anything but a linear phase response for an FIR filter - it's a simple linear equation with no tricks to pull.

(BTW - a transport delay in control systems is anything that has a transfer function anything like e^(-a*s) in s domain.)

Last edited by Vas Crabb; 01/09/09 09:29 PM. Reason: greek characters didn't work