On 16 Jul, 23:30, "orien1202" <saharmonfa...@yahoo.ca> wrote:
> Hi,
> I have a question about anti-aliasing filters which I thought would fit in
> this discussion thread. *

That thread is a year and a half old. What's wrong with starting
a new thread?

> I have an RC circuit and my ultimate goal is to estimate the value of the
> capacitor using least square estimation.

Any particular RC cirquit, or just *some* RC cirquit?

>*In order to carry out this
> estimation I need to discretize the system transfer function and find the
> ARMA model,

What does ARMA models have to do with RC networks?

> however to do that I need to use an anti-aliasing filter first
> to make the transfer function bandlimited. *

If you want to sample the data, then yes, you need an anti-alias
filter.

> I tried using a Butterworth filter, which does very well is preserving the
> magnitude response of the system however the phase is really not preserved
> and since I'm trying to estimate the value of the capacitor in the systemI
> thought preserving the phase is important for my estimation purposes.

For your information, the phase is lost in ARMA models.

> However I am now not certain any more.

Whenever you implement an RC network you necessarily mess
with both magnitude and phase.

> So my question is how do I know if preserving phase is important in an
> application or not?

Usually, it is not. Simply because it is impossible to
preserve phase in any useful cirquits. The best you can
hope for is to state what kind of phase degradation
is acceptable, and how much.

> and if it is what is the best anti-aliasing filter that
> would preserve phase?

Again, you need to find out what is acceptable in the end
applications. The phase is lost in ARMA models, so if that's
what you are going to find, there is no reason to worry
about phase in the anti-alias filter.

Rune