Anti-Aliasing filter

Scott Seidman wrote:
> "orien1202" <saharmonfared@yahoo.ca> wrote in
> news:PpWdncfkZLIY9ePVnZ2dnUVZ_qPinZ2d@giganews.com: 
> 
>> Hi,
>> I have a question about anti-aliasing filters which I thought would
>> fit in this discussion thread.  
>> I have an RC circuit and my ultimate goal is to estimate the value of
>> the capacitor using least square estimation.  In order to carry out
>> this estimation I need to discretize the system transfer function and
>> find the ARMA model, however to do that I need to use an anti-aliasing
>> filter first to make the transfer function bandlimited.  
>> 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 system I thought preserving the phase is important for my
>> estimation purposes. However I am now not certain any more.
>> So my question is how do I know if preserving phase is important in an
>> application or not? and if it is what is the best anti-aliasing filter
>> that would preserve phase?
>> thanks 
> 
> The Bessel filter produces linear phase-- i.e., a simple time delay.  Best 
> practice is to filter all your channels through the same filter.

It approximated linear phase better than most analog filters, but it 
isn't very good compared to symmetric FIRs. The rolloff can be 
disappointing. http://en.wikipedia.org/wiki/Bessel_filter

Jerry
-- 
Engineering is the art of making what you want from things you can get.
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Jerry Avins <jya@ieee.org> wrote in news:
_IKdnZSWmsGJxOLVnZ2dnUVZ_v3inZ2d@rcn.net:

> 
> It approximated linear phase better than most analog filters, but it 
> isn't very good compared to symmetric FIRs. The rolloff can be 
> disappointing. http://en.wikipedia.org/wiki/Bessel_filter

The man wants an anti aliasing filter.  I suppose you could sample fast 
enough so the phase in the frequency band of interest is close enough to 
zero, and then do whatever FIR/decimation you want to, but I assumed we 
were still in the analog world.

-- 
Scott
Reverse name to reply

Scott Seidman wrote:
> Jerry Avins <jya@ieee.org> wrote in news:
> _IKdnZSWmsGJxOLVnZ2dnUVZ_v3inZ2d@rcn.net:
> 
>> It approximated linear phase better than most analog filters, but it 
>> isn't very good compared to symmetric FIRs. The rolloff can be 
>> disappointing. http://en.wikipedia.org/wiki/Bessel_filter
> 
> The man wants an anti aliasing filter.  I suppose you could sample fast 
> enough so the phase in the frequency band of interest is close enough to 
> zero, and then do whatever FIR/decimation you want to, but I assumed we 
> were still in the analog world.

The FIR was just for comparison. The Bessel can be cuite goog well below 
cutoff, but the plot in the link I posted shows that one needs a lot of 
oversampling to use it.  Bessel can beu useful with the right tradeoffs.

Jerry
-- 
Engineering is the art of making what you want from things you can get.
&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;&#4294967295;