FPGA Central

Hi,
I have a question about anti-aliasing filters which I thought would fit i
this discussion thread.
I have an RC circuit and my ultimate goal is to estimate the value of th
capacitor using least square estimation. In order to carry out thi
estimation I need to discretize the system transfer function and find th
ARMA model, however to do that I need to use an anti-aliasing filter firs
to make the transfer function bandlimited.
I tried using a Butterworth filter, which does very well is preserving th
magnitude response of the system however the phase is really not preserve
and since I'm trying to estimate the value of the capacitor in the system
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 a
application or not? and if it is what is the best anti-aliasing filter tha
would preserve phase?
thanks

orien1202 wrote:
> 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?

I'm thoroughly confused. Are you trying to guess the size of the
capacitor in an R-C rolloff you can't examine? Do you want to design a
filter and need to determine the appropriate capacitor value? Are you
hunting mosquitoes with a shotgun? :-)

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

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

On Jul 16, 5:30*pm, "orien1202" <saharmonfa...@yahoo.ca> wrote:

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

I am involved in many applications where preserving phase is
important. To preserve phase the type of anti-aliasing filter is not
that important, but how you implement it is. We use very stable
precision caps/resistors that have very low drift over time/
temperature, then model the inverse of the transfer function of the
anti-aliasing filter in software, then compensate for any phase errors
introduced by the filter. We also generally use a delta-sigma A/D for
phase important applications, as it simplifies the anti-aliasing
filter, and therefore the cost (since the precise resistors/caps are
not cheap or physically small)

"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.

--
Scott
Reverse name to reply

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.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯

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.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯ ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯