CIC Group Delay

I would like to know if I can predict what the group delay of a CIC
filter is.

I have an agressive FIR BPF where the sample rate is 2000 x the filter
bandwidth. The group delay is longer than I would like it to be. Could
a CIC filter offer a lower group delay?

I'll need to interpolate back to the original sample rate, so I need to
take that into consideration as part of the answer.

Thanks, in advance.

Jim

jim_nospam_beasley@yahoo.com wrote:
> I would like to know if I can predict what the group delay of a CIC
> filter is.

What is a CIC filter?


Bob
-- 

"Things should be described as simply as possible, but no 
simpler."

                                              A. Einstein

"Bob Cain" <arcane@arcanemethods.com> wrote in message 
news:dii59h19o4@enews4.newsguy.com...
>
> jim_nospam_beasley@yahoo.com wrote:
>> I would like to know if I can predict what the group delay of a CIC
>> filter is.
http://www.google.com/search?hl=en&q=%22cic+filter%22+%22group+delay%22
>
> What is a CIC filter?
>
http://www.google.com/search?hl=en&q=cic+filter
>
> Bob
> -- 
>
> "Things should be described as simply as possible, but no simpler."
>
>                                              A. Einstein
Simple enough, Bob? ;-)
Cheers, Syms. 

<jim_nospam_beasley@yahoo.com> wrote in message 
news:1129092761.120373.45800@z14g2000cwz.googlegroups.com...
>I would like to know if I can predict what the group delay of a CIC
> filter is.
>
> I have an agressive FIR BPF where the sample rate is 2000 x the filter
> bandwidth. The group delay is longer than I would like it to be. Could
> a CIC filter offer a lower group delay?

I assume your current filter is linear phase.  The delay through a linear 
phase FIR filter is half the filter length.  Since a CIC filter is also a 
linear phase, the delay through the CIC will be half the length of its 
impulse response, which is the sum of the lengths of each stage.

For a given delay, a CIC will not filter as well as a low-pass FIR designed 
without restrictions.  Similarly, for a given set of performance criteria, a 
well designed low-pass will require less delay than a CIC to meet them.

If you need to minimize group delay in the passband, you want a minimum 
phase IIR.  Keep in mind, though, that a filter as narrow as you indicate is 
going to have a lot of passband delay no matter what you do.

--
Matt

Jim,

A CIC filter is only good for the start of the decimation or end of the
interpolation, you can't reasonably replace your filter with only a CIC
filter or filters; a CIC filter is only a good filter if you are
GROSSLY oversampled.  For a multirate implementation (which your 1000x
oversampling screams for) you might start with some CIC filters
followed by some halfband filters and a final FIR filter, take the rate
back up using the halfband and CIC filters in reverse order.

I am curios how long is the FIR you have designed?  What are the
required design specs (normalize to Fs if you like)? Is linear phase a
requirement?

Dirk


jim_nospam_beasley@yahoo.com wrote:
> I would like to know if I can predict what the group delay of a CIC
> filter is.
>
> I have an agressive FIR BPF where the sample rate is 2000 x the filter
> bandwidth. The group delay is longer than I would like it to be. Could
> a CIC filter offer a lower group delay?
>
> I'll need to interpolate back to the original sample rate, so I need to
> take that into consideration as part of the answer.
> 
> Thanks, in advance.
> 
> Jim

Thanks, all, for your replies.  Yes, it's pretty clear I can't get
something for nothing. While addressing some system level concerns, to
test some basic principals, the order of the FIR was about 5700. A
multi-rate filter is preferable, and will reduce the resource
requirements tremendously. I was most concerned with what to do about
the group delay.  A quick check with cascaded multirate filters didn't
give much improvement in delay.

I'll probably explore some compromises, such as with filter bandwidth,
and delay is going to be on the order of 1/BW.

If you can ease up on whatever transition bandwidth requirements you
have, or the stop band requirements, that will help too.

Dirk

It's a damn good article - one of the best I've ever read within the
class of engineering magazine articles. 

--Randy

On Tue, 11 Oct 2005 22:00:37 -0700, Bob Cain
<arcane@arcanemethods.com> wrote:

>
>
>jim_nospam_beasley@yahoo.com wrote:
>> I would like to know if I can predict what the group delay of a CIC
>> filter is.
>
>What is a CIC filter?
>
>
>Bob

Hi,
  ya' have a look at:

http://www.embedded.com/shared/printableArticle.jhtml?articleID=160400592

[-Rick-]

On 11 Oct 2005 21:52:41 -0700, jim_nospam_beasley@yahoo.com wrote:

>I would like to know if I can predict what the group delay of a CIC
>filter is.
>
>I have an agressive FIR BPF where the sample rate is 2000 x the filter
>bandwidth. The group delay is longer than I would like it to be. Could
>a CIC filter offer a lower group delay?
>
>I'll need to interpolate back to the original sample rate, so I need to
>take that into consideration as part of the answer.
>
>Thanks, in advance.
>
>Jim

Hi,

  as far as I recall, the delay through a 
single-stage CIC filter having D delay elements 
in its comb filter will be

  Delay = (D - 1)/2   samples.

[-Rick-]