Forums

how to get rid of ringing with IIR notch filters?

Started by Unknown December 22, 2003
Hello,

I have an IIR notch filter ( i.e. bandstop ), that rings like crazy
whenever a transient signal comes through ... which is very unwanted
for my application.  Anybody have any ideas how to get around this?  I
have to stick with IIR form filters.  I've tried all the various
standard design flavors, i.e. Butterworth, Bessel, Elliptic, etc ...
but they all seem to ring unacceptably about the same.  

Regards,

Robert

www.gldsp.com

( modify address for return email )

www.numbersusa.com
www.americanpatrol.com
Hello,

I have an IIR notch filter ( i.e. bandstop ), that rings like crazy
whenever a transient signal comes through ... which is very unwanted
for my application.  Anybody have any ideas how to get around this?  I
have to stick with IIR form filters.  I've tried all the various
standard design flavors, i.e. Butterworth, Bessel, Elliptic, etc ...
but they all seem to ring unacceptably about the same.  

Regards,

Robert

www.gldsp.com

( modify address for return email )

www.numbersusa.com
www.americanpatrol.com
"Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote:

> ><r_obert@REMOVE_THIS.hotmail.com> wrote in message >news:407duvga6086tosc3tbf17nnpegf3ibqru@4ax.com... >> Hello, >> >> I have an IIR notch filter ( i.e. bandstop ), that rings like crazy >> whenever a transient signal comes through ... which is very unwanted >> for my application. Anybody have any ideas how to get around this? I >> have to stick with IIR form filters. I've tried all the various >> standard design flavors, i.e. Butterworth, Bessel, Elliptic, etc ... >> but they all seem to ring unacceptably about the same. > >How about trading band edge steepness for ringing? There are established >ways to get zero ringing with maximum rise time - associated with band edge >steepness.
Fred, The ringing would be expected to be reduced with increased band edge steepness, or vice versa? I've played with the band edge settings a bit, but without much luck. But I wasn't really doing it with any set strategy, or expected relationships in mind. Do you have any further information on this, or these established ways to get zero ringing with maximum rise time ( or any other related info )? Thanks, Robert www.gldsp.com ( modify address for return email ) www.numbersusa.com www.americanpatrol.com
Hello Robert,

Try a higher order filter.

Clay


<r_obert@REMOVE_THIS.hotmail.com> wrote in message
news:407duvga6086tosc3tbf17nnpegf3ibqru@4ax.com...
> Hello, > > I have an IIR notch filter ( i.e. bandstop ), that rings like crazy > whenever a transient signal comes through ... which is very unwanted > for my application. Anybody have any ideas how to get around this? I > have to stick with IIR form filters. I've tried all the various > standard design flavors, i.e. Butterworth, Bessel, Elliptic, etc ... > but they all seem to ring unacceptably about the same. > > Regards, > > Robert > > www.gldsp.com > > ( modify address for return email ) > > www.numbersusa.com > www.americanpatrol.com
Hi Clay,

I did attempt that route a little, with not much improvement.  I'll
review it again though.

Thanks,

Robert

www.gldsp.com

"Clay S. Turner" <CSTurner@WSE.Biz> wrote:

>Hello Robert, > >Try a higher order filter. > >Clay > > ><r_obert@REMOVE_THIS.hotmail.com> wrote in message >news:407duvga6086tosc3tbf17nnpegf3ibqru@4ax.com... >> Hello, >> >> I have an IIR notch filter ( i.e. bandstop ), that rings like crazy >> whenever a transient signal comes through ... which is very unwanted >> for my application. Anybody have any ideas how to get around this? I >> have to stick with IIR form filters. I've tried all the various >> standard design flavors, i.e. Butterworth, Bessel, Elliptic, etc ... >> but they all seem to ring unacceptably about the same. >> >> Regards, >> >> Robert >> >> www.gldsp.com >> >> ( modify address for return email ) >> >> www.numbersusa.com >> www.americanpatrol.com >
( modify address for return email ) www.numbersusa.com www.americanpatrol.com
Hi Robert,

I would really like to know a rigorous way to design FIR filters according
to ringing constraints, in addition to other criteria.... but I don't.

The length of the impulse response, i.e., the amount of ringing, is the
bandwidth of the frequency response.  Use a higher order filter as Clay
suggests, and use those extra poles and zeros to smooth the frequency
response as much as possible. And as Fred says, let your transition regions
be as wide as possible.

By the DSP uncertainty principle, it's clear that a very narrow notch has to
have a very ringy impulse response.  It would seem that the frequency
response you want to approximate to get the most compact impulse response is
1-some_gaussian, like the picture on your web site.  You might try using
yulewalk to approximate this sort of response directly, or maybe just use a
Bessel bandstop.


"Matt Timmermans" <mt0000@sympatico.nospam-remove.ca> wrote in message
news:WtKFb.4467$d%1.983990@news20.bellglobal.com...
> Hi Robert, > > I would really like to know a rigorous way to design FIR filters according > to ringing constraints, in addition to other criteria.... but I don't. > > The length of the impulse response, i.e., the amount of ringing, is the > bandwidth of the frequency response. Use a higher order filter as Clay > suggests, and use those extra poles and zeros to smooth the frequency > response as much as possible. And as Fred says, let your transition
regions
> be as wide as possible. > > By the DSP uncertainty principle, it's clear that a very narrow notch has
to
> have a very ringy impulse response. It would seem that the frequency > response you want to approximate to get the most compact impulse response
is
> 1-some_gaussian, like the picture on your web site. You might try using > yulewalk to approximate this sort of response directly, or maybe just use
a
> Bessel bandstop.
Well, I don't agree with this exactly. Although there can be cases like this. The ringing has to do with the transition widths more than anything. Increasing the order of the filter can make that worse unless there is some deliberate shaping going on. Take a lowpass filter as an example. The sharper the cutoff, the higher the ringing amplitude - the more the ringing approximates what would result from an ideal lowpass = the integral of a sinc. Ringing amplitude and ringing duration are two different things of course. Fred
<r_obert@REMOVE_THIS.hotmail.com> wrote in message
news:1ndeuvkavn69a88mgtvgnq17q8nkkgsujs@4ax.com...
> "Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote: > > > > ><r_obert@REMOVE_THIS.hotmail.com> wrote in message > >news:407duvga6086tosc3tbf17nnpegf3ibqru@4ax.com... > >> Hello, > >> > >> I have an IIR notch filter ( i.e. bandstop ), that rings like crazy > >> whenever a transient signal comes through ... which is very unwanted > >> for my application. Anybody have any ideas how to get around this? I > >> have to stick with IIR form filters. I've tried all the various > >> standard design flavors, i.e. Butterworth, Bessel, Elliptic, etc ... > >> but they all seem to ring unacceptably about the same. > > > >How about trading band edge steepness for ringing? There are established > >ways to get zero ringing with maximum rise time - associated with band
edge
> >steepness. > > Fred, > > The ringing would be expected to be reduced with increased band edge > steepness, or vice versa? > > I've played with the band edge settings a bit, but without much luck. > But I wasn't really doing it with any set strategy, or expected > relationships in mind. Do you have any further information on this, > or these established ways to get zero ringing with maximum rise time ( > or any other related info )?
Robert, Take a look at page 199 of Temes, Barcilon and Marshall "The Optimization of Bandlimited Sytems" Proc IEEE Vol 61 No. 2 Feb 1973 pp 196-234. Here, Victor Barcilon discusses time spread and frequency spread, deltaT and deltaOmega respectively, and shows that the Gaussian is optimum, that deltaT*deltaOmega>=1/2, etc...... These are optimum functions and not system functions. He treats the optimum function that minimizes rise time while being monotonic (which surely doesn't ring). Fred
In article 407duvga6086tosc3tbf17nnpegf3ibqru@4ax.com,
r_obert@REMOVE_THIS.hotmail.com at r_obert@REMOVE_THIS.hotmail.com wrote on
12/22/2003 02:28:

> I have an IIR notch filter ( i.e. bandstop ), that rings like crazy > whenever a transient signal comes through ... which is very unwanted > for my application. Anybody have any ideas how to get around this? I > have to stick with IIR form filters. I've tried all the various > standard design flavors, i.e. Butterworth, Bessel, Elliptic, etc ... > but they all seem to ring unacceptably about the same.
hmmm. this must be more than a 2nd order IIR notch, because there wouldn't be any differentiation between Butterworth, Bessel, etc. do you need a broad stopband? because if you don't, i think a 2nd order would do fine for just eliminating a single frequency. anyway, since you can model any notch as a straight wire and a BPF subtracted from it, if the notch is narrow (usually the desired property), then the BPF passband is narrow also, and then, because of the preservation of the time-bandwidth product, there is nothing that i can imagine that you can do to reduce the ringing except for widening the notch. rots o' ruck. r b-j
"Fred Marshall" <fmarshallx@remove_the_x.acm.org> wrote:

>> I've played with the band edge settings a bit, but without much luck. >> But I wasn't really doing it with any set strategy, or expected >> relationships in mind. Do you have any further information on this, >> or these established ways to get zero ringing with maximum rise time ( >> or any other related info )? > >Robert, > >Take a look at page 199 of Temes, Barcilon and Marshall "The Optimization of >Bandlimited Sytems" Proc IEEE Vol 61 No. 2 Feb 1973 pp 196-234. Here, >Victor Barcilon discusses time spread and frequency spread, deltaT and >deltaOmega respectively, and shows that the Gaussian is optimum, that >deltaT*deltaOmega>=1/2, etc...... These are optimum functions and not >system functions. > >He treats the optimum function that minimizes rise time while being >monotonic (which surely doesn't ring).
Fred, Thanks for the followup info. I'll have a look. Robert www.gldsp.com ( modify address for return email ) www.numbersusa.com www.americanpatrol.com