f



Finding the group delay of a filter

Has anyone implemented a function to find the group delay (the time between=
 the filter's initial response and its peak response)of the different digit=
al filters in LabVIEW?<br><br>I need to track the arrival of a certain part=
 of a signal. I know approx. when it is to arrive, however locating it gets=
 complicated due to the variable delay caused by the necessary filtering (d=
elay depends on the frequencies, filter order, type etc.) .<br><br>I see th=
at MatLAB / MatWorks has a group delay function, but perhaps someone has ma=
de a G equivalent? <br><br>The digital filter design toolkit is said to hav=
e analysis tools that can give you the group delay...but I'm not sure if th=
at could be used to find the delay in any case programmatically? Even if it=
 can I'm a bit reluctant to buy the toolkit just for that functionality...<=
br><br>I though I had solved it when I found a description of how to calcul=
ate the delay of a Butterworth filter (at least) on: http://www.mathcad.com=
/Library/LibraryContent/MathML/group_b.htm<br><br>The results are in the ri=
ght ballpark...but sometimes it fails by more than 5 microseconds and that'=
s not good enough for my application (could be an incorrect implementation =
off course). <br><br>If I use correlation to find the delay I get extremely=
 good results, however that will not be robust enough (there could be other=
 strong features, e.g. due to reflections, that might mislead the correlati=
on.). <br><br>An alternative might be to let the application generate a nic=
e signal with the right frequencies internally, run it through the chosen f=
ilter setup and find the group delay by correlation on that signal...then a=
pply it on the real signal. The most elegant thing would be to be able to d=
o it mathematically though.
0
x9561 (148463)
4/1/2005 5:10:13 PM
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