Looking for details about the usage of group delay

[neo]

about the group delay

:arrow: i am examining the filter i designed these days, it is a 8th order LPF design, my instructor told me to pay attention to the group delay, 8O i have got a plot of the group delay, but i dont know how to use it, is it really important about the group delay? anybody give me guidance about the group delay? something like the advantage and disadvantage, how to use it or reduce it. thanks a lot. :?:

 

[Mazz]

Re: about the group delay

Group delay is not really relevant, infact it represents the absolute delay that a signal will have when passing through your filter.
More important is group delay ripple, because, having an high ripple, the different harmonic components of your modulated signal will be delayed in different way, giving a distortion to the signal itself.
Mazz

 

[Humungus]

Re: about the group delay

What Mazz want to say is take a look at your group delay versus signal frequency plot and see if group delay is "almost" constant throughout the signal spectre.

 

[neo]

about the group delay

yeah, i understand u completely, i think i'd reconsider my design now, my filter seems to form a huge ripple throughout the useful signal, ^_^， it is from 45n to 67n vs. freq. from 1megaHz to 10mega, is it a normal one? oh, it is a 4th LPF designed for the baseband of a RF tranceiver. need i redesign it to a 2nd one? does it matter?
by the way, mazz, y are always waiting for answering my stupid questions, ha? u are always the first one to reply.
thanks a lot!

 

[flatulent]

Re: about the group delay

You have several options. One is to make the filter wider than you need and use more poles to get the attenuation you want out of band.

If you can take some amplitude variation, the Gaussian to X dB down type will work better.

Finally, you can delay equalize the filter.

What are the amplitude-frequency points you have to meet?

 

[neo]

about the group delay

to flatulent: the cut-off freqency is about to be 10M, and the useful signal freqency covers from 7M to 9M.......,,,,

 

[flatulent]

Re: about the group delay

to flatulent: the cut-off freqency is about to be 10M, and the useful signal freqency covers from 7M to 9M.......,,,,

There should be a more detailed set of specifications you are designing to. They should read something like:

1 - 10 MHz no more than Y dB down from midband.

ZZZ MHz and above, qqq dB or more down from midband.