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Design >> RF Design >> group delay
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Message started by aaron_do on Aug 5^th, 2007, 6:18am

Title: group delay
Post by aaron_do on Aug 5^th, 2007, 6:18am

Hi all,

I have a system where I plotted the output phase versus frequency from the PXF analysis. So it should be w.r.t the input phase. Anyway I put the waveform into the calculator and took a derivitive (deriv function), and i'm thinking the result is the group delay right? So for the desired band, I found the maximum devitation in GD to be 70 u. So i assume this is 70 microseconds right? The thing is my data rate is up to 250 kbs or 4 us. Does that mean that i'm going to have a problem? I'm assuming FSK modulation with an IF of 2 MHz...don't know if that helps.

Anyway does anyone have any advice?

thanks,
aaron

Title: Re: group delay
Post by aaron_do on Aug 5^th, 2007, 7:36am

I narrowed the problem down to my IF filter. I guess i need to improve the group delay...does anybody know how to relate group delay to ISI/BER? My guess would be to take the percentage of the power with a group delay more than 1/BR (BR = bit rate) given the modulation scheme and consider that the SNR.

any thoughts?

cheers,
Aaron

Title: Re: group delay
Post by aaron_do on Aug 5^th, 2007, 10:06pm

ok...sily mistake...i forgot to normalize...need to divide by 360. Thats what i get for working late.

Title: Re: group delay
Post by ACWWong on Aug 6^th, 2007, 3:28am

the group delay of the filter is set by the filter response (poles and zeros) and is often plotted in filter tables (a book I find very good for this is Williams "Electronic-Filter-Design-Handbook"). More attenuation generally means more group delay variation, so classically in order of least delay variation Bessel < Butterworth < Cheby < Ellipitic.

Then a simple ac simulation can give you the group delay. Out-of-interest, why/how are you doing PSS+PXF ?
For a particular modulation you can inspect the eye diagram, and see the impact of group delay variation...

For BER you're best simulating in something like matlab (which also has a v. good filter support), with a model of your demodulator.

Title: Re: group delay
Post by aaron_do on Aug 6^th, 2007, 6:56am

Hi,

I'm was using PSS/PXF to find the gain of the system including downconversion and IF filtering. And i figured i could use the phase plot from the PXF form to view the group delay.

How would i use an eye diagram? Wouldn't i need a modulator and demodulator? I haven't done any baseband design yet so doing the simulation in Matlab will have to wait...

thanks,
Aaron

Title: Re: group delay
Post by ACWWong on Aug 6^th, 2007, 7:22am

you can view the eye diagram in Cadence (there is a calculator function).. basically you wrap around the transient waveforms onto a time axis of 1 symbol period... From the superposition you can see the "eye" and any noise/distortion in it likely to impact your demodulation process... so you don't need a demodulator, but you will need a modulated (in your case FSK) input signal...
For example, with an ideal schematic in a transient simulation you will see closure of the eye i your eye diagarm as your IF filter bandwidth is narrowed... getting the true value with noise and alike from an eye diagram is not efficient in Cadence (transient noise etc is inefficient) and is best suited to something like matlab...

Title: Re: group delay
Post by aaron_do on Aug 6^th, 2007, 7:54am

ok thanks for the help,

Aaron