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Measuring Group Delay -- HP Vector Network Analyzer

Doing some research to understand a bit more about a measurement I am required to make, Group Delay. From internet reading, the definition of group delay: - derivative of the change in phase over the change in angular velocity. Tg = -d(theta)/d(omega)

It seems relevant to make as it will give some indication of the signal distortion for a particular modulated signal. In my case OQPSK.

Now for praticality. Need a bit of help interpretting the results of the measurement made using an HP8722D per the manual.

my case has a BW of 500MHz. The plot is a waveform with ripple over the range. The spec I am comparing against has three components: Linear (time/Bandwith), parablic (time/BandWidth^2), Ripple (time).

How do I get these results from a trace? The parabolic is the hardest for me to conceptualize. Do I need to make some kind of interpolation?

Thanks.

Reply to
thegtwo
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This is not easy without pictures, but let's look at the trace. The trace you have is very probably NOT straight. The curvature of that trace is the parabolic you are seeking. (Note that it's not always a parabolic curve, but the below is close enough for most measurements - not always, but usually)

To get the actual value, take a straight line from the low end of the trace to the high end. Now measure the tangential 'distance' to the edge of the curvature part. That's the parabolic.

Cheers

PeteS

Reply to
PeteS

PeteS, Thanks for the response. I am having a bit of trouble visualizing what you described. I have post a jpg that represents the traces of the group delay measurement on an 8722. In my case its a 500Mhz BW and the the time scale is in nS.

http:\\\\

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\\groupdelay.jpg

If possible, annotate the graph or use it as a reference to discuss. Thanks for the help

Reply to
thegtwo

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