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Group Delay Testing Issues - RF Cafe Forums
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| dvryan
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Post subject: Group Delay Testing Issues
Posted: Wed Apr 19, 2006 1:35 pm
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Joined: Wed Apr 19, 2006
1:27 pm
Posts: 3 |
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I'm testing a fiber optic TX/RX pair with a long
(~5km) fiber link. I'm using the 8510C to get the
group delay, but it's giving me a negative number.
When I test the same TX/RX pair with a short (1m)
fiber, I get a 15ns delay. With the 5km fiber, the
delay is ~-80ns. I don't know why I'm getting this
result . . . any advice would be appreciated.
Thanks,
Dan
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Joe |
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Post subject: Group Delay
Posted: Wed Apr 19, 2006 4:10 pm
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Joined: Fri Feb 17, 2006
6:25 pm
Posts: 11
Location: US
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Dan,
Your problem is due to insufficient
sampling points in the measurement. Group delay
is measured as the slope of the phase vs frequency.
If there are too few sampled points, the instrument
is confused. This occurs because a typical phase
plot slopes down to -180 degrees and then restarts
at +180 degrees. Without several points within each
360 degree cycle the phase plot will sometimes slope
upward giving a negative delay. Also, long delays
have a very high phase slope, changing rapidly vs
frequency.
To illustrate: a typical phase
plot will have many points near the 180 degree crossing.
( -175,-176,-177,-178,-179,-180,+179,+178...) a
phase plot with too few samples may go -120,+140,-10...
It becomes difficult to get the correct slope because
of the missed crossings at -180/+180 degrees. The
worst case may be 1 point or less in each 360 degree
cycle. In that case the delay is no longer a usefull
measurement. The result is a plot with incorrect
slope resulting in smaller than actual delay or
even negative delay.
To get a better reading,
increase the number of frequency sampling points
and and reduce the bandwidth. For long delays it
is often necessary to measure in narrow bandwidths
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dvryan |
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Post subject: Group Delay
Posted: Mon Apr 24, 2006 11:14 am
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Joined: Wed Apr 19, 2006
1:27 pm
Posts: 3 |
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Joe,
Thanks for your advice. I've done what
you suggested and now get a positive measurement.
There's still an issue, though- the signal is very
noisy (+/-10ns), and I'd like to get that down to
1ns to see the delay ripple across any given band.
Is this possible with the 8510C? I've done grop
delay using spec ans and not had this kind of issue
. . .
Thanks,
Dan
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Joe |
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Post subject: Group Delay
Posted: Tue Apr 25, 2006 1:55 pm
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Joined: Fri Feb 17, 2006
6:25 pm
Posts: 11
Location: US
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Dan,
You didn't mention how much loss you
had. If there is a lot of loss and the output level
is low, it will be noisy. You could increase the
source power (watch for saturation or compression
of the unit you are measuring).
You could
also use averaging to clean up the signal. This
will average several sweeps and limit the effects
of random noise. Don't use smoothing because this
distorts the actual measurement.
Joe
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dvryan |
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Post subject: Group Delay
Posted: Tue Apr 25, 2006 2:31 pm
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Joined: Wed Apr 19, 2006
1:27 pm
Posts: 3 |
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Joe,
Power is actually relatively high- near
compression. I read elsewhere that, due to the length
of the line, I won't be able to use an 8510C to
measure within +/- 1 ns because I have to reduce
the BW so much that it affects the measurement accuracy.
This was supported in older HP documentation, claiming
that I wouldn't be able to get beyond 10ns accuracy
for the smaller (kHz) BWs. So, it looks like I'll
have to live with what I have. Thanks for your help!
- Dan
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Posted 11/12/2012
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