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

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

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

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

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