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DUT group delay measurment uncertain

Question asked by abdallahLB on Jan 16, 2016
Hi guys,

I am abdallah, I have a problem in measuring the group delay of a DUT. first of all I need to tell you about the idea. the DUT is a linear equalizer continuous time HMC6545 fro Hittite technologies. so this equalizer filter has 9 taps and fixed delay between taps about 18ps. the taps are non ideal and every tap distort the signal after through especially at hight frequency. so I need to measure the frequency response of each tap individually by set it to a positif value and all others to zeros.
the non desired non prospected result is the group delays of the taps is too higher than the 18 ps (about 1.7 ns)!!!!!!
notice that if I calculate the impulse responses of every tap using inverse Fourier transformer the and I plot the 9 impulse responses on the same figure, it is obvious that the difference between the pics is exactly 18 ps ans this is good results.

the question is why the group delay for every  taps it different from n*18 ps and too higher than the value (about 1.7ns) where n is the number the measured tap.

I think that it is caused by electrical delay of VNA's cables (about 0.5 meter for each one).

But the logic that the VNA should remove this delay.

how can help me??

Thanks  

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