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A basic calibration question

Question asked by zjxie on Sep 13, 2006
Latest reply on Oct 9, 2006 by opticyclic
I just started to use VNA. After I done a full insertable 2 port calibration with SMA cable. I measured group delay with port 1 and port 2 reference plane connect to each other. I expected 0s group delay with certain variation. The average group delay is 0s, but the variation is about +/- 4ps with a single measurement, for 16 trace average, the variation reduced to +/- 2ps . Is this variation normal? Do I need to change cable or fix my VNA (8753ES)  ? 
Thanks in advance.