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Global phase offset between linear & segmented sweep over same freq range

Question asked by lrc on Jul 23, 2012
Latest reply on Jul 23, 2012 by Dr_joel
Using a N5230c VNA, I perform both a linear and a segmented S21 measurement over a 1 MHz span, centered near 6 GHz. The start and stop frequencies are the same; however, the segmented sweep has several segments, each with different spacing.  There is a global phase shift such that the linear sweep is 0.05 degrees lower in phase than the segmented sweep.  Note that the segmented sweep is taken such that the frequencies increase from segment to segment. 

This small shift was also visible when acquiring a "segmented sweep" by taking a series of linear sweeps, each successively higher in frequency, and stitching them together in software (not efficient, but a quick test using simple code before I had segmented sweeping working).  The phase shifted randomly from segment to segment.

Is this a calibration error.  It seems unlikely over the same span.  What is the origin of this global phase shift?