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Phase sign in S21 measurement

Question asked by lalalu on Jul 1, 2010
Latest reply on Jul 1, 2010 by daras

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Hi,

Why does the phase measurement is negative?
I mean - I cal the VNA with thru and measure 0 deg. Now I add (lambda/4) length cable and measure -90 deg ?
Why the measured value is negative the logic says the measure should be +90 deg.

I'm sorry if it sound like a silly question but I ask few "experts" and they said that the way the VNA works. WHY ???

Thanks,
L.L.

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daras Jul 1, 2010 2:14 PM
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There is nothing odd about a negative phase value. VNA's measure phase in +/- 1st and 2nd quadrants (i.e +/- 180 degrees). depending on the initial phase value and the nature of the transmission medium, the phase values could be negative at some frequencies and positive at others. However, it is the phase slope that must follow a certain rule. All physical transmission paths have a certain physical length and therefore a signal is always delayed when transmitted through that path. given that the equation of group delay is Delay = - dPhi/dw, then phase slope must always be negative in order to get a positive delay, which is what we would expect in a real physical transmission path. So unless your phase slope is heading North instead of South, then there is nothing to worry about.
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Phase sign in S21 measurement

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