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PNA-X. How can I best display phase angles close to +/- 180 deg?

Question asked by drkirkby on Jul 27, 2017
Latest reply on Jul 27, 2017 by drkirkby

PNA-X display of phase angles near 180 degrees

 

VNA is an N5247A. Firmware  A.10.49.08

 

What the best way to display phase angles, with good resolution (say 1 deg/division), where the phase angles are wrapping around at close to +/- 180 deg?

 

By way of example, I attach a screen shot, where I'm trying to determine the delay of an offset short. (This is actually from a recalled .s1p file, but I doubt that makes any difference to my question.)

 

  • The window in the top left shows the phase variation with frequency of an offset short. No port extensions are used.
  • The window in the bottom left shows the phase variation with frequency. A port extension of 57.9 ps is applied. The phase is close to +/- 180 degree, so one might conclude the delay of the offset short is close to 57.9 ps. But one can't see just how far the phase angle varies from +/- 180 degrees, as the scale is so large (50 degrees/division).
  • On the window at the top right, the reference is +180 degrees, but with a resolution of 1 deg/division. We can see that the phase drops to as low as about 179 degrees.
  • On the window at the bottom right, the reference is -180 degrees, again with a resolution of 1 degree/division. The phase rises as high as -179 degrees.

 

Is there a way to display the phase, such that angles that are close to -180, get 360 degrees added to them, so allowing phase angles close to +/- 180 degrees to be shown at a reasonable scale, allowing small departures from +/- 180 degrees to be seen?

 

What I really want to do is find what port extension will cause the minimum departure of the phase from the ideal 180 degrees.

 

Strangely, on my 8720D,. setting the reference as +180 degrees, but 1 degree/division, phase angles that are close to -180 are actually shown close to +180 degrees. I've never quite understood why that happens, but it is quite convenient if one wants to look at phase angles which are very close to +/-180 degrees.

 

Phase wrapping is often an annoying problem. It's even more tricky if it happens in 2 or more dimensions, but at least this is a 1D problem.

 

Dave

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