I am a novice with network analyzers and I have some confusion interpreting the wrap-around phase-shift and want to understand how to fundamentally interpret this.

A method of measuring Velocity of Propagation for a cable is measuring the total phase-shifts across a frequency range and applying it to some formulas to obtain VOP. But before we even talk about the formula, I just want better understanding of the phase-shift. When I think of phase-shifts, I think of a sinusoidal signal lagging/leading a reference signal. So when I see the wrap-around phase-shift every 360 degrees, is it correct to interpret the phase-shifts as a lag/lead in the signal?

And for the cable we are measuring to calculate VOP via the phase-shifts, I've been informed at the starting frequency we are measuring, the phase should start at a NEGATIVE phase in order to get the 'correct' total phase-shift for VOP calculation. Why negative?

Thanks !

A method of measuring Velocity of Propagation for a cable is measuring the total phase-shifts across a frequency range and applying it to some formulas to obtain VOP. But before we even talk about the formula, I just want better understanding of the phase-shift. When I think of phase-shifts, I think of a sinusoidal signal lagging/leading a reference signal. So when I see the wrap-around phase-shift every 360 degrees, is it correct to interpret the phase-shifts as a lag/lead in the signal?

And for the cable we are measuring to calculate VOP via the phase-shifts, I've been informed at the starting frequency we are measuring, the phase should start at a NEGATIVE phase in order to get the 'correct' total phase-shift for VOP calculation. Why negative?

Thanks !

I am not sure what the actual question is. Anytime you add electrical length, you will see a phase change...it's just simple physics.

You did not mention how you are looking at this....is this a reflection measurement or transmission measurement? I assume you first performed a cal to eliminate any setup variations. When you then connect a cable you are looking at the difference in phase between the reference receiver (usually inside the VNA) and the test port receiver (for the device under test.)

If you are looking at transmission measurement, and you have performed a cal, then any cable length you add will make the phase display start from 0 and go negative from there with increasing frequency. The number of 360 deg phase shifts you see are dependent upon the frequency span and electrical length of the cable.

The wrap-around phase shifts you see are just a display convenience to see the entire amount of phase shift on one screen. If you have a PNA, you can also look at unwrapped phase and you will see that the phase (in this case) goes more and more negative with frequency.....perhaps thousands of degrees or more depending (again) upon cable length and frequency span.

If you give us a bit more info, such as length/type of cable, freq, and how you are looking at it we may be able to help more. Also, whenever any significant cable length is involved, make sure you are using stepped-sweep mode or just slow the sweep down until you see no more changes with sweep speed. See: http://na.tm.agilent.com/pna/files/eld.html