Wrap Phase and Unwrap Phase (Trace Format)

Menu Path: Trace > Format > Format:

Selecting the Wrap Phase or Unwrap Phase trace format displays wrapped or unwrapped phase for the active trace. The y-axis unit is either degrees or radians.

Wrapped Phase

Wrapped phase means that all phase points are constrained to the range -180 degrees ≤ Phase Offset < 180 degrees. When the actual phase is outside this range, the phase value is increased or decreased by a multiple of 360 degrees to put the phase value within +/- 180 degrees of the Phase Offset value.

For example when Phase Offset = -10 degrees, the phase range is [-190, 170). In this case, 171 degrees is shifted to -189 degrees (171 - 360 = -189), and -400 degrees is displayed as +40 degrees (-400 + 360 = -40).

Unwrapped Phase

Unwrapped phase lets you designate a point (x-axis) value about which phase values are to be unwrapped. This point is called the Unwrap Reference. This would typically be a well-known point in a measurement such as the passband of a band pass filter.

When Unwrap Phase is selected, you can also specify a Phase Offset, which offsets the phase trace by the amount of phase shift that you specify.

Phase and Spectrum Measurements

In a spectrum measurement, the usefulness of phase information is often not obvious. Because phase is a relative concept, you may ask "phase with respect to what?"

Phase displayed on an FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. analyzer depends on the relative position of the waveform in the time record. Shift the waveform 90 degrees in the time record, and the answer on the display will change 90 degrees.

Triggering gives you control over the start of the time record. If triggering is used, then the phase of a particular signal can be stabilized. This (for example) allows the phase of harmonics to be compared to the fundamental.

If no triggering is used, the VSA will acquire a time record when ready, which will be uncorrelated to the input signal. In this case, the phase of the signal will vary randomly from measurement to measurement.

You can also examine the relative phases between multiple signals in a single time record, as with a modulated signal.