My TDS460 says in the manual:
Phase Timing measurement. The amount of phase shift, expressed in degrees of the target waveform cycle, between the /MidRef/ crossings of two different waveforms. Waveforms measured should be of the same frequency or one waveform should be a harmonic of the other.
Phase is a dual waveform measurement; that is, it is measured from a target waveform to a reference waveform. To get a specific phase measurement, specify the target and reference sources.
Phase is determined in the following manner:
- The first /MidRefCrossing (MCross1Target)/ and third (/MCross3/) in the source (Target) waveform are found.
- The period of the target waveform is calculated (se "Period" above).
- The first /MidRefCrossing (MCross1Ref)/ in the reference waveform crossing in the same direction (polarity) as that found /MCross1Target/ for the target waveform is found.
- The phase is determined by the following: /Phase = ((MCross1Ref - MCross1Target) / Period) x 360/ If the target waveform leads the reference waveform, phase is positive; if it lags, negative.
Phase is not available in the Snapshot display.
Ref levels (thresholds) are set in a menu, typically 50% between max and min (and 10 and 90% for rise/fall measurements). Crossings are counted starting from time 0, or from the left of the window of a gated measurement. A 10% (of amplitude) hysteresis is applied after the first crossing, and so on.
The accompanying diagram seems to suggest that the result is calculated in floating point, so has sub-sample accuracy for rapidly changing signals (presumably, following whatever interpolation is set?).
So, just a basic edge detector method. Fragile, but general enough not to care what the waveform is, and to work in most cases. (Noisy waveforms usually give an "unstable histogram" warning but remain useful, except when an unstable trigger or incomplete acquisition in EQ sampling mode happens...)