Rigol DS1000C User Manual page 145

Ellipse method to observe the phase difference
Sinθ = A/B or C/D, where θ = phase shift (in degrees) between the two signals.
From the formula above, you could get:
If the main axis of the ellipse is at I and III quadrant, θ must be in the range of
(0~π/2) or (3π/2~2π). If the main axis is at II and IV quadrant, θ must be in the
range of (π/2~π) or (π~3π/2).
© Copyright RIGOL Technologies, Inc. 2007.
Signal must be
centered in X
D
A
C
Figure 3-6
θ = ±arcsine (A/B) or ±arcsine (C/D)
User's Guide for DS1000 Series
B
RIGOL
3-9