Rigol DS1302CA User Manual page 125

Sin = A/B or C/D, where  = 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,  must be in the range of
(0~π/2) or (3π/2~2π). If the main axis is at II and IV quadrant,  must be in the
range of (π/2~π) or (π~3π/2).
User‟s Guide for DS1000CA Series
Signal must be
centered in X
D
C
Ellipse method to observe the phase difference
 = arcsine (A/B) or arcsine (C/D)
B
A
Figure 3-6
RIGOL
3-9