RIGOL DSA875 - Manual (Page 10)

RIGOL
DSA800 Options and Accessories 6
VSWR Measurement Kit
VSWR-DSA800 option (used together with the tracking generator function and the
VB1020, VB1040 or VB1080 option) provides measurement functions of S11-related
specifications (such as the return loss, reflection coefficient and VSWR).
Press Meas at the front panel and then press VSWR to enable the VSWR
measurement function. The screen is divided into two windows with the upper
window (the basic measurement window) displaying the sweep trace and the lower
window displaying the measurement wizard and measurement results. Perform two
measurements respectively according to the measurement wizard in the lower
window: measurement with the device under test disconnected (Meas Setup 
Cal Open, represented by trace 2) and measurement with the device under test
connected (Meas Setup  VSWR, represented by trace 1). The return loss is
determined by the difference (represented by the math trace) of the results of the
two measurements and the reflection coefficient and VSWR are determined by the
return loss.
VB1020/VB1040/VB1080
Device under Test