HP 8712ES User Manual page 56

Making Measurements
Measuring Devices with Your Network Analyzer
Refer to Figure 3-2, "Simplified Block Diagram," for the following
discussion regarding detection schemes and modes. The forward
transmitted signal (routed to input B) and the reflected signal (input A)
are measured by comparison to the incident signal. The reverse
transmitted signal (routed to input A) and the reflected signal (input B)
are also measured by comparison to the incident signal. The analyzer
couples off a small portion of the incident signal to use as a reference
signal (routed to input R). Sweeping the source frequencies, the analyzer
measures and displays the response of your test device.
Figure 3-2 Simplified Block Diagram
3-4 ES User's Guide