In Impedance Measurement Handbook 6th Edition, pages 2-7, Figure2-4 (c)Auto-balancing bridge for frequency above 100 kHz.

The purpose of the figure is to measure the impedance of the DUT.I have some questions, as shown in the picture.

A. What is used to mix wiht Lp? multiply OSC with Lp?

B. Low-pass filter outputs DC value ? proportional to phase difference?

C. If the low-pass filter outputs DC, what is the role of the vector modulator? proportional correction?

D. Why use negative 90 degrees?

B. Low-pass filter outputs DC value ? proportional to phase difference?

C. If the low-pass filter outputs DC, what is the role of the vector modulator? proportional correction?

D. Why use negative 90 degrees?

Can anyone give me some tips.

Thanks.