As mentioned above, the thermistor

As mentioned above, the thermistor resistance depends on the surrounding temperature.
This effect is compensated in an instrument based on two self-balancing bridges.
The RF power is input only to one of these, as shown in Figure 3.33.
The equilibrium voltages Vc and VRF feed a chopping and summing circuit, whose output Vc + VRF goes to a voltage-to-time converter.
This produces a pulse train V1, whose width is proportional to Vc + VRF.
The chopping section also generates a signal with an amplitude proportional to Vc – VRF, and a frequency of a few kilohertz, which is further amplified.
The signals V1 and V2 enter an electronic switch whose output is measured by a medium value meter M.
This measure is proportional to the RF power because: