The working principle of the thermistor power meter is illustrated in Figure 3.29 [15]: two thermistors (RT1 and RT2) are connected (1) in parallel, for measurand signals appearing at the RF input (PRF); and (2) in series, for the following measuring circuit (e.g., a bridge).
The capacitance C prevents the power dc component from flowing to the thermistors; the C2 stops the RF power toward the bridge.
A bridge with a thermistor or a barretter in one arm is called a bolometer.
Bolometer-based measurements can be performed with (1) a manual bolometer with variation of the bias current, (2) a manual bolometer with substitution method, or (3) a self-balancing bolometer.