HP 431C Operation The detector mount contains two thermisor sensors, the Rd detector and the Rc compensator. The Rd detector is coupled to the RF power to be measured. The Rc compensator measures the ambient temperature and acts as a reference for the Rd measurements. In coaxial mounts the Rd and Rc sensors each consist of a pair of thermistors. In waveguide mounts the Rd and Rc are each a single thermistor. Each of the thermistors has a resistance at room temperature of about 1500 ohms. As current is passed through the thermistors they warm up and drop in resistance. In operation of the meter, current is passed through each of the thermistors to bring their individual resistances to about 100 ohms. This gives the series connected Rd and Rc sensors a metering resistance of 200 ohms. Through capacitive coupling to ground, the two Rd thermistors are in parallel to the applied RF being sensed, giving an RF load of 50 ohms. The Rd is in a bridge circuit which is also part of a 10 KHz oscillator circuit. This 10 KHz oscillator provides the power that brings the 2 Rd thermistors to the series operating resistance of 200 ohms. When the Rd thermistors reach 200 ohms, the Rd bridge is in balance and the 10 KHz oscillator, through feedback from the bridge, holds its power output at this level. The Rc thermistors are part of another balanced bridge. The same amount of 10 KHz current is coupled from the 10 KHz oscillator output to both the Rc and Rd bridges by the series connected primaries of the A1T1 and A1T2 transformers. This identical current in the two bridges causes them to both operate at 200 ohms series resistance, balancing both bridges. As RF power to be measured is coupled into the Rd sensors, less 10 KHz power is needed to keep the sensing bridge balanced at 200 ohms. This lowered 10 KHz power is also coupled into Rc bridge causing it to unbalance. The metering circuitry corrects this imbalance by providing a DC current into the Rc bridge to bring it back into balance. The amount of DC power needed to balance the Rc bridge is equivalent to the RF Power that was introduced into the Rd bridge. The meter indicates the amount of DC power that is provided to the Rc bridge which is identical to the RF power that has been dissipated in the Rd bridge.