Thermistor Bead Matching for Temperature Compensated RF Power Thermistor Mounts

The temperature compensated thermistor-power meter of the dual bridge design has for many years been the most widely used instrument for microwave power measurement. During these many years very little if anything has been written with regard to the manner or criteria for matching the thermistor beads. This paper attempts to fill that void. The procedure to match thermistor beads for less than 2 ¿W/°C drift is to be accomplished by pairing the dR/dt characteristic of each bead and adjusting the dP/dR characteristic for a match. dR/dt of the beads may be determined from resistance data at 90°C and 110°C, and the expression dR/dt = 9.4497 [ln Ra-1.11648 ln Rb + 0.61749] which is derived for 200 ¿ operating resistance thermistors. The thermistor beads are then paired by selecting such that dR1/dt and dR2/dt are within 1 percent of each other. The conditions necessary for matched dP/dR of the thermistor beads are developed and procedures indicated. The paired beads are installed in the thermistor mount and the constant dP/dR of each bead adjusted for equality by moving the heat sink of one of the thermistors until a balance of the slave bridge and master bridge is obtained simultaneously.