A Transient Response Error in Microwave Power Meters Using Thermistor Detectors

Broadband coaxial thermistor mounts are commonly used in automated precision microwave measurement systems such as six-port networks. To reduce the effect of temperature drift and to decrease the total measurement time, it is desirable to measure the dc bias voltage on the thermistor mount very quickly after turning the rf on or off. However, investigation has revealed that a coaxial mount may take much longer to settle to a stable dc bias voltage than the thermistor element time constant or the associated power meter servo bandwidth would indicate. If the bias voltage is measured before this transient ends, the error in the calculated rf power can be very large; as much as 1.4% has been observed. This paper describes these transients and gives measured durations and maximum error for a number of different bolometer mounts.