Coaxial Thermistor Mount
The Keysight model 478A coaxial thermistor mount is designed for use with Keysight N432A or 432A power meters to measure microwave power from 1μw to 10 MW. The design of the mount minimizes adverse effects from environmental temperature changes during measurement. For increased measurement accuracy, Effective Efficiency and Calibration Factor are measured for each mount, and at selected frequencies across the operating range; the results are marked on the label of the instrument. Model 478A can be used over the 10-HMZ to 10-GHz frequency range. Throughout the range, the mount terminates the coaxial input in a 50Ω impedance and has an SWR of not more than 1.75 without external tuning.
Each mount contains two series pairs of thermistors, which are matched to cancel the effects of drift with ambient temperature change. Thermal stability is accomplished by mounting the leads of all four thermistors on a common thermal conductor to ensure a common thermal environment. This conductor is thermally insulated from the main body of the mount so the thermal noise or shocks applied externally to the mount, such as those handling the mount manually, cannot significantly penetrate to disturb the thermistor. This thermal immunity enables the thermistors to be used in the measurement of microwave power down to the microwatt region.