System for Transfer of Calibration Factor for Coaxial Bolometer Mounts with One Percent Transfer Inaccuracy

A system of coaxial RF power transfer standards (directional couplers with thermistor mounts on their side arms) and associated instrumentation has been developed for accurate transfer of calibration factors from primary standards laboratories to working power meters. The frequency range is 0.1 to 12.4 GHz and calibration can be transferred to terminating or feedthrough power meters. The system includes six RF power sources, five coaxial RF power transfer standards, a dc substitution power standard, and feedback circuits (including chopper and amplifier). The sources are stabilized by dc substitution feedback so that the RF power is either ½, 1, 5, or 10 milliwatts. Since no corrections are made for the impedances of the source and load, the maximum possible error in transferring a calibration depends upon the VSWR of the bolometer mount or feedthrough power meter to which the calibration is transferred. This error approaches 1 percent as the VSWR approaches 1.5. Calibrations can be transferred at fixed frequencies or using swept frequency methods. High precision swept frequency techniques for measuring insertion loss and input and equivalent output VSWR were developed in connection with this system. These are described and the results of system performance measurements are given.