Superheterodyne Measurement of Microwave Attenuation at a 10-kHz Intermediate Frequency

The excellent relative frequency stability of the output of two microwave oscillators phase-locked to a common reference signal permits the use of an audio intermediate frequency in the superheterodyne measurement of microwave attenuation. The phase-lock feature also permits the measurement of microwave phase at the audio frequency. The choice of the audio instead of the more conventional 30-MHz intermediate frequency is made on the basis of the low cost and convenient operation, for similar accuracy, of an audio ratio transformer and audio phase shifter in comparison with a 30-MHz cutoff attenuator and 30-MHz phase shifter. A measurement system of this type has been operated at frequencies ranging from 2.5 to 18 GHz. The basic reference signal for phase lock is fed into harmonic mixers associated with the signal source and local oscillator. The outputs of the harmonic mixers go to phase discriminators that control the microwave oscillator frequencies. The precision of measurement at a signal frequency of 10 GHz varies from ±0.0002 dB for an attenuation step of 10 dB or less, to ±0.001 dB for a 50-dB step.