Equipment for the measurement of the insertion loss of waveguide networks in the frequency range 3.8ߝ4.2 Gc/s

The concept of insertion loss as applied to microwave networks is discussed and methods of measurement are reviewed. It is concluded that superheterodyne substitution methods enable precise measurements of insertion loss to be made at frequencies for which no standard attenuators exist. Equipment of this type for use in the range 3.8ߝ4.2 Gc/s uses a principle developed by C. M. Burrell. An intermediate frequency of 34.5 Mc/s is employed, and the 34.5 Mc/s output from the waveguide path, into which the network being measured is inserted, is compared with a signal at 34.5 Mc/s obtained from a crystal oscillator followed by a piston attenuator. Switching between the two signals is effected at a repetition rate of 1 kc/s, and detection of the composite 34.5 Mc/s signal yields a 1 kc/s output whose amplitude is proportional to the difference between the two 34.5 Mc/s signals; detection in a phase detector gives the sense of the difference. The piston attenuator is adjusted for zero output from the phase detector, and the difference of the attenuator readings, with and without the network inserted, gives the required insertion loss. The accuracy of measurement is ±0.05 and ±0.1 dB for losses of 6 and 30 dB, respectively, and losses of up to 60 dB, can be measured with decreased accuracy.