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

摘要

In the equipment described, group delay is measured by means of a carrier which is frequency modulated with a signal at 1 Mc/s. The change of phase delay of the demodulated signal relative to the original 1 Mc/s signal, before and after the insertion of the network in the signal path, is the group delay of the network under test. The systematic accuracy of the method is considered in relation to the accuracy required for measuring waveguide networks suitable for use in microwave multi-channel telephony links. A superheterodyne system is employed in the equipment with an intermediate frequency of 70 Mc/s. For measurements on a display basis the signal- and local-oscillator frequencies are mechanically varied in synchronism to give a constant intermediate frequency. Automatic gain control is provided in the i.f. amplifier so that networks with losses up to 20dB can be measured; limiters reduce the effect of amplitude modulation produced by networks having a large attenuaion/frequency slope. The phase stability of the equipment is ensured by the use of a feedback amplifier in the 1 Mc/s path. A phase comparator of the modulator type is used in which errors due to the variation of the input level have been reduced. An accuracy of ±0.1 ns is obtained for point-by-point measurements on networks such as filters. Group-delay/frequency characteristics over a band 40 Mc/s wide may be displayed on an oscilloscope with an accuracy of about ±0.2 ns.