A Precise Direct Reading Phase and Transmission Measuring System for Video Frequencies

HE evolution of transmission networks for communications systems progresses through three fairly well-defined phases--design, synthesis and final adjustment. The design phase ordinarily involves no problem of measurement. In the synthesis stage, during which the physical model is constructed from the paper design, precise equipment is often needed for measuring the magnitude of the various components comprising the network. The adjustment stage, in which the network is actually tested as an element in a transmission circuit, generally requires the most complex instrumentation. In the latter category we m a y include insertion loss, gain, and phase measurement systems. Television and broad-band carrier facilities, such as the New YorkMidwest video cable link, employ vast numbers of transmission networks. These include, for example, filters, equalizers, and repeaters. T h e final adjustment of these networks requires a large number of precise insertion phase and transmission measurements during both development and manufacturing stages. Consequently, the measurement equipment must combine laboratory accuracy with speed of measurement suitable for use in production testing. The quantities measured are defined in Fig. 1. Conforming with current usage, the term Transmission is used herein to designate insertion loss and gain. T h e performance of the system with respect to frequency range, measurement range and accuracy is as follows: Frequency Range: 50-3600 kilocycles Generator and Network Termination Impedance: 75 ft Transmission Range: + 40 db to -- 40 db; Accuracy ± 0 .