A Precise Direct Reading Phase and Transmission Measuring 

 System for Video Frequencies 



By D. A. ALSBERG and D. LEED 



THE evolution of transmission networks for communications systems 

 progresses through three fairly well-defined phases — design, syn- 

 thesis 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 may include insertion 

 loss, gain, and phase measurement systems. 



Television and broad-band carrier facilities, such as the New York- 

 Midwest video cable link, employ vast numbers of transmission networks. 

 These include, for example, filters, equalizers, and repeaters. The fmal 

 adjustment of these networks requires a large number of precise insertion 

 phase and transmission measurements during both development and manu- 

 facturing stages. Consequently, the measurement equipment must com- 

 bine 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. 



The performance of the system with respect to frequency range, measure- 

 ment range and accuracy is as follows: 

 Frequency Range: 50-3600 kilocycles 

 Generator and Network Termination Impedance: 75 Q 

 Transmission Range: -|-40 db to — 40 db; Accuracy ±0.05 db 



— 40 db to — 60 db; reduced accuracy. 

 Insertion Phase Shift Range: 0-360°; Accuracy ±0.25 degree (+40 db 



to -40 db) 



The measuring circuit is based on the heterodyne principle whereby the 

 phase and transmission of the unknoicn are translated from the variable 

 frequency to a constant intermediate frequency at which the phase and 

 transmission standards operate. Accurate phase-shifters and variable 

 attenuators with negligible phase shifts are constructed readily for fixed 



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