Impedance Bridges for the 

 Megacycle Range 



By H. T. WIT-1IK1.M 



(Maiui.scrii)f received August 19, 1952) 



This paper reviews ac bridges developed for use in the Bell System for the 

 measurement of impedance parameters, particularly at frequencies in the 

 megacycle range. Three recent bridges designed for measuring networks 

 and components for coaxial systems are described. 



INTRODUCTION 



The need during recent 3'ears for increased accuracy of impedance 

 measurement in the megacycle range has led to advances in the art of 

 l)ridge measurement. A particular stimulus has been the development 

 of a new coaxial system, designated L-3, for transmitting over distances 

 up to several thousand miles a continuous frequency band extending 

 roughly from 0.3 to 8 megacycles per second. Such a system will be 

 capable of providing on a single coaxial unit the combination of a single 

 television chainiel and as many as GOO one-way telephone channels. 

 The large loss inherent in transmitting this wide frequency band o\'ei' 

 the cable makes it necessary to provide an amplifier about every four 

 miles, and these amplifiers and associated networks have created diffi- 

 cult measurement problems. 



MEASUREMENT PROBLEMS 



The measurement problems arise partly from the wide frequency 

 l)and, approximately thirty times the minimum frequency. This makes 

 equalization of the system for satisfactory transmission very difficult, 

 particularly in transmitting a television signal which covers a frecjuency 

 Inuid equivalent to about a thousand telephone channels and which 

 must be equalized for phase as well as loss. 



Even more important, however, ai'c the problems arising from the 

 close spacing of the amjjlifiers, with the i-esult that a transcontinental 

 circuit re(iuires up to a thousand amplifiers in its path. Departures in 



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