1000 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



individual transmission characteristics will produce cumulative errors, 

 making it necessary to maintain close control over the manufacture 

 and adjustment of all of these amplifiers and associated networks. This 

 calls for networks of highly refined design and requires ancillary meas- 

 urement facilities of greater precision than heretofore available at these 

 higher frequencies. 



The design of transmission networks to meet exacting requirements 

 is a subtle art, embracing on the one hand the use of complex mathe- 

 matical manipulation to produce theoretical networks having the de- 

 sired loss and phase characteristics, and requiring, on the other hand, a 

 down-to-earth knowledge of the properties of the actual components 

 used including parasitic effects and interaction of the various elements 

 when assembled into a network. To furnish this knowledge, to measure 

 the component resistors, capacitors, inductors and transformers which 

 are the building blocks of the networks, to evaluate the ever-present 

 parasitic effects, to determine simplified circuit equivalents of the more 

 complex components such as transformers, and to answer other ques- 

 tions too numerous to mention, measurements of impedance parameters 

 - precise measurements - are required. 



EXISTING BRIDGE TECHNIQUE 



For measuring impedance and admittance parameters, that is R,. L, 

 C and G, suitable ac bridges, ordinarily simply designated as impedance 

 bridges, have long held a high place in the Bell System because of their 

 inherent reliability and precision, and their ability to cover a wide 

 range of A^alues. The development of many of the original bridges^' ^' ^' ^ 

 for frequencies above the audio range stemmed from the needs of the 

 earlier carrier systems. With this development came also analysis of 

 shielding technique, standardization of capacitance,^' ^ and a syste- 

 matic classification of bridge methods^ by J. G. Ferguson in 1933, in 

 which bridges were grouped into two major types designated as ratio- 

 arm and product-arm, respectively. Following this classification, com- 

 bined impedance and admittance bridges were developed,^' ^° utiliz- 

 ing a single set of bridge standards for both kinds of parameters by 

 changing the configuration of the bridge network. There have also 

 been special purpose bridges^ , 12. i , h ^^^^ ^^^ ^^ audio and the lower 

 carrier frequencies. More recently, coaxial impedance standards^^ having 

 values calculable from physical dimensions have been developed. 



Bridges for frequencies above one-half megacycle were used in the 

 Bell System as early as 1919,^ but relatively few bridges were built 

 until the mid 1930's when new carrier systems required bridges in the 



