Electrical Measurement of Communication 

 Apparatus^ 



By W. J. SHACKELTON and J. G. FERGUSON 



Synopsis: This paper describes precision high-frequency measurements 

 of a fundamental type, special emphasis being placed on the measuring 

 circuits rather than on the types of apparatus measured. Standards of 

 frequency, resistance, capacitance, and inductance are discussed briefly. 

 Bridge measurements are described for the measurement of frequency, 

 inductance, efTective resistance, capacitance, dielectric loss, capacitance 

 balance and inductance balance. Circuits for the measurement of other 

 high-frequency characteristics such as attenuation, gain, and cross-talk 

 are included. 



Introduction 



LONG DISTANCE electrical communication is now being eflfected 

 by means of frequencies embracing the audible range and extend- 

 ing from there to the so-called short wave-lengths employed in radio 

 transmission. According to the field of usefulness, this whole range 

 has been subdivided into the audio, the carrier, and the radio ranges. 

 From the viewpoint of the power engineer, all of the frequencies 

 embraced in these ranges are high frequencies, but to the communica- 

 tion engineer, only those frequencies in the upper regions are con- 

 sidered high. 



This paper discusses methods of measurement and measuring instru- 

 ments adapted to the measurement of communication apparatus over 

 this complete range. Most of the measuring apparatus described is 

 designed particularly for use at audio and carrier frequencies. The 

 measuring methods which are discussed are intended primarily for 

 laboratory use in connection with the development and inspection of 

 telephone apparatus prior to its application in the field. 



Many of the transmission problems in the communication field 

 involve the impedance characteristics of apparatus and circuits. In 

 the manufacture of apparatus, impedance limits are used to a very 

 great extent in inspection tests. Consequently, quantities of prime 

 importance are those defining impedance characteristics; that is, 

 inductance, capacitance and resistance at specified conditions, of 

 course, such as temperature, frequency, and current or voltage. Other 

 characteristics, of a less fundamental nature but nevertheless of con- 

 siderable importance, are attenuation, gain, inductance and capaci- 



1 Presented at the Regional Meeting of District No. 1 of the A. I. E. E., Pittsfield, 

 Mass., May 25-28, 1927. 



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