An Electrical Frequency Analyzer' 



By R. L. WEGEL and C. R. MOORE 



SvNorsls: An .ipiMr.itus has Ik.-cii (li-vcli>|>i'<l by means of which it 

 is possihli- to measuic and olitain a permanent record of the frequency 

 components of an electric current wave. The device has two frequency ran- 

 ges: 20 to 1250 cycles and 80 to 50(X) cycles; the amount of power required 

 docs not in general exceetl 500 microwatts; and the time nccessar>' for 

 making a record is about 5 minutes. An attachment is provided which 

 permits of the making of simultaneous harn\onic analyses of two complex 

 waves in the s;ime length of time. 



In principle, the pr<x'css consists in feeding the complex wave to be 

 analyzed into a seleitive network, the essential feature of which is a 

 sharply tuned circuit whose frequency of tuning is controlled by varying 

 the capacitance in small steps with a pneumatic apparatus similar to 

 that in a player piano. A maximum of res|X)nse of the circuit occurs at 

 each fretjuency of tuning which coincides with a component of the com- 

 plex wave. An automatic photographic recorder of the response to each 

 frequency of tuning is provided by means of which the frequency and 

 magnitude of each com|)onent of the complex wave may be obtained. 

 For convenience of operation, an automatic control apparatus is pro- 

 vided, so that it is only necessary to connect the complex source or sources 

 to be analyzed and press a starting button. The completed record of the 

 analysis is delivered after the machine has passed through the entire range 

 of frequencies. 



The application has so far been principally to problems in the com- 

 munication field such as the analysis of performance and distortion at 

 audio lre<iuencies of vacuum tube and mechanical oscillators and ampli- 

 fiers, analysis of complex telephone waves and speech sounds, and the 

 effect on a complex wave of transmission through electrical and acoustic 

 apparatus. In the power field many applications are obvious, such as 

 for example, quantitative comparison as to frequency content of the 

 voltage and current supplied to and delivered by transformers, voltage 

 and magnetic flux studies in generators and motors, commutation, and 

 the effect of wave-shapes in power transmission line problems and con- 

 trol apparatus. 



Introduction 



THE harmonic analyzer described in this paper consists of a 

 variable tuned circuit into which the complex current wave 

 to be analyzed is introduced, and an automatic recording apparatus 

 to register its response as the frequency of tuning is changed. 



The first recorded use of a tuned circuit as an analyzer was by 

 Pupin in 1894.- He analyzed power waves by measuring the response 

 of circuits tuned to each of the harmonic frequencies. It has been 

 the practise for a number of years to determine the frequency char- 

 acteristics of currents and voltages on power circuits and noise on 

 telephone lines by means of a variable resonant circuit which includes 

 a telephone receiver for listening. 



' Presented at the Midwinter Convention of the A. I. E. E., Philadelphia, Pa., 

 February 4-8, 1924. 



' Resonance .-\nalvsis of Alternating and Polyphase Currents, Trans. .A. I. E. E., 

 Vol. XI, p. 523. 



299 



