An Analyzer for the Voice Frequency Range 



By C. R. MOORE and A. S. CURTIS 



[Editorial Note: The frequency analyzers described in this paper and 

 in the paper immediately following, demonstrate in an unusual manner 

 how a single fundamental principle may be employed to accomplish quite 

 dissimilar results. The analyzers described in both papers employ a 

 resonating element of fixed frequency and translate the wave components 

 under study to this frequency by heterodyning them with the output of 

 variable frequency oscillators. In the analyzer described in the first paper, 

 the wave components under study are translated to a higher frequency 

 while in that described in the second paper the translation is downward to a 

 lower frequency. In view of these differences in design it is desirable to call 

 particular attention to the reasons which have led to the working out of the 

 two designs. 



The analyzer discussed by Moore and Curtis has been so designed as to 

 sweep through the voice frequency range to as high as 5,000 cycles by the 

 manipulation of a single control. To accomplish this end, it was found 

 desirable to heterodyne upward by employing a variable frequency oscillator 

 of considerably higher frequency than 5,000 cycles. The frequency of this 

 oscillator can be varied continuously throughout the range from about 

 11,000 cycles to 16,000 cycles, and the fixed frequency resonating element 

 is tuned to about 1 1,000 cycles. As translation of the wave under study to a 

 higher frequency range reduces the percentage separation of the various 

 components, it was necessary to choose a very sharply tuned resonating ele- 

 ment. This takes the form of a steel rod which is loosely coupled magneti- 

 cally to a driving circuit at one end and a registering circuit at the other. 

 As the modulator used to accomplish the heterodyning process produces 

 many frequencies other than the first of the "sum" and "difference" terms, 

 it has been necessary to choose the frequency ranges such that all undesired 

 frequencies which can not be made extremely small will be well removed 

 from the single difference frequency under observation. 



The analyzer described in the paper by Landeen is capable of working 

 over the range from about 3,000 cycles to 100,000 cycles. A requirement 

 of this design was that very high resolution be obtained. To assist in ac- 

 compUshing this end, the frequencies under study are translated downward 

 in the frequency scale to the resonating element which consists of a circuit 

 tuned to 800 cycles. This downward translation increases the percentage 

 difference of frequency separation of the components under study. Because 

 of the great range of frequencies covered by the analyzer it is not possible 

 to have sum and difference terms other than those of the second order fall 

 outside of the range of sensitivity of the resonator. The modulator has 

 therefore been so designed as to preclude formation in the higher order 

 terms. To increase its discrimination, the analyzer makes use of two tuned 

 circuits and amplifiers arranged in tandem and placed before the modu- 

 lator. The frequency to which these circuits are tuned must of course be 

 variable and is set to coincide with the component under study.] 



THE present analyzer was designed to aid in the solution of certain 

 problems arising in the study and development of commercial 

 telephone transmitters. These problems require high discrimination 

 and the accurate measurement of frequency components in the presence 

 of much larger components. 



The present analyzer differs fundamentally from that described by 

 R. L. Wegel and one of the present authors about two years ago.^ 



^ "An Electrical Frequency Analyzer," by C. R. Moore and R. L. Wegel, A. I. 

 E. E. Journal, September 1924; Bell System Technical Journal, October 1924, 



217 



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