218 



BELL SYSTEM TECHNICAL JOURNAL 



It is of the type employing a single resonating element of fixed fre- 

 quency, the component waves under study being translated in fre- 

 quency to it by heterodyning with the output of a variable frequency 

 oscillator. It was deemed essential that the analyzer be so designed 

 as to cover the entire voice range up to 5,000 cycles by the manipula- 

 tion of a single control. In the presenta analyzer this control is the 

 variable condenser of the heterodyning oscillator. Two of these 

 analyzers have been built and are in use in the Bell Telephone Labora- 

 tories. 



The development of this new form of analyzing device was under- 

 taken only after a careful review of existing types. Such a study led 

 to the conclusion that none of the available forms was applicable to 

 the solution of our type of problem. This problem which had to do 



SOURCE OF 

 UMKNOWN WAVE 



CARRIER 

 FREQUENCY 

 OSCILLATOR 



AUXILIARY j j j 



OSCILLATOR I T"! I , ^ „| 



SET AT \—\ t— 1A"^TEN^AT0R}- 



FREQUENCroH l-^-' [ | 



RE50NANTaRCun| "icitn I 



Fig. 1 — Functional Diagram 



with the study of commercial telephone transmitters did not require 

 the analysis of a complex wave containing a fundamental frequency 

 and its associated harmonics. What we were interested in was the 

 measurement of the transmitter output at a particular frequency in 

 the presence of a much greater output at other frequencies. For 

 example, the measured component may be only one per cent of the 

 magnitude of the disturbing component and separated from it by only 

 30 to 40 c.p.s. It is evident that if the frequency of the component 

 which we wish to measure is close to that of one of the disturbing com- 

 ponents and much smaller in magnitude a very high degree of fre- 

 quency discrimination on the part of the analyzer is necessary. 



A functional diagram of the analyzer is shown in Fig. 1. The wave 

 to be analyzed is impressed at the " voice ' ' input terminals of a balanced 

 modulator. The variable carrier frequency is also supplied to the 



