ANALYZER FOR THE VOICE FREQUENCY RANGE 219 



modulator and in conjunction with the wave to be analyzed produces 

 the familiar upper and lower sideband frequencies in the modulator 

 output.^ The modulator output is connected to a resonant element, 

 the natural frequency of which is left unchanged during the analysis, 

 and the response of the resonant element is measured by a suitable 

 amplifier and a meter. The tuned element of the analyzer makes use of 

 longitudinal vibrations in a steel bar which in the apparatus herein 

 described has a natural frequency of approximately 11,000 c.p.s. The 

 frequency range of the carrier oscillator is from the natural frequency of 

 the resonant element to 5,000 c.p.s. above. It will readily be seen that 

 if, for instance, there is a 1,000 c.p.s component in the unknown wave 

 and the carrier frequency oscillator is set at a frequency of 1,000 c.p.s. 

 above the natural frequency of the resonant element, the lower side- 

 band or difference component from the modulator will be at the fre- 

 quency of this resonant element. The process of analysis is then to 

 vary the frequency of the carrier oscillator gradually and to determine 

 the output from the resonant element at each desired frequency. 

 Inasmuch as the frequency range of the carrier oscillator is relatively 

 small, the entire variation of frequency can be accomplished by a single 

 air condenser. Therefore, the frequency setting of the analyzer may 

 be varied continuously instead of in discrete steps. The frequency 

 calibration chart of the carrier oscillator is arranged to show the input 

 frequencies of the unknown wave to which a given frequency of this 

 oscillator will correspond rather than to indicate the frequency of the 

 oscillator itself. 



It has been found convenient, for comparing the magnitudes of the 

 various frequency components in the unknown wave, to use an auxiliary 

 oscillator supplying current to a potential attenuator. The frequency 

 of this oscillator is maintained at the frequency of the tuned element. 

 The input terminals of the amplifier may be connected either to the out- 

 put of the resonant circuit or to the output of the potential attenuator. 

 Theprocedure is to note the deflection on theoutput meter of the amplifier 

 produced by the resonant element and then switch the amplifier to the 

 attenuator and to reproduce this deflection. The frequency compo- 

 nents can readily be compared in this manner and their relative magni- 

 tudes determined directly in TU by reference to the attenuator dial 

 setting. 



In order to give a clearer idea of the operation of the analyzer the 



pertinent theory of the vacuum tube modulator will be discussed in 



the Appendix. 



^ A. I. E. E. Journal, April 1921 — "Carrier Current Telephony and Telegraphy," 

 by E. H. Colpitis and O. B. Blackwell. 



15 



