ANALYZER FOR THE VOICE FREQUENCY RANGE 



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cally. Moreover, inasmuch as this selective circuit would consist of a 

 number of highly resonant elements, it would be rather questionable 

 whether these elements would all be affected alike by ordinary varia- 

 tions in room temperature. Some experiments were then made using 

 mechanical resonance and these have given a very satisfactory solution 

 of the problem. 



The resonant element now in use consists of a steel rod clamped at 

 the center having the magnetic element of a telephone receiver at each 

 end with its poles separated a few mils from the end of the rod as shown 

 in Fig. 2. One of the receiver units is connected to the output of the 

 modulator and is used for driving the bar while the other receiver unit 



Fig. 2 — Mechanical resonant circuit 



is connected across the input of a suitable amplifier having a thermo- 

 couple and meter at its output. In order to minimize the effect of 

 other extraneous frequency components the amplifier is tuned to have 

 its maximum efficiency at the resonant frequency of the bar. The bar 

 is approximately 9 inches long and resonates to longitudinal vibrations 

 at 11,350 c.p.s. A frequency response curve of the bar, showing varia- 

 tion of the output of the driven receiver with frequency, is shown in 

 Fig. 3. It will be seen that a departure of 10 c.p.s. from the resonant 

 frequency gives a loss of over 25 7 f/ corresponding to a voltage ratio 

 of approximately one to twenty. Therefore, even when frequencies in 

 the unknown wave are as low as 50-100 c.p.s. the frequency dis- 

 crimination is quite satisfactory. It may be of interest to note that 

 the value of the reactance-resistance ratio Q, calculated from the curve, 

 is about 15,000 whereas the construction of an electrical inductance to 

 operate at this frequency having a value of Q over 200 would be diffi- 

 cult and expensive. 



