AN ELECTRICAL FREQUENCY ANALYZER 301 



core which, by means of a switch, may be thrown in scries or in 

 parallel, thereby chaiiyinj; the value of the inductance in the ratio 

 of U) to 1. With the same range of capacitance values this change 

 in intluctance gives the two frequency ranges of tuning, 20-12.50 

 c>cles and 80-5000 cycles. By means of the high-ratio trans- 

 former T the response of this circuit is applied to a vacuum tube 

 amplitier-rectifier and registered by means of the recording meter. 



This circuit arrangement will analyze a complex wave by virtue 

 of the selective shunting of current by the tuned circuit from the 

 input network. The impeilance of the source of the complex wave 

 is in practise maintained high in value at all frequencies compared 

 to that of the input network so that the input wave-shape is inde- 

 pendent of the small changes in impedance of the analyzer due to 

 the var>ing of condenser C. The current fed into the analyzer trav- 

 erses two paths, the input netw'ork and the tuned circuit. The 

 impedances of these paths are respectively, 



{Ri+jwLi)/jc^Ci 

 ' Ri+jwLi + 1/jwCi 

 a nd Z = R +ju}L + 1 /j(^C. 



The transformer T introtluces into the tuned circuit a small resistance 

 and inductance, both of which are negligible. The input network 

 impedance Zi varies gradually from 0.4 ohms for direct current to 

 about 10 ohms at 5000 cycles. The values of the elements are: 

 /?i = 0.4 ohms, Li = 0.075 milhenries, Ci = about 15 microfarads. 

 Impedance Z of the tuned circuit depends on the setting of the vari- 

 able condenser C. The resistance R of the iron-core coil, varies 

 with frequency; its values for the parallel connection are 0.7 ohms 

 for direct current, 1.5 ohms at 2500 cycles and 4.2 ohms at 5000 cycles 

 The value of the inductance L for the parallel connection is 23.4 

 milhenries and is practically constant with change of frequency. 

 For the series connection both R and L are sixteen times as great. 

 The capacitance is varied from about 200 microfarads to about 

 0.05 microfarads. It will be seen that for each capacitance value 

 there is a frequency, /,= l/(2ir V LC), for which the tuned cir- 

 cuit impedance, Z, is R. For other frequencies Z is much greater 

 due to the reactance. An incoming current of frequency /r is, there- 

 fore, largely shunted through the tuned circuit while current of any 

 other frequency passes through the input network. In this way 

 if the capacitance C is varied gradually the tuned circuit will shunt 

 selectively from the input network the successive components of 

 the complex wave. 



