ANALYZER FOR COMPLEX ELECTRIC WAVES 



237 



the low frequency output and a given input component regardless of the 

 presence of other interfering currents in the input side. Through the 

 use of a balanced circuit intermodulation is reduced considerably 

 below the limit possible with a single tube modulator. 



The output of the modulator is connected directly to a double tuned 

 circuit which selects the low frequency modulation product correspond- 

 ing to the component of the complex wave being examined. The 

 frequency to which this circuit is adjusted is, as already mentioned, 

 800 cycles. 



Low Frequency Amplifier-Rectifier. The 800 cycle output from the 

 modulator is generally too small to measure on a meter of the usual type 

 without first being amplified. For this reason a low frequency ampli- 

 fier has been added and the output of this rectified so that all measure- 

 ments could be made on a sensitive D.C. meter, having a full scale 

 deflection of 1 or 2 milliamperes. 



RECTIFIED 

 CURRENT OUTPUT 



Fig. 6 — Low frequency amplifier-rectifier for heterodyne current analyzer 



The combined low frequency amplifier-rectifier circuit is shown in 

 Fig. 6. A step-up transformer is used between the amplifier tube and 

 the rectifier in order to increase the amplification so that, for a given 

 A.C. output, a smaller input to the first amplifier might be used. 

 Since this circuit is always to be operated at the one frequency (800 

 cycles), its overall frequency range characteristic is not of particular 

 interest and its performance was studied only at the one frequency. 

 The rectifier is of the grid leak and condenser type. Its performance 

 differs from the usual type of rectifier since it is operated over that 

 portion of its characteristic which gives a linear relation between input 

 voltage and direct current output. By suppressing the space current 

 corresponding to zero input the accuracy with which data can be taken 

 is greatly increased. As shown by the circuit arrangement in the 

 output side, part of the "A" battery current is used to oppose the space 

 current through the meter. This permits using a meter of high sen- 

 sitivity, having a full scale deflection of one or two milliamperes, on 

 which 1/lOOth part of a milliampere can easily be read. 



