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BELL SYSTEM TECHNICAL JOURNAL 



240 volts since its tubes are subjected to the heaviest input, being pre- 

 ceded by only one selective circuit which can but partially eliminate 

 large interfering currents. This seems a most unusual arrangement 

 until it is remembered that, although the amplitude of a particular 

 part of the current may be increased, the total load on the first stage 

 may well be greater than that on the final stage. This is the reverse 

 of the situation in cascade amplification where the first tubes handle 

 only a small current and the succeeding ones a proportionately larger 

 current. 



The discrimination of one of the tuned circuits when a coupling 

 resistance of 1 ohm is used is given by curve A of Fig. 4. Curve B 

 shows the effect of adding the second tuned circuit. If regenerative 

 amplification were employed, both the selectivity and amplification 

 could of course be greatly increased, but this has not been used because 

 of the necessity for high stability and measurement precision. 



Modulator. As previously mentioned, the second amplifier unit 

 works into a modulator, the circuit of which is shown in Fig. 5. This is 



LOW FREQUENCY 

 TUNED CIRCUIT 



TO LOW FRE- 

 QUENCY AMPLIFIER- 

 RECTIFIER 



Fig. 5 — Modulator for heterodyne current analyzer 



of a two tube balanced type In which modulation, or frequency trans- 

 formation, takes place in the grid circuit. The heterodyning fre- 

 quency is applied in the common input lead across a suitable resistance. 

 The input from the amplifier is applied through a transformer across 

 the grids of the two tubes in series with a high resistance in each side. 

 No biasing potential is applied on the grids. A modulator operated in 

 this manner has the property of giving a modulation output propor- 

 tional to the smaller of the two input currents and independent of the 

 larger. The amplitude of this output may, therefore, be determined 

 entirely by the amplitude of the component being measured. Another 

 desirable characteristic of this type of modulator is that its efficiency is 

 not affected by interference, hence it will show a fixed relation between 



