ANALYZER FOR COMPLEX ELECTRIC WAVES 241 



effective resistance in the selective circuits. When these are made 1 

 ohm each, it requires only 1 microampere to give one milliampere of 

 rectified current. With 10 ohms coupling resistance only 10~^ amperes 

 input would be required. This, however, is a larger coupling than it is 

 desired to use, since the analyzer becomes too sensitive and susceptible 

 to mechanical vibrations as well as to electrical interference from out- 

 side sources. 



One desirable feature is that the sensitivity characteristic of the 

 complete current analyzer is a straight line so that doubling the input 

 will give twice the deflection in the meter reading the rectified current. 

 This is an advantage since, if the deflections and input amplitudes do 

 not change by the same ratio, the presence of interfering currents is 

 indicated. 



Selectivity. The selectivity of the current analyzer depends upon 

 the time constants of both the high and low frequency tuned circuits, 

 and to some extent upon the coupling resistances, but the latter are 

 usually relatively small and do not have an appreciable effect. The 

 discrimination obtained by the use of the heterodyne method and fixed 

 low frequency selective circuit is shown by curve C of Fig. 4. This 

 curve may be compared with curve A , which shows what can be done 

 with high grade elements in a single tuned circuit. The discrimination 

 of the complete analyzer, including the initial stages and the heterodyne 

 stage, is given by curve D. Tests with two frequencies show that if 

 one is 250 times as large as the other the smaller may be measured 

 without appreciable error if the difference in frequency is not less than 

 1 per cent; if the ratio of amplitudes is 1,000 to 1, the frequency differ- 

 ence need not be less than 2 per cent. 



Modulation. As to modulation in the current analyzer there are 

 two sources which contribute, the vacuum tubes and the tuning coils 

 and transformers. Of these the tubes are the most troublesome, since 

 they furnish both even and odd order modulation products, whereas 

 the coils contribute only to the odd orders. Of these the third is 

 generally the only one that is of any interest since higher odd orders 

 are too small to produce any interference. Modulation need be con- 

 sidered only when measurements are made of small components in the 

 presence of very large ones, as it is under these circumstances that 

 conditions for modulation are the most favorable. This condition 

 requires high sensitivity which is obtained by increasing the coupling 

 resistance, and large resistance means greater interference voltage on 

 the first amplifier and also less selectivity in the tuned circuits. The 

 result is an increased load on all sections of the current analyzer, which 

 causes modulation and may produce an error in the readings. It is. 



