242 BELL SYSTEM TECHNICAL JOURNAL 



therefore, important to know the Hmitations which this imposes upon 

 it as a measuring device. Then with this information data can be 

 taken within known limits of accuracy. Measurements have been 



F,G.8 



Fig. 10- — Second and third order modulation in heterodyne current analyzer to- 

 gether with 5,000 and 13.000 cycle band-pass filters with one or two input frequencies 



made on the combined modulation occurring in the analyzer and also in 

 the filters which were necessarily associated with the measurements. 

 These were made with one input current and also with two input 

 currents of different frequencies, which were applied simultaneously 

 to the analyzer. One or two filters were therefore necessary to suppress 

 all components except the fundamental currents desired, but any small 

 amount of modulation which would occur in the filters would add to 

 that produced in the analyzer so that the results shown by the curve iri 

 Fig. lo represent the total modulation in both the filters and the ana- 

 lyzer. The modulation amplitude is expressed in terms of transmis- 

 sion units with respect to the current into the analyzer. This curve is 

 quite irregular and depends somewhat upon the frequency. It is the 

 lowest at 39,000 cycles where the modulation current is shown to be 86 

 7'C/^ down or 0.00005 as large as the current into the analyzer. Meas- 

 urements can, therefore, be made at this frequency of the modulation 

 occurring in any device when its amplitude is not less than 66 TU 

 below the amplitude of the fundamental, with a possible maximum 

 error of 10 per cent. At other frequencies this amplitude may be less, 

 as for instance at 21,000 cycles, measurements may be made up to 80 



^ For a discussion of this method of expressing current ratios see "The Trans- 

 mission Unit and Telephone Reference Systems" by W. H. Martin, Journal A. I. E. 

 E., June 1924, Vol. XLIII, No. 6; also Bell System Technical Journal, July 1924, 

 Vol. Ill, No. 3; also "The Transmission Unit," R. W L. Hartley, Electrical Com- 

 munication, July 1924, Vol. Ill, No. 1. 



