122 



BELL SYSTEM TECHNICAL JOURNAL 



emphasis on accurate reproduction has recently led to the introduc- 

 tion of improved technique for checking this important characteristic 

 under dynamic conditions. The method employed consists of im- 

 pressing a pure sine-wave input on the transmitter at various fre- 

 quencies throughout the audio range and subjecting the output of 



-si \ I LU I I I I Mill 



30 



100 1000 



FREQUENCY IN CYCLE.S PER SECOND 



10,000 



Fig. 1 — -Frequency-response characteristic of Western Electric 7-A (50-kw) radio 



transmitter. 



a Straight-line rectifier to harmonic analysis. One type of harmonic 

 analyzer which has been used with excellent results is that due to 

 Wegel and Moore. ^ This device produces a photographic record, an 

 example of which is shown in Fig. 2. Measurements of this type are 



2ND HARMONIC -10.5% 

 3RD HARM0NIC= 9.0% 



75 100 125 



FREQUENCY IN CYCLES PER SECOND 



200 



Fig. 2- 



-Harmonic analyzer graph indicating overloading (2nd harmonic, 10.5 per 

 cent; 3rd harmonic, 9 per cent). 



of particular importance under present conditions since current Ameri- 

 can practice is tending toward the extensive use of transmitters in 

 which modulation is accomplished at relatively low power levels and 

 the required power output is obtained by means of subsequent stages 

 amplifying modulated radio-frequency power. Such amplifying stages 



2 R. L. Wegel and C. R. Moore, "An Electrical Frequency Analyzer," Bell Syst. 

 Tech. Jour., p. 299-323, April, 1924. 



