BACKGROUND NOISE IN BROADCASTING 



341 



made at signal levels which were low enough to stay well within this 

 range. 



The other set was provided with a diode rectifier which functioned 

 as a linear detector. In order to improve the linearity of the charac- 

 teristic, an initial bias was used and was adjusted to obtain the best 

 characteristic as indicated by the following test: 



If a large unmodulated carrier is impressed on a linear rectifier, 

 together with a much smaller unmodulated carrier, the beat frequency 

 output should be independent of the amplitude of the larger carrier 

 over a wide range. This phenomenon was observed experimentally 

 and the initial bias was altered until the range, over which the large 

 carrier could be adjusted without changing the output, was a maxi- 

 mum. In Fig. 4, the horizontal curve shows the magnitude of the 



y 80 





3 5 



2 o; 



20 30 40 50 60 70 80 90 100 110 



IMPRESSED ALTERNATING VOLTAGE IN ARBITRARY UNITS 



Fig. 4 — Characteristics of radio frequency amplifier and linear detector. 



audio-frequency output, while the sloping curve shows the direct 

 current flowing in the detector circuit. The dashed curve is due to the 



