BACKGROUND NOISE IN BROADCASTING 



339 



cast band. The output of the second of the units was fed through a 

 radio frequency attenuator to the grid of a single stage amplifier, the 

 output circuit of which contained a step-down transformer bridged 

 across the transmission line feeding the radio receivers. With zero 

 loss in the attenuator the noise energy fed to the line was ample for the 

 purposes of the present study. 



An additional description of some of the pieces of equipment used 

 in the foregoing set-up may be of interest. 



Source of Constant Unmodulated Carrier Frequency 

 The oscillator contained in the No. lA Frequency Monitoring Unit 

 is of unusual frequency stability. The piezo-electric crystal is mounted 

 in a specially designed thermal insulating chamber which reduces the 

 temperature fluctuations to an extremely small fraction of a degree. 

 Voltage regulating equipment is included in the unit, giving further 

 assistance in stabilizing the frequency. Detailed descriptions of the 

 oscillator ^ and of the frequency monitor ^ have been published. 



A similar oscillator is used as a control unit at Station WABC. 

 Hence, it was expected that a very constant beat frequency could be 

 obtained between that station and the local oscillator. The frequency 

 of the latter was adjustable over a narrow range by means of a vernier 

 condenser in the crystal circuit. Figure 2 shows a number of plots of 



5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 



TIME IN MINUTES 



Fig. 2 — Beat frequency between WABC and Western Electric No. lA Frequency 



Monitoring Unit. 



the beat frequency against time. These curves indicate an extremely 

 slow drift, and experience has shown that the beat frequency would 

 hold to within 0.4 cycle over a period of at least five minutes, and 

 usually considerably longer. This high stability greatly facilitated 

 work which required a very small difference in frequency of the two 

 carriers. 



