76 



USRL TEST STATIONS 



INNER CONDUCTOR, 



OUTER CONDUCTOR 

 OF COPPER 



<r — INNER 



\l CONDUCTOR 



WE CO DUAL COAXIAL CABLE WITH TWO QUADS 



RUBBER COVERED 

 COAXIAL CABLE 



RUBBER COVERED 

 TWISTED PAIR 



COTTON COVERED 

 COAXIAL CABLE 



STRANDED INNER 

 CONDUCTOR 



RUBBER- 

 COPPER BRAJD 



ROPE FILLERS 

 RUBBER 



WE CO. 720 CABLE FOUR COAX RUBBER COVERED CABLE 



Figure 11. Types of coaxial cable used by USRL. 



tubes, heated bv regulated direct current, which have 

 a maximum gain of approximately +60 db (variable 

 in 10-db steps) between the 135-ohm transformer 

 input and a 600-ohm cathode-follower output cir- 

 cuit. The gain is controllable from —20 db to +60 

 db by a split attenuator, comprising two 40-db sec- 

 tions, connected at the grids of the first and third 

 stages. The attenuation preceding the first stage is 

 completely inserted before attenuation of the second 

 section is introduced, though both are operated from 

 a single shaft. Improved signal-to-amplifier noise 

 margin and higher undistorted output levels are ob- 

 tained by this method of gain control. 



In the receiving amplifier of system 2 is a second 

 attenuator covering 10 db in 1-db steps. There is also 

 included a supplementary amplifier, continuously 

 adjustable from approximately +20 db to +25 db, 

 that is used with the primary receiving amplifier 

 when additional gain is required. 



Detectors 



Detector circuits are used with each system for 

 obtaining frequency discrimination against back- 

 ground and inherent noise, harmonics, and water- 

 borne interference, particularly that from the other 

 system. 



The general principle of operation is shown in the 

 system block diagrams (Figures 6 and 7). The input 



signal / is impressed on the grid of a balanced carrier 

 suppression-type modulator through an attenuator 

 and a 150-kc low-pass filter circuit. The carrier fre- 

 quency, 650 kc — / kc, is brought to this modulator 

 from the signal generator through a tuned buffer 

 amplifier, controlled by an automatic volume control 

 circuit. The buffer amplifier is used primarily to ob- 

 tain an adequate margin between the carrier level 

 and the maximum signal level in order to minimize 

 the unwanted modulation products other than the 

 sum frequency. The output of the first modulator is 

 then passed through a buffer stage incorporating 

 tuned circuits. The tuned circuits pass only the sum 

 frequency, (650 — /)+/ = 650 kc, which is impressed 

 on the grid of the second modulator. The second 

 carrier frequency, from the detector tuning oscillator 

 circuit of the signal generator, is brought to this 

 modulator through a tuned buffer amplifier, which 

 has primarily the function of providing an adequate 

 carrier-to-signal level margin. In system 1 the detector 

 tuning frequency is 678 kc. The 28-kc difference fre- 

 quency from the second modulator is then impressed 

 on a crystal filter having an essentially square-top 

 pass band of about 12 c centered at 28 kc. The filter 

 is followed by one stage of tuned amplification termi- 

 nating in 135 ohms. 



The detector circuit of system 2 has three accept- 

 ance band widths provided by three crystal filters. 



