LONG DISTANCE CABLE CIRCUIT 



585 



of current. No extraneous frequencies should appear in the output 

 and all the frequencies in the band should be transmitted from input 

 to output with equal velocity. With an average spacing of 50 miles 

 for the repeaters, 40 of these are required on a circuit 2,000 miles 

 long so that obtaining proper performance allows only very small 

 departures of the individual repeaters from the ideal characteristics. 

 With respect to equality of gain at different frequencies, if the top 

 and bottom frequencies of the band transmitted over a 2,000-mile 

 circuit are not to drop more than, say, 2 db, below frequencies in the 

 middle of the band, each amplifier is permitted to be only .05 db down 

 at the edges of the band. (This corresponds to a power difference of 

 1 per cent.) By the use of resistance coupling and high mutual 

 inductance transformers throughout, the amplifiers developed for this 

 system have been given the characteristics shown in Fig. 13. It will 

 be observed that between 100 and 10,000 cycles the gain differences 

 are less than .05 db while at 35 cycles the gain is only .2 db below 

 the gain at 1,000 cycles. 



+2 



O UJ +4 



UJ D 



a _i 



IT < 



UJ > 



UJ UJ 



Z 



z 

 < 



o 

 2 -4 



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I- -6 



CYCLES PER SECOND 



Fig. 13 — Gain-frequency characteristic of non-regulating repeater without line 



equalizer. 



With respect to departure of the amplifier from linearity, the effects 

 produced are largely caused by the vacuum tubes. Very little of 

 such distortion is introduced by the amplifier coils. Measurements 

 on one of these amplifiers have shown that with a single frequency 

 output of 1 milliwatt, which is about the average power corresponding 

 to the maximum program volume, the second harmonics are about 

 50 db weaker than the fundamental, i.e., differ in power from the 

 fundamental in the ratio 1 to 100,000. Other harmonics are lower 

 in magnitude. 



Non-linearity in the amplifier also manifests itself by change in 

 gain with current strength. In this amplifier a variation in load from 

 1 milliwatt to a much weaker load causes a change in gain of only 



