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BELL SYSTEM TECHNICAL JOURNAL 



may or may not pass the intermediate-frequency amplifier and the 

 associated filters depending on their design. If the interfering com- 

 ponent lies on the opposite side of the wanted carrier from the oscillator 

 and differs from the former by the intermediate frequency, it will be 

 passed by the receiver, subject only to the attenuation due to the 

 radio-frequency circuits (the input circuits tuned to the wanted 



Schematic of triple detection receiver for television channel 



carrier). This characteristic must be given careful consideration in 

 the design of selective receivers of the superheterodyne type and has 

 led to the introduction of carefully designed, loosely coupled, input 

 circuits or an initial tuned radio-frequency stage for this purpose. 

 Neither of these expedients were possibilities in the television receiver, 

 however, because of the extraordinary width of the required trans- 

 mission band. Recourse was had, therefore, to a triple detection 

 arrangement. Speaking somewhat in the vernacular, the desired 

 signal was "beat up" to 5000 kilocycles where it was passed through 

 sharply tuned coupled circuits, then "beat down" to 120 kilocycles, 

 amplified, filtered and rectified, finally passing through a suitable low 

 pass filter, audio amplifier and output transformer to the television 

 reproducing apparatus. 



The circuit arrangement is shown schematically in Fig. 6. Two 

 tuned circuits with capacity coupling were connected to the input of 

 the first detector or modulator. A relatively tight coupling was 

 employed to produce the well-known double-peaked resonance curve 

 capable of affording the required band width. The antenna was not 

 tuned but was loosely coupled to the selective circuits by means of an 

 adjustable capacity. The incoming radio signal was impressed upon 

 the grid of the modulator tube along with a suitable voltage from an 

 oscillator operating at 6575 kilocycles. The 5000-kilocycle com- 

 ponents which resulted were selected by means of two carefully 

 designed tuned circuits also capacity coupled. The purpose of this 

 stage in the process will be evident if it is appreciated that at 1575 



