418 BELL SYSTEM TECHNICAL JOURNAL 



kilocycle while the mechanism is in motion. It can be seen under the 

 hood in the center of the panel. 



Below the coarse frequency dial is seen a small dial connected to a 

 variable condenser which permits the operator to vary the frequency of 

 the oscillator up to ± 50 cycles from the frequency to which it is set 

 and to read the frequency change with an accuracy of about 3 cycles. 

 This feature is found to be useful in locating peaks of frequency char- 

 acteristics of sharply resonant circuits. 



The frequency checks are made by operating a key which throws the 

 oscillator output across a telephone switchboard lamp and a 100 kc 

 crystal across the grid of the output stage. For the low frequency 

 check another key superposes the 60 cycles power main frequency on 

 the oscillator output and a screwdriver adjustment operating a con- 

 denser in the fixed oscillator adjusts the oscillator frequency to syn- 

 chronism with the scale set at 60 cycles. For the high frequency check 

 a minimum signal is obtained on the lamp with the scale set to 100 kc 

 by adjusting a padding condenser in the variable oscillator. 



In the modulator a pentode type vacuum tube is used, which has a 

 control grid-plate current characteristic which over nearly the entire 

 region from zero bias to cut-off approaches a parabola so closely, that 

 where modulation products lower than 40 db down on the useful out- 

 put can be neglected, only first and second modulation products need 

 be considered. The bias is placed in the middle of the parabolic range 

 and t"he two input signals are adjusted to equality and to a value 

 covering the entire parabolic range. This gives the maximum useful 

 modulation output necessitating the smallest amount of gain in the 

 output stage at little sacrifice in efficiency. The modulator being 

 parabolic, the only products of modulation other than the useful output 

 are the two high frequency input signals, their harmonics and sum 

 frequencies. These are eliminated from the output by inserting a filter 

 between the modulator and the output stage. Advantage is taken of 

 phase discrimination since the circuit is arranged in push-pull to de- 

 crease the filter requirements for some of the products, which are 

 generated in phase. 



The plate supply is obtained from a rectifier operating on the 60-cycle 

 main supply. It is provided with a vacuum tube regulator circuit 

 which keeps the plate and screen voltages constant over a ± 5 volt 

 variation of the power line voltage. The output control is obtained 

 by means of a potentiometer in the output amplifier input. Two out- 

 put impedances, 600 and 135 ohms, may be selected by operating a key. 



The apparatus is mounted on a standard 19-inch panel 28 inches high. 

 The bottom, the coolest part, is occupied by the oscillators; the middle 



