322 BELL SYSTEM TECHNICAL JOURNAL 



There are two opportunities for undesirable interchanges of power 

 in the parallel phase shifter system; there can be an interchange be- 

 tween antenna branches through condensers in the same diversity 

 group, and an interchange between diversity branches through con- 

 densers in the same antenna branch. Both of these can be reduced 

 to the required values by a proper proportioning of input and output 

 impedances with respect to the condenser reactance. This results in 

 a large loss, about 40 db, through the phase shifting system. 



Input Circuits and First Demodulators 



Obtaining uniformity of phase shift in the circuits preceding the 

 first detectors is facilitated by reducing the number of circuit elements 

 and selectivity to a minimum. No high-frequency amplification is 

 used for this reason. The selectivity required to avoid image reception 

 is materially lowered by the choice of a high operating frequency for 

 the first intermediate frequency amplifiers. In addition to the usual 

 requirement of high selectivity and high voltage transformation ratio, 

 the input circuits must have uniformity in gain and phase, and must 

 properly terminate the transmission lines so that multiple reflections 

 will not shift the effective input amplitude or phase. 



These factors seemed to preclude use of the usual variable tuned 

 circuits on account of the time required to change from one frequency 

 to another, and consequently fixed tuned circuits were used. Five 

 input circuits are mounted on a panel and the switches for changing 

 the input and output circuits of the two panels of each receiver which 

 are mounted on a single bay are ganged. Each input circuit consists 

 of two anti-resonant circuits capacity-coupled to each other and to 

 the transmission line. It operates into the grids of the two first 

 demodulator tubes which are paralleled. The first beating oscillator 

 input is applied in push-pull between the cathodes of the tubes. 



Oscillators and Automatic Tuning System 



Three highly stable oscillators are used in the receivers, one for 

 beating the signal frequency down to 2,900 kc, the second for beating 

 it down further to 100 kc, and the third for use as a reference frequency 

 for the automatic tuning system, or it may be used in the final de- 

 modulator when the automatic branch selector is used. 



The first beating oscillator is of the coil-and-condenser type and 

 covers the range from 7,000 kc. to 17,000 kc. Automatic tuning is 

 used to compensate for long-time variations in its frequency, as well 

 as any variations in the transmitter frequency, but every effort has 

 been made to keep short-time variations to a minimum. The oscil- 



