TD-2 MICROWAVE RADIO RELAY SYSTEM 



1063 



C. FM Receiver 



The FM receiver contains an IF amplifier, limiter, discriminator, and 

 video amplifier, as indicated in Fig. 17. The input ampUfier consists of two 

 stages, each using a 404A pentode, with broad-band interstage networks. The 

 two-stage instantaneous amplitude limiter has biased siUcon varistors shunt- 

 ing the single-tuned plate loads of each of the 41 8A tubes. The bias voltages 

 are so adjusted that the load impedance is high for signal voltages less than 

 about one volt, and very low for any larger signal. 



1. Discriminator 



The discriminator circuit follows early conventional practice, in that 

 two separately driven antiresonant circuits are used. The signal at the limiter 

 output is fed to two 404A amplifier stages, one tuned above the signal band, 



DISCRIMINATOR 



VIDEO 

 AMPLIFIERS 



Fig. 17— Block diagram of FM terminal receiver. 



the other below. The frequency-modulated signals produce amplitude varia- 

 tions of the voltage across these tuned circuits which are detected by diode 

 rectifiers and applied to the video amplifier. A potentiometer in the cathode 

 interconnection of the amplifier tubes provides a balance adjustment for 

 the discriminator. 



2. Video Amplifier 



The video amplifier is a three-stage resistance-capacity coupled unit 

 having negative feedback in each symmetrical half, and negative feedback 

 to longitudinal voltages through a common cathode resistor. The gain is 

 adjustable over a range of several db by means of a dual potentiometer 

 which varies the common cathode resistance in each half of the ampUfier. 

 Whenever such an adjustment is made, a constant loop gain is maintained 

 in the feedback system by varying simultaneously the local cathode de- 



