406 



REGULATORY CIRCUITS 



function of the tuning accuracy of the receiver. To obtain the maximum 

 performance from a radar system the IF bandwidth must be matched to 

 both the received pulse and the tuning error of the receiver. Fig. 8-8 shows 

 the loss in signal-to-noise ratio (sensitivity) as a function of a static tuning 

 error in the receiver. The video bandwidth is considered to be very much 

 larger than the IF bandwidth. 



s 1 

 o 

 fe 2 



UJ 

 Q- 



^ 3 



CO 5 

 o 



0.5 1.0 1.5 2.0 



AfT (TUNING ERROR x PULSE LENGTH) 



Fig. 8-8 Loss in Sensitivity with Tuning Error. After R. P. Scott, Proc. IRE 

 (Feb. 1948) p. 185. 



To obtain maximum system performance a compromise between AFC 

 performance and bandwidth must be made to provide maximum SjN. 

 Maximum performance is obtained with Bt ==1.0 when the tuning error 

 is negligible. In a MOPA (master oscillator power amplifier) system it is 

 feasible to realize this condition. In the case of a separate-pulsed trans- 

 mitting oscillator and a continuous-wave klystron local oscillator, some 

 allowance for the static error must be made. The reference is a frequency 

 discriminator and associated envelope rectifiers. The reference must be 

 tuned to the center frequency of the receiver IF and must be stable within 

 environmental conditions encountered. With an IF frequency of 30 Mc 

 the reference can be made to be accurate within ±50 kc of the IF. When 

 a master oscillator, operating continuously, is employed in the radar to 

 obtain the transmitted signal, much greater accuracy can be obtained by 

 the use of crystal control of the IF frequency. The static accuracy that can 

 be realized in the usual radar employing two oscillators is dependent on the 

 accuracy of the reference and the amount of zero frequency gain it is 

 practical to employ in the AFC feedback loop. 



The dynamic error characteristic must also be considered in choosing the 

 bandwidth. Since the AFC of a pulsed radar set is a sampled-data feedback 

 control device, the error reduction that can be achieved is dependent on the 

 sampling rate. In practice, the effective bandwidth of a continuous propor- 

 tional error AFC is limited to about one-twentieth the PRF. Tuning errors 



