THE RADAR RECEIVER 733 



sideband signal at this point. In the case of fire-control radar equipments, 

 where precision range measurement is required, it is desirable to determine 

 the range by reference to the leading or lagging edge of the radar received 

 pulse. Here the optimum band width may be considerably greater than 

 the value indicated above for a simple search system to assure a 

 minimum rise time of the displayed pulse and an accordingly more precise 

 determination of the position of the pulse. Additional factors which influ- 

 ence the radar receiver band width value in a particular system design 

 involve the frequency stability of the microwave generator and the local 

 beating oscillator, the sensitivity characteristics of the automatic frequency 

 control system, the frequency stability characteristics of the IF amplifier 

 itself, and finally the desire to permit ease of tuning by the radar operator. 

 The phase distortion introduced by the IF amplifier is of secondary in- 

 terest in the case of radar systems. The faithful reproduction of all char- 

 acteristics of the received radar pulse is usually not of extreme importance, 

 suice with few exceptions the criterion of presence is all important. The 

 detailed form of the transmission characteristic is likewise not extremely 

 critical, the usual "rounded" IF transmission characteristic, however, con- 

 tributing somewhat to ease in tuning the radar receiver. 



Gain Characteristics 



A consideration of the converted input signal levels encountered and the 

 video output level desired indicates the IF amplifier gain requirement. The 

 input signal to the IF amplifier is determined by the type of converter em- 

 ployed and the presence or not of radio-frequency amplification preceding 

 the IF section and the absolute noise power resulting. The video level at 

 the second detector must be maintained at a sufiiciently high level so that 

 microphonic disturbances within the remaining video components are neg- 

 ligible and low enough to assure satisfactory detection without serious over- 

 load effects. Undesired feedback at the IF frequency also tends to limit 

 the practical gain which can be introduced into the IF amplifier. In the 

 military radar systems of the past war period the usual maximum gain 

 associated with the IF amplifier was of the order of 110 db with a maximum 

 detector output level of approximately 1 volt rms of input circuit noise. 

 The extreme variation of the level of the desired radar signal makes neces- 

 sary that provision for a large gain control variation be included in the IF 

 amplifier design. This gain control often involves automatic, as well as 

 manual, adjustment and commonly a gain variation range of the order of 

 80 db is required. 



Another consideration which enters into the IF amplifier design and is 

 associated with gain features, is the behavior of the amplifier in the presence 

 of extremely large radar signals or enemy "jamming" signals. Optimum 



