THE RADAR RECEIVER 735 



The Second Detector 



The final conversion of the IF signal to a video form is accomplished by 

 simple detection. This process is usually associated with the IF amplifier 

 because of the relative ease by which the video signal can be transmitted to 

 the following display circuits, usually physically removed from the IF ampli- 

 fier, as compared with the transmission problem which exists at the inter- 

 mediate frequency. 



Two types of second detector circuits which are commonly employed in 

 radar receiver design are the linear diode rectifier and the plate circuit detec- 

 tor, both similar in form to those employed in prewar television practice. 

 Several factors must be considered in the choice of the second detector 

 operating characteristic. Linear detection of the signal is desirable from 

 the standpoint of realizing the greatest possible visibility of weak radar 

 signals in the presence of noise of comparable amplitude. In the case of 

 lobing radar signals where bearing determinations are made by comparison 

 of the return signal amplitude for two bearing conditions of the antenna 

 radiation, the characteristic of the second detector enters to affect the sensi- 

 tivity of azimuth response in two ways. The lobing sensitivity is increased 

 by the use of a square law detector, however, the presence of a "jamming" 

 signal of the CW type which may be located off axis can introduce a false 

 bearing indication, which is not present when a purely linear detector is 

 employed. In general, the linear detector has been employed in most radar 

 systems developed during the past war period. 



The detailed circuit design of the IF amplifier can be conveniently sepa- 

 rated into three quite distinct parts. These include the input circuit design, 

 the interstage arrangement, and the design of the second detector circuit. 



2.22 IF Amplifier Input Circuit Design 



The primary consideration in the IF amplifier input circuit design is the 

 effect of this stage on the over-all receiver noise figure. As indicated pre- 

 viously, this over-all receiver noise figure is dependent on the performance 

 characteristics of the first or input IF amplifier stage to a degree dependent 

 on the loss of the converter stage preceding it. In the military radar field, 

 employing microwave frequencies of 3000 mc and greater, the crystal con- 

 verter is universally employed and the over-all noise performance is deter- 

 mined largely by the IF input stage. The use of high-gain pentodes in the 

 IF amplifier assures that noise contributions from the following stages 

 are negligible. 



Figure 26 represents an equivalent input circuit of the IF amplifier con- 

 venient for discussion of the noise performance of this circuit. Here the 

 noise contribution, exclusive of the signal source, is observed to be composed 

 j of two sources, one due to shot noise of the first IF stage referred to the grid 



