362 THE RADAR RECEIVER 



The source admittance can be designed on this basis when the IF cou- 

 pling circuit is as specified. 



When the image frequency signal generated at the mixer is allowed to 

 be radiated by the antenna or dissipated in the local oscillator source 

 admittance, the value of /><, is that which is normally specified by the crystal 

 manufacturer as conversion loss. 



In many airborne radar receivers a short-slot hybrid junction is employed 

 in a balanced mixer. When the crystals are matched in such a mixer, all 

 of the image frequency signal generated in the mixer propagates out the 

 local oscillator port. Normally this port is matched, therefore the image 

 signal energy is lost. 



In some radar sets a filter may precede the mixer to reduce interference 

 from other radar sets. Such a filter may appear as a susceptance at the 

 image frequency and reflect the image signal originating in the mixer. If 

 the signal arrives in the correct phase at the mixer crystals, the performance 

 is improved. The phase depends on the distance between the mixer and 

 the filter. However, the distance between the mixer and the filter is also 

 dependent on the mixer to IF coupling circuit, since the filter would be 

 situated so as to give the optimum mismatch of the source to the mixer. 

 To obtain lowest receiver noise figure, design of the RF and IF circuits 

 must therefore be considered jointly, not separately. One solution to this 

 problem might be the use of the short-slot hybrid with a filter in both signal 

 and local oscillator paths. 



7-7 IF AMPLIFIER DESIGN 



The IF amplifier consists of a cascaded arrangement of vacuum tube 

 amplifiers which employ band pass coupling networks. Frequency of 

 operation is a compromise between several factors such as noise figure, 

 circuit stability, spurious responses, and receiver tuning characteristics. 

 Consideration of these factors usually leads to the choice of an IF frequency 

 between 30 and 60 Mc in the ordinary pulse-type airborne radar receiver. 



The IF amplifier is a filter amplifier, and its small-signal transfer function 

 is given by 



G(s) = H — — —^ ;^;^37-r - . (7-18) 



In this expression // is a constant depending on the number of tubes, 

 their transconductance, and the capacitance values of the circuits; s is the 

 complex frequency variable a -\- j(ji-, n is the number of circuits; q and m 

 are determined by the network complexity. The transfer function vanishes 

 when s = because of the numerator term. The function thus has a zero 

 of order nq at the origin. The denominator can be factored into 



{s - s,){s - s,*){s - s,){s - S2*)(s - s,)(s - .^3*) - • (7-19) 



