10-15] MISCELLANEOUS MICROWAVE COMPONENTS 551 



with a local oscillator signal in a crystal diode to obtain the IF or difference 

 frequency signal which then passes to the IF amplifier. 



The ultimate sensitivity of a radar receiver is limited not by the gain of 

 the receiver, but by the magnitude of losses and noise power present in the 

 input circuitry. The overall receiver noise figure is given by the expression 



A^, = Lc{Nu + yVc - 1) (10-27) 



where A^^ is the receiver noise figure 



Lc is the crystal diode conversion loss 



Nif is the IF amplifier noise figure 



and Nc is the crystal diode noise ratio or noise temperature, 

 all expressed as power ratios. 



The receiver noise figure is usually expressed in db, and as such is 



Noise figure (db) = 10 log Nr (10-28) 



This represents the best noise figure which theoretically can be realized with 

 the given values of L^ Nif, and Nc. There are factors not included in this 

 definition of noise figure which can deteriorate the receiver noise figure 

 realized in practice. Thus any loss between the antenna and the mixer will 

 reduce practical receiver sensitivity a corresponding amount; hence, 

 duplexer losses, etc., should be kept to a minimum. A further deterioration 

 of the practical noise figure can be caused by noise sidebands present in the 

 local oscillator (LO) output. In the simple single-diode mixer, this klystron 

 noise produces extraneous noise components in the IF output of the mixer 

 which result in a deterioration of receiver noise figure. Much of this noise 

 could be eliminated by use of a very narrow-band filter between klystron 

 and mixer, although this is not generally practical. The preferred method 

 of eliminating the effect of local oscillator noise is to use a balanced mixer. 

 Balanced mixers commonly employ some form of 3-db hybrid junction 

 and two crystal mixer diodes in a manner which permits the desired IF 

 output signals to add, while the klystron noise components cancel by 

 subtraction.-*" Most common are the short-slot hybrid mixers, ^^ the folded- 

 tee mixers, and the magic-tee mixers. Fig. 10-24 illustrates a balanced 

 mixer employing an /7-plane folded hybrid tee. In the mixer shown, the local 

 oscillator signal divides equally in the hybrid and arrives at the crystals 

 in phase. The input signal, isolated from the local oscillator, also divides 

 equally, but arrives at the crystals out oj phase. By using a push-pull IF 

 input and reversed balanced crystals in the mixer, the local oscillator noise 

 components will cancel in the mixer output circuit. A variety of input, 

 output circuit, and crystal arrangements are possible in balanced mixers. 



40L. D. Strom, "Noise Cancellation in Microwave Mixers," Tele-Tech 15, No. 3 86fF. 

 (March 1956). 



