7-5] 



MIXERS 



357 



appears to be the best suited for radar receivers, although the noise figure 

 is somewhat higher than the reflection type. Many practical problems 

 associated with these amplifiers, such as stabilization of the loaded ^,'s of 

 the resonant circuits and regulation of pump power level, must be solved 

 before these amplifiers find large use in airborne radar sets. However, 

 these negative-resistance amplifiers appear to be a final step in attaining 

 receivers whose sensitivity is truly limited by external noise. 



7-5 MIXERS 



The SHF (super high frequency) mixer in the majority of airborne radar 

 receivers incorporates crystal diodes. Properties of the crystal mixer which 

 are important to radar system operation are: 



1. The effective noise temperature 



2. The conversion loss 



3. The intermodulation components 



A crystal mixer can be represented by an equivalent circuit, as is shown 

 in Fig. 7-3a. The nonlinearity of the crystal arises from the variation 



Zj Local Oscillator 

 Source Impedance 



Crystal 



Image Zero Frequency 



Impedance Impedance 



(a) (b) 



Each Impedance Shown External to the Crystal 

 is Zero to All Frequency Components Except 

 the One to Which it Refers 



Fig. 7-3 (a) Equivalent Circuit of Crystal Mixer and (b) F-/ Characteristics of 



a Mixer. 



in the barrier resistance Rb which is a function of the voltage applied to the 

 crystal. A typical transfer characteristic is shown in Fig. 7-3b. The 

 spreading resistance Rs and barrier capacitance Cb are detrimental parasitic 

 elements. Because of these elements, not all of the heterodyne signals' 

 energy can reach the IF and image termination. 



