7-6] COUPLING TO THE MIXER 361 



A frequency-dependent part of tm has been observed to vary as {filfi)n, 

 where/2 and/i are IF frequencies and n is between 0.5 and 1.^ Since the IF 

 noise figure varies approximately as/i//2 at high IF, the IF frequency 

 at which minimum Free is realized is not critical. 



The local oscillator is usually a klystron with a wide electronic tuning 

 range. Such oscillators exhibit shot noise whose spectra are determined by 

 the ^ of their resonators. To minimize intermodulation components be- 

 tween such noise and the local oscillator signal a high IF frequency is 

 desirable. The use of a balanced mixer, however, reduces this noise sig- 

 nificantly. 



In any mixer design, the objective is to provide minimum tm and L. 

 The conversion loss L depends on the match between the signal source at 

 both the signal and image frequency, the RF signal frequency, the IF 

 frequency, the crystal biasing, and the local oscillator signal level. The 

 noise temperature tm is also dependent on L, the IF frequency, and local 

 oscillator signal. 



7-6 COUPLING TO THE MIXER 



To obtain minimum noise figure, minimum mixer conversion loss must be 

 realized. Conversion loss is defined on an available power basis; therefore 

 the conversion loss does not depend on the actual IF load admittance 

 connected to the mixer. The conversion loss, however, is dependent on 

 the RF signal source admittance. To obtain minimum conversion loss at 

 the principal beating frequency (signal frequency beating against local 

 oscillator frequency) a mismatch is required between the mixer and the 

 source. The input admittance of the mixer, however, depends on the IF 

 conductance seen by the mixer. This in turn depends on the design of the 

 first IF stage of the receiver and the network which connects it to the mixer. 



A condition frequently encountered in airborne radar receivers is that 

 the IF admittance is very large incident to the use of a double-tuned trans- 

 former between the mixer and first IF tube. The secondary circuit is usually 

 damped only by the coil losses and circuit losses. A very large admittance 

 is therefore coupled into the primary circuit near the resonant frequency 

 of the secondary circuit. 



For this type of coupling between mixer and IF amplifier, an optimum 

 mismatch between the signal source and the mixer is given approximately 

 by 



" = zi^ (^-"' 



where p is the VSWR (voltage standing wave ratio) at the signal frequency 

 and Lo is the optimum conversion loss. 



3P. D. Strum, "Some Aspects of Crystal Mixer Performance," Proc. IRE 41, 876-889 (1953). 



