360 THE RADAR RECEIVER 



The image frequency signal appears across the crystal and propagates 

 down the waveguide toward the local oscillator and the antenna. If the 

 image wave sees a match, such as would exist if it were allowed to enter 

 the local oscillator channel, the energy in this signal is dissipated and energy 

 that could have appeared in useful IF output is lost. Proper reflection of 

 the wave can cause it to enter the mixer and arrive at the crystal in proper 

 phase so that the output IF is increased. Optimum handling of the image 

 can improve the noise figure about 1 or 2 db. In general, however, con- 

 ventional pulse type airborne radar receivers have broad band mixers. 

 The image conversion is terminated and the lowest possible noise figure 

 is not obtained. 



A number of other intermodulation components involving the second 

 harmonic of the local oscillator occur and can be significant when the RF 

 acceptance bandwidth is great. 



The crystal diode voltage-current relationship is given by 



'( 



exp|^-l) (7-15) 



where e = electronic charge 



V = applied voltage 



K — constant depending on crystal 



T = temperature of the junction. 



Shot noise is exhibited by the crystal; the mean square fluctuation current 

 is Pdf = 2eIo4f, where /« is the d-c current through the crystal. Equation 

 7-15 indicates that a given conversion loss could be obtained with a lower 

 d-c current by reducing the temperature and therefore producing less shot 

 and granular noise. In addition to the shot noise there is a frequency- 

 dependent noise. All of this noisiness of the crystal is specified by the 

 crystal noise temperature ratio tx- The mixer noise temperature is /„ and 

 is given by 



21 



H-h'l 



a- i'-'« 



for the broadband mixer. L is the conversion loss; U is the value specified 

 by crystal manufacturers. The tm of an actual mixer may be difi^erent, 

 depending on the termination of the image conversion which affects L. 

 Equation 7-8 shows that a large value of Fip causes the conversion loss of 

 the mixer to be the dominant parameter of the mixer contributing to the 

 noise figure. In fact even for a low IF noise figure the conversion loss is 

 more dominant than the noise temperature. The mixer therefore yields 

 lowest noise figure when it is designed for minimum conversion loss. 



