552 



ANTENNAS AND RF COMPONENTS 



To IF Amplifier 



Crystals 



Fig. 10-24 Balanced Mixer Using Folded-Tee Hybrid Junction. 



The principal advantage expected from using a balanced mixer is an 

 improvement in receiver noise figure through reduction or elimination of 

 local oscillator noise. The improvement in receiver noise figure which can 

 be realized from a balanced mixer depends, of course, on the klystron noise 

 level and on the degree of noise suppression achieved, which is dependent 

 upon the quality of balance achieved in the mixer, although adequate 

 suppression can be obtained with moderate crystal unbalance. Balanced 

 pairs of crystals should always be used in balanced mixers, and replaced 

 with a balanced pair when change is necessary. Although in a well-balanced 

 mixer the local oscillator noise suppression may be near total, the improve- 

 ment to be expected in receiver noise figure is of the order of 2 db at X band. 

 Greater advantage can be realized at higher frequencies because local 

 oscillator noise increases with frequency. Certain secondary advantages 

 are realized from use of a balanced mixer, including reduced local oscillator 

 power requirement. 



Generally it is desirable in a mixer that reflection of any input signal 

 should be back out the port through which it entered. Among the reasons 

 for this is the prevention of local oscillator radiation through the antenna, 

 and variations of crystal current which would arise with variations in 

 antenna impedance. Also, local oscillator power reflected back into the 

 signal input of the mixer introduces local oscillator noise to the signal 

 channel; hence some of the noise-suppressing advantage of the balanced 

 mixer is lost. The symmetrical hybrid tee mixers have the desired charac- 

 teristics while the ordinary slot hybrid mixers do not. In these, signal 

 reflections enter the LO channel, while LO reflections travel toward the 

 antenna by the signal channel. In many applications this may be of no 

 consequence, while in others it may be of importance. LO radiation and 

 effects of antenna variations can be significantly reduced, if necessary in 

 such instances, by employing a low-level ferrite isolator (see Paragraph 



