10-15] 



MISCELLANEOUS MICROWAVE COMPONENTS 



553 



10-16) in the signal input arm of the mixer. Because of the bandwidth 

 advantage of the slot hybrid it has been proposed that slot hybrid mixers 

 incorporate a broadband 90-deg phase shift section in one crystal arm to 

 obtain the preferred characteristic. 



As balanced mixers for the higher frequencies are readily available 

 commercially, and as they are often more convenient to use than the single 

 crystal mixer, they are finding rather general application in new system 

 designs. Depending principally on how well crystal balance is maintained, 

 the noise figure improvement may or may not be realized. Improvements 

 in crystal diodes have resulted in measurable reductions in crystal conver- 

 sion loss (Lc) and in crystal noise ratio (Nc) such that an overall receiver 

 noise figure of 8 db or less should be realizable at X band. Complete 

 cancellation of local oscillator noise is assumed, however. 



Microwave Noise Sources. Unless some means of checking is pro- 

 vided, the operator of a radar may be unaware of a deterioration in system 

 performance, particularly if it is due to loss in receiver sensitivity. One of 

 the simplest ways of providing a quick overall check on receiver perform- 

 ance is with a built-in calibrated microwave noise source, which serves as a 

 standard signal with which an absolute measure of receiver sensitivity can 

 be made with reasonable accuracy. Such noise sources, which are special 

 gas discharge tubes in a section of transmission line, are now available in 

 compact form for incorporation as a component of the radar system. These 

 discharge tubes typically generate an excess noise power of 15 to 18 db 

 above random noise power at room temperature, depending on the kind 

 of gas fill in the tube. A simple comparative measurement of receiver noise 

 output level, with and without the noise tube turned on, provides a valuable 

 check of receiver sensitivity. Fig. 10-25 illustrates how one type of gas tube 

 noise source can be inserted directly between the duplexer and the mixer; 



Fig. 10-25 Simplified Block Diagram of Radar Incorporating Double-Ended 

 Noise Source for Monitoring Receiver Noise Figure. 



