356 THE RADAR RECEIVER 



7-4 LOW-NOISE FIGURE DEVICES FOR RF 

 AMPLIFICATION 



The crystal mixer Is a rather fragile element, and its electrical character- 

 istics deteriorate when large-signal inputs are applied to it. Because of the 

 conversion loss of the crystal mixer, the receiver noise figure is highly de- 

 pendent on a low IF noise figure. From a consideration of the noise figure of 

 cascaded networks it is seen that a large available power gain in the first 

 network minimizes the noise contributions of the later networks. 



A low-noise RF amplifier preceding the crystal mixer can provide the 

 power gain required to oflFset the loss of the mixer. Two types of RF 

 amplifiers that might be employed in an airborne radar receiver are the 

 traveling wave tube (TWT) amplifier and the variable parameter amplifier. 

 These devices are discussed in more detail in Chapters 10 and 11. 



Noise in the traveling wave tube results from noise in the electron beam. 

 Theoretically the noise can be reduced to about three times kTB. At 

 present such tubes are not available for airborne radar receivers. Tubes are 

 available, however, that are nearly competitive in noise figure with the 

 microwave crystal mixer in the frequency range employed by the airborne 

 radar set. 



A TWT will produce a saturated output under strong signal conditions 

 at maximum gain, alleviating many TR switching difficulties. Since the 

 tube provides gain, the noise figure of the elements which follow is not 

 nearly as important as in the conventional airborne radar receiver. There- 

 fore, much higher intermediate frequencies are feasible without degrading 

 the noise figure. A higher IF results in fewer spurious signal outputs from 

 the receiver. The tube can be gain-controlled by changing the beam current 

 so that it can produce an attenuation equal to the cold loss of the tube if 

 required. This is an advantage when attempting to amplify strong signals 

 with minimum distortion. 



One disadvantage in the traveling (forward) wave tube results from the 

 wide bandwidth. The noise spectrum is very wide and this results in more 

 noise at the mixer than desired. An RF filter between the traveling wave 

 tube and the mixer can, however, eliminate this condition if necessary. 

 Another disadvantage is that a number of spurious signals can be generated 

 in the tube, and are likely to be encountered in practice due to the wide 

 RF acceptance bandwidth of the tube. The backward wave amplifier has 

 a narrower bandwidth than the forward wave amplifier and may prove 

 to be the most desirable type of traveling wave tube for use as an RF 

 preamplifier in an airborne radar set. Traveling wave tubes may also be 

 constructed with two slow wave structures to provide mixing. 



Variable parameter amplifiers — also called parametric amplifiers — are 

 much simpler than the TWT amplifiers. The transmission type of amplifier 



