7-3] RECEIVER NOISE FIGURE 355 



to FGkTBn, where F is the overall noise figure expressed as a power ratio. 



+ -. (7-6) 



(7-7) 



In the common airborne radar set, RF amplification is not employed. 

 Instead, the signal is heterodyned to some intermediate frequency and 

 then amplified. Microwave crystal mixers are passive nonlinear devices. 

 Their noisiness is characterized by the amount of noise produced by the 

 mixer compared with the noise from a resistance at the same external 

 temperature. The noise is thus expressed as a temperature ratio tm- A 

 mixer acts as a switch, and in terms of available power exhibits a loss at 

 the conversion frequency. This loss is designated as a power ratio Lx- 

 Following the previous notation the noise figure of a mixer is then tmLx. 



The noise figure of a superheterodyne radar receiver is then 



Fr.. = 1 -f- ^m - 1) + ^L,[(/. - 1) + (Fi, - 1)] (7-8) 



J a J a 



where Free is the receiver noise figure expressed as a power ratio 



/m-1 is the excess noise of the mixer 



FiF-\ is the excess IF noise figure expressed as a power ratio 



Lt is the product of the conversion loss of the mixer and loss in 

 the microwave transmission circuitry between the antenna 

 and mixer expressed as a power ratio 



/„ is the effective noise temperature ratio of the mixer 



T is the noise temperature of the receiver 



Ta is the temperature of the antenna. 



The noise figure is usually defined with respect to room temperature 

 (about 291° K).^ A radar receiver is, however, connected to a directional 

 antenna which can be represented as an equivalent generator at a tempera- 

 ture less than room temperature when the antenna is directed toward 

 space. In fact the equivalent antenna temperature under this condition 

 may be about 4°K. 



When referred to the antenna temperature under this condition, the 

 noise figure of the best airborne radar sets is on the order of 30 db. 



2Under such a definition T/Ta is unity. 



