70S HELL SYSTKM TF.CIIMCAL JOIRXAL 



of the noise disturhaiues al the in])ut teriniiials of the receiver. Xoise 

 disturbances ma}- be defined as the resultant unwanted interfering electrical 

 energy at the y)oint under consideration and includes contributions due to 

 atmospheric disturbances, unwanted radiation from adjacent electrical 

 efjuipment, microi)honic disturbances, and noise due to vacuum tubes and 

 thermal agitation. At microwave frequencies we are usuallv concerned 

 only with the thermal agitation and tube noise rontribulion. .Atmospheric 

 disturbances at radar frequencies are negligible and microphonic and elec- 

 trical interferences from adjacent electrical equipment can, bv proper and 

 sufficient engineering, be reduced to any desired level. 



It has been shown- ■'' that the thermal noise (Johnson noise) voltage which 

 appears at the input terminals of a radio or radar recei^'er is determined bv 

 the value of the resistance component of the generator imj^edance at this 

 point. For ma.ximum transfer of signal power the load termination is re- 

 quired to be equal to the internal impedance of the generator and for this 

 condition the total thermal noise power delivered to the load is given by 



Ps = KTB (watts) 

 where: 



A' = Boltzman's constant = 1.38 X !()"-•' Joule/degree abs. 

 T = Absolute temperature in degrees 



B = Bandwidth under consideration in cycles i)er second; and the signal- 

 to-noise ratio is given by 



d" "" IF^rii ^'^ numeric) 



where Ps = ma.ximum available signal jjower. 



If the signal generator referred to is followed by any 4-terminal network 

 representative of a converter element, an amplifier, or a passive network, 

 the effective signal-to-noise ratio at the output terminals of the network will 

 be modified. To obtain a measure of this ef^"ect we may assign a figure of 

 merit, /•', to the network called the "noise figure" of the network and deline 

 this as the ratio of the available signal-to-noise ratio at the signal generator 

 terminals to the a\ailable signal-to-noise ratio al the output terminal of 

 the network. 



'"The .Misolutf Sinsitivilx of Radio Rcici\HTs," 1), (). Ndiih. A'. ( '. .1. Rcviri,'. \'ol. 

 \1. January 1^42. 



^"Xoisc Fif^urt-s of Radio RirriviTS." II. T. I'liis. I'rm-. I. K. E.. X'ol. M. Xo. 7. Jiii\ 

 1944. 



