398 BELL S YSTEM TECHNICA L JOURNA L 



IF tube. The noise performance of the first IF stage will be discussed in 

 some detail.** 



A convenient method of expressing the departure from ideal performance 

 is the use of the "noise figure" proposed by Friis*^ The noise figure of a 

 network or amplifier may be defined as follows: 



_ Available output noise power /j^-j 



GKTAf 



where G is the "a\-ailable gain" which is defined as the ratio of the available 

 signal power at the output terminals of the network or amplifier to the 

 available signal power at the terminals of the signal generator. KTAf is 

 the available noise power from a passive resistance, where K is Boltzmann's 

 constant, T is absolute temperature and A/ is the incremental bandwidth. 

 This follows from consideration of a noise generator of resistance Ro working 

 into a load resistance Ro . This is the condition for maximum power into 

 the load, half of the noise voltage appearing across the source and half 

 across the load. The open-circuit noise voltage appearing across the 

 terminals of a resistance Ro is 



V- = AKTRoAf (16) 



The noise power delivered to the load by the source resistance will be 



^- = 5 = ^^^^ '''> 



If there were no source of noise other than that of the resistance of the signal 

 source itself, the noise figure would be unit}'. If the signal source works 

 dnectly into a matched load resistance at the same temperature, the noise 

 figure is 2 since the available output noise power is KTAf and the available 

 gain is one-half. 



The importance of the noise figure of the radar receiver is obvious since a 

 reduction in the noise figure is equivalent to the same percentage increase in 

 transmitter power. In recent radar systems the noise arising in the first IF 

 stage constituted a substantial part of the total receiver noise. 



If the first IF tube provides at least 15 db gain, noise introduced by its 

 plate load impedance, and by anyother sources in the rest of the amplifier, 

 is usually negligible. The departure of the noise figure of the IF amplifier 

 from unity is then due to noise arising in the first tube and in its input 

 circuit. 



In^he very high frequency (\'HF) range, essentially all of the noise arising 



** It will be assumed in all of the discussion about noise that there is no noise due to 

 flicker effect, emission of positive ions, microphonics, sputter, ionization, secondary emis- 

 sion, or reflection of electrons from charged insulators. 



« "Noise Figure of Radio Receivers," H. T. Friis, /. R. E. Proceedings, July 1944. 



