BRUM) BAM) MU l«)\\ W !■: XOISI'l SOURCE 



609 



and gain,^ and discuss the factors involved in making noise ligure measure- 

 nienls hv means of a noise source. 



Notes on Noise Figure 



Definition: The Noise Figure of a network, with a generator connected 

 to its input terminals, is the ratio of the available signal-to-noise power ratio 

 at the signal generator terminals (weighted by the network bandwidth) 

 lo the available signal-to-noise power ratio at its output terminals. 



Definition: The G.\ix of a network is the ratio of the available signal 

 power at the output terminals of the network to the available signal power 

 at the output terminals of the signal generator. 



INPUT 

 TERMINALS 



OUTPUT 

 TERMINALS 



en = 4KTiRiB 



T,R 



NETWORK 



GENERATOR 



TERMINALS GAIN = G 



NOISE FIGURE - f 

 Fig. 1 — Schematic diagram of generator, network and output power meter. 



These defmitions apply to a circuit consisting of a generator, a network 

 and an output jKjwer meter as shown schematically in Fig. 1. The signal 

 power available from the generator, having an open circuit voltage e and 

 an internal resistance Ri , is: 



SA 



e 

 4Ri 



(1) 



The noise power available from the signal generator resistance, Ri , at ab 

 solute temperature Ti , is 



AKTiRiB 



Pf/A — 



4i?i 



= kTiB 



(2) 



where B is the effective bandwidth of the network, by which the generator 

 noise is weighted in this case. 



3 H. T. Friis, Froc. I. R. E., Vol. 32, # 17, pp. 419-422, July, 1944. 



