 It therefore adds 1/10 of the noise power caused by the source, and the noise figure is 0.4 dB.For a low-impedance source of 250 Ω, on the other hand, the noise voltage contribution of the tube is 10 times larger than the signal source, so that the noise power is one hundred times larger than that caused by the source.The noise figure in this case is 20 dB.