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PDFThe noise figure (NF) is the ratio, in decibels (dB), of the actual output noise to the noise that would remain if the instrument did not contribute its own thermal noise. If you terminate a unit under test (UUT) input with a matched resistive load (typically 50 Ω) and measure the noise power density (watts/Hz) at its output (No), the NF is given by the following equation:
where G is the power gain (in linear units) of the UUT, k ≈ 1.38 ×10-23 is Boltzmann’s constant, and T ≈ 290° K is the ambient temperature.
In heterodyne systems, output noise power includes spurious contributions from image-frequency transformation, but the portion attributable to thermal noise in the input termination includes only that which appears in the output via the principal frequency transformation of the system, and excludes that which appears via the image frequency transformation.
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