Define noise figure (NF) and describe its role in predicting the overall noise performance of a RF receiver chain.

• A. NF is a measure of how much the SNR degrades from input to output; NF_dB = SNR_in - SNR_out; cascading adds in dB.
• B. NF is the ratio of input noise power to output signal power.
• C. NF describes the amplitude of noise.
• D. NF is the ratio of bandwidths.

Answer: (A)

Noise figure tells you how much a device degrades the signal’s quality as it passes through. It captures the change in the signal-to-noise ratio from input to output, not just the amount of noise or the bandwidth.

By definition, NF is the input SNR divided by the output SNR. In linear terms, F = SNR_in / SNR_out; in decibels, NF_dB = SNR_in_dB − SNR_out_dB. This means a higher NF means the device adds more noise, making the output SNR worse than the input SNR.

This concept is essential for predicting the overall noise performance of a receiver chain. When multiple stages are cascaded, the total noise figure depends on the noise figures of each stage and the gains of the preceding stages. The precise relationship is given by Friis’ formula, showing that the first stage often dominates the total NF because its gain strongly suppresses the contribution of later stages’ noise. In practice, talking about cascaded NF in dB is a helpful intuition, with the exact calculation using the Friis expression.

The other statements aren’t correct because NF is not simply a ratio of input noise power to output signal power, it does not describe the amplitude of noise alone, and it is not about a ratio of bandwidths.