Define Noise Figure (NF) and its significance in receiver sensitivity.

• A. NF equals SNR_out / SNR_in; a measure of how much a receiver degrades signal quality; lower NF means better sensitivity
• B. NF equals SNR_in / SNR_out; a measure of how much a receiver degrades signal quality; lower NF means better sensitivity
• C. NF equals the ratio of bandwidth to noise bandwidth
• D. NF measures impedance matching quality

Answer: (B)

The main idea is how much the receiver’s own noise degrades the signal as it passes through the front end. Noise Figure is defined as the ratio of the input signal-to-noise ratio to the output signal-to-noise ratio, NF = SNR_in / SNR_out. If the receiver adds no noise, the output SNR equals the input SNR and NF equals 1 (0 dB). In real receivers, NF is greater than 1, and the smaller the NF, the less the signal is degraded and the better the receiver’s sensitivity. This matters for detecting weak signals because a lower NF means the front end contributes less extra noise, allowing the system to discern weaker signals. NF is usually expressed in decibels as NF(dB) = 10 log10(F), where F is the linear Noise Factor. The first active stage (like the LNA) largely sets the NF, so improving that stage yields the most improvement in sensitivity. While impedance matching and bandwidth affect overall performance, they are separate concepts from the Noise Figure itself.