Which statement best defines the noise figure of a receiver?

• A. The ratio of input power to output power
• B. The ratio of bandwidth to noise figure
• C. The ratio of output SNR to input SNR
• D. The ratio of carrier to noise

Answer: (C)

Noise figure tells you how much a receiver degrades the quality of a signal by adding its own noise. It does this by comparing the signal-to-noise ratio (SNR) at the input of the receiver to the SNR at the output. A perfect receiver would pass the signal without adding noise, giving an NF of 1 (0 dB); real receivers have NF greater than 1, meaning the output SNR is worse than the input SNR by that factor.

The option describing the ratio of the output SNR to the input SNR best communicates this relationship—the way the device changes the SNR as the signal passes through. In standard terms, the noise figure is the ratio of input SNR to output SNR, but the essence is the same: it’s about how the SNR is reduced by the receiver. The other statements talk about input/output power, bandwidth, or carrier-to-noise in ways that don’t describe how the receiver’s noise affects SNR.