If a received carrier is -60 dBm and the thermal floor in 1 Hz bandwidth is -100 dBm, what is the SNR in dB?

• A. 60 dB
• B. -40 dB
• C. 40 dB
• D. 0 dB

Answer: (C)

SNR in dB is the difference between the signal power and the noise power within the same bandwidth. Here, the signal power is -60 dBm and the thermal noise floor in 1 Hz is -100 dBm. Subtracting: -60 dBm minus (-100 dBm) equals +40 dB. This means the signal is 40 dB above the thermal noise in that 1 Hz band. The other values don’t fit because they would imply different relationships between signal and noise: a 60 dB result would require the noise to be -120 dBm, -40 dB would mean the noise is stronger than the signal, and 0 dB would mean they are equal.