Noise Floor and Signal-to-Noise Ratio

Noise floor changes as signal amplitude changes. For instance, the noise floor may be –150 dBm/Hz for a signal level of –5 dBm, and it may increase to –140 dBm/Hz for a signal level of 5 dBm. It may seem that the latter signal has less integrity, but this is not the case because (assuming all other factors are equal) the signal dynamic range is still 145 dBc/Hz. Consequently, the signal-to-noise ratio (SNR) is specified, but the noise floor of the product is not. While SNR is not the same for all signal levels, it remains relatively constant until the absolute noise density is at or below the theoretical limit of –173.8 dBm/Hz at room temperature. Beyond this limit, the SNR decreases one-to-one with signal level.