Accurate measurement of low signal-to-noise ratios using a superheterodyne spectrum analyzer

A modern superheterodyne spectrum analyzer measures RF power by the process of envelope detection, log amplification, and video averaging. This process is known to give an accurate power measurement for a noise-free sinusoidal tone signal and an error of 2.5 dB for additive white Gaussian noise (AWGN). For a tone combined with AWGN, the measurement of noise power N together with signal power S results in the need for two correction factors: one to compensate for the error in the measurement of combined signal and noise power, and another to take into account (on the logarithmic decibel scale) the subtraction of N from (S+N)/N to get the true signal-to-noise ratio (SNR identical to S/N). The first factor ranges from 0 to 2.5 dB; the second factor is >or=3 dB for SNR/0 dB. The results are extended to the measurement of SNR for phase-shift-keyed (PSK) digital communications signals combined with AWGN.>