Signal-to-Noise Ratio in AM Receivers

Experimental tests have been made to determine the effect of linear detectors upon the signal and the signal-to-noise ratio obtained from the demodulation of an rf carrier. The experimental data confirm theoretical results and indicate that: 1. The concept of excess noise figure must be used with care. It cannot be employed to calculate the noise at the output of a receiver unless due consideration is given to the presence of a nonlinear device following the if stage. The reason for this is the effect of the presence of a carrier on the noise output of a detector. This noise increases when a carrier is present; the maximum increase is 4 to 7 db, depending upon the shape of the if filter. 2. For sine-wave amplitude modulation and if bandwidths at least 3 or 4 times larger than the af bandwidth, a universal curve can be given that shows the relation between the signal-to-noise ratio at the output and the carrier-to-noise ratio at the input of a second detector. 3. If two AM carriers are simultaneously present at the input of a linear second detector, this discriminates against the modulation of the weaker carrier.