Some Aspects of Radio Reception at Ultra-High Frequency: Part III. The Signal-to-Noise Ratio of Radio Receivers

The signal-to-noise ratio of a radio receiver can best be analyzed by finding all the various noise sources and then referring them to a single point. When the equivalent-noise-resistance concept is used at the input to express the output noise of the first tube of a series, noise sources beyond this point are also conveniently referred back to this same input. Thus, a single equivalent noise resistance Reqcan be used to express all the noise sources at or beyond the first tube. In the simplest case, where noise sources at the input of the first tube can be neglected, the maximum signal at the input also gives best signal-to-noise ratio. The signal-to-noise ratio then depends on the ratio of tube input resistance (which determines the maximum signal) to equivalent noise resistance. When there is induced noise in the input of the first tube, the optimum signal-to-noise ratio is obtained by coupling the antenna somewhat more tightly to the tube input than for maximum power transfer. Although this reduces the signal at the grid from its maximum value, the reduction of impedance is more marked and causes an even greater reduction in induced noise. The optimum signal-to-noise ratio depends on the product of induced noise and equivalent noise resistance. Even when the input resistance is infinite or negative, the optimum signal-to-noise ratio is limited either by this induced noise or by the bandwidth of the input circuit.