Sensitivity and Circuit Noise
Author: J.B. Hoag
The sensitivity of a receiver is defined as the minimum input signal voltage which will deliver a standard output signal voltage. A receiver will be more sensitive if it has a large radio-frequency amplification. The limit of sensitiveness is set by the various noises which are picked up in the antenna, be they man-made or natural atmospherics, and by the noises developed in the receiver itself, particularly in the first-stage r.f. amplifier. Hence, particularly for long-distance reception, where the incoming signal comes in very weak, studies of the signal-to-noise ratio become of importance. Man cannot do much about the external natural noises, but he can reduce the internal noises of the receiver considerably. However, even with a very carefully constructed receiver, where all joints have been properly soldered and where all parts have been designed to have high stability, a definite amount of noise will be heard. This will sound like a hiss. It is due to the thermal motions of the electrons in the input circuit of the first tube, and to the short-time sporadic changes which occur in the space charge of the first tube (a phenomenon called "tube noise"). In addition, any mechanical vibration of the circuit parts, or of the internal electrode structures, especially in the first tube, will change the normal current flow in these circuits. Upon amplification through the rest of the receiver, these appear in large volume in the loudspeaker. These are called "microphonics", and are eliminated by properly supporting and shielding the first stage from mechanical vibrations.
It is found that these noises are distributed more or less uniformly over the entire frequency spectrum. Hence the signal-to-noise ratio will be improved if the receiver is adjusted to respond to only an exceedingly small band of frequencies.