CAUSES OF NOISE

Evening Post, Volume CXX, Issue 112, 7 November 1935, Page 30

CAUSES OF NOISE

SOME CONTROLLABLE

There is generally a certain amount of "noise" associated with the reception of broadcast programmes. Some of it is within the direct control of the designer or operator of the receiver, as, for example, imperfections which may be due to faulty connections or the use of parts which are defective or of inferior quality. Noise due to power supply disturbances and interference is. more or less under control, if not at the receiver, at any rate at the source of the interference. Natur-ally-occurring static disturbances are beyond control, although their effects can be lessened, except in extreme cases, by increasing the power of transmitters. But even if every precaution is taken to eliminate disturbances due to the cause above-mentioned, there still remains the type of noise. which may be best described as a "hiss," and which is most apparently when the receiver is brought to its most sensitive state. This type of disturbance is of consequence at the present time when sensitivity is one of the primary qualities of a receiver and a high degree of amplification is expected and obtained from practically every set. The hissing, rushing noise which is almost invariably present in a'receiver which has a high gain is considered to be due primarily to two causes, known respectively as "thermal agitation" and "shot effect." The first is put down to the uneven movement of electrons in a conductor which is carrying a current. The condition may be approximately likened to that of water flowing along a pipe. If the pipe has a roughened internal surface that part of the stream which is in proximity thereto is diverted from its path and thrown towards other sections of the stream, so that the uniformly smooth flow is disturbed. It might be imagined that the flow of electrons which form a current along a wire may also be subject to disturbances, the resuit o fwhich is to create small irregular series of pulses in the output. "Shot effect" occurs within the valves. Theoreticall}', electrons are emitted from the hot cathode of the valve in an even stream, the density of which depends upon the properties of the cathode, the plate voltage, and other characteristics of the valve. In practice, however, the emission is not perfectly even, and is subject to small local variations which cause the plate of which is to create small irregular both sides of its mean value. The process here may be compared to the boiling of water. For example, a saucepan of water which is boiling vigorously may, in equal periods of time, produce equal volumes of steam, but it will be noticed that from instant to instant the number ot bubble;: rising to the surface of the water varies. The emission of electrons from a hot cathode is. so to speak, a boiling out process, and, like the boiling water, is not perfectly even for every minute fraction of a second. The difficulty is that the two effects just mentioned

occur in every circuit and valve in a receiver. In the later stages tlvj strength of the signal is usually such that it completely overrides these small variations and imperfections, but in the I early stages, e.g., the aerial circuit or j the frequency changing stage of .1 superheterodyne receiver, the disturbances due to thermal agitation and shot effect are relatively great compared to a weak signal. The signal variations and the noise variations become inextricably mixed in the early stages. and in succeeding stages the noise is amplified at the same rate' as the signal. Thus in th? output the hiss level may be comparable with the signal. With the demand for sensitive receivers, particularly those which are designed to pick up the relatively weak impulses from oversea short-wave stations, this inherent noise has become a matter of considerable importance to the designers of" valves and associated circuits. In a superheterodyne receiver the frequency changing valve is a comparatively prolific source of this type of trouble, and it is for this reason that much research has been carried out in the production of valves which will perform the frequencj" changing function with a minimum o£ added disturbance and circuit arrangements which will result in the signal strength'at that stage being comparatively high, and, therefore, less affected.