Noise
All signal noise is caused by random motion of electrons or other current- carriers in a circuit. Even in a piece of wire consisting of the same metal throughout there will be some noise caused by the irregular movement of the electrons at room temperature. This type of noise can be greatly reduced by lowering the temperature, but is usually negligible compared to the other noise sources in a circuit. In general, noise is generated mainly when a small number of electrons are moving comparatively rapidly. The main sources of noise are therefore semiconductor junctions and bad contacts, and it is this latter source that is the main concern for the switch user. In addition, however, noise can be generated at junctions that are not semiconductor junctions. ;\’herever any two different metals meet there will be a build-up of electrons that causes a measurable voltage difference, called the contact potentiaL. This contact potential is mainly DC, but because of irregularities in the movement of the electrons around the contacts it will have an AC component, which is noise.
Any type of switching circuit will therefore introduce noise into a signal that is switched. The noise arises to some extent from variations in contact resistance, but can also be caused by the fluctuating contact potentials if the contacts are not clean and of identical chemical composition. A switch is not a major noise-producing component in circuits unless it has been used in a circuit that operates at very low signal levels. Switching at such low levels is unusual, because it is normally possible to amplify a signal before switching it, but if such switching is unavoidable, it should make use of switches with gold-plated contacts. In addition, if switching at low signal levels is completely unavoidable, no DC should pass through the switch, because DC flow is by far the most likely cause of noise in a switch (as it is in a potentiometer).
The type of noise that is caused by the switch contacts while they are closed is the usual broad-band noise, but there can be an additional problem of impulse noise when switch contacts close, caused by the charge and discharge of stray capacitance by the DC levels in the circuit. A familiar effect of this at one time was the noise pulse that accompanied audio circuit switching, for example when changing an amplifier input from disc to tape.
The root cause of this problem is the presence of DC on one of the switch contacts, or the use of different DC levels between contacts. The easy way out of this is, of course, to design the circuit so that switching is carried out at zero DC level at all times, but this may require the use of more coupling capacitors in the circuit path than is thought desirable, and in any case if the switch contacts are isolated to ground there will be a build-up of DC through the leakage resistance of any capacitor. The usual cure, if a purely mechanical switch is to be used, is to equalize the DC levels at the switch contacts, usually by the use of high-value resistors (Figure 12.3). This will lead to some signal breakthrough unless the signal circuits are at low impedance.