Signal-carrying switches:Special requirements

Special requirements

A switch intended to carry signals, as distinct from DC or AC supplies, must inevitably be subject to several requirements in addition to the normal needs already described. The type and frequency range of the signals that the switch is required to carry determine to a very great extent the special needs that will prevail: a signal-carrying switch for low-frequency AC in the range 5-240 V and with currents that are not less than 50 mA or so will be identical, to all intents and purposes, to a switch for AC supply use. The important differences in design appear when a switch is intended to carry signals of high frequency, or signals in which rapid changes of voltage are important, such as digital signals. A feature of signal switching that is of no importance in supply switching is that the switch can itself add noise to the signal or even generate spurious signals. Capacitance between contacts (and from contacts to ground) is of considerable importance, and the main- tenance of low contact resistance becomes more difficult just as it becomes more important.

Contact resistance can be particularly important if signals of low-voltage level are being switched. At low signal voltage levels, there is no sparking or arcing which might expose fresh surfaces now and again, and unless a wiping action can be obtained, as is often the case, the contact resistance can become quite high. This in itself is not necessarily a hazard if the signal current levels are low, but if the switch is used for low-voltage levels at which current levels are not low, then the voltage drop across the switch contacts will be appreciable.

Since the greatest problem in low-level circuits is usually the maintenance of an adequate signal-to-noise ratio, any impedance in series with the signal current is very undesirable. For low-level circuit switching, gold-plated contacts are usually recommended. Although the resistance is never as low as can be obtained with silver, gold has the great advantage of being chemically inert, so that contact resistance is very stable. The softness of the metal also tends to ensure a greater area of contact, which compensates for the higher resistivity.

Other important contact considerations are capacitance and nonlinearity. If the contact resistance is high, then a thin non-conducting film probably exists on the contacts, and this film will form a capacitor coupling between the contacts. For signals at very high frequencies, the reactance of this capacitive coupling may be lower than the resistance of the contacts, but like any other reactance will introduce a phase shift. Because of the low reactance of the layer, an increase in contact resistance may be unnoticeable in terms of signal strength, but the phase changes can cause problems that will not necessarily be attributed to the switch. For digital signals at lower frequencies that have short rise and fall times, the capacitive coupling will cause some differentiation of the signals, which may have no noticeable effect until it causes loss of amplitude.

A rectifying contact is considerably more troublesome. This can arise where there is metal to oxide contact on a switch, and in some cases it can be the result of severe arcing which has left a sharp whisker of metal making contact against an oxidized surface on the opposite contact. The result is a diode whose forward resistance and back resistance values are not very greatly different, but which differ nonetheless. If appreciable signal currents flow across this part-rectifying contact then the diode action will have the usual effects on the signal. One effect will be distortion of the signal, such as differences between positive-going and negative-going portions of the signal.

The other effect, which can be very serious, is DC bias. If the switch feeds a DC-sensitive input, such as a directly-coupled input to an operational amplifier, the amount of DC bias produced by the rectifying action of the switch contacts may be enough to bias the amplifier off or into non-linear action. For digital circuits, the effect can be to prevent any signal input from being effective. Modern contact materials make this condition unlikely, but there is always a possibility of a rectifying action arising through contamination, particularly in humid atmospheres, and its effects are seldom immediately blamed on the switch contacts.

For the rather special case of RF switching at high power, the problem of arcing is much more severe than it is for supply current switches. This is because the time of opening the switch contacts will inevitably be greater than the time of a cycle, so that the waveform will be at its peak voltage at least once while the contacts are opening. Since the contacts have capacitive reactance in any case, a considerable current can flow, and this will cause ionization of the air that, because of the short time between peaks of a high-frequency signal, cannot easily be damped out. Switches for high- power, high-frequency applications are therefore a very specialized branch of switching, and one for which a one-off switch may have to be designed to suit the particular application.

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