Many components in electronic equipment do not have a completely rigid structure but consist of parts that can vibrate when the component is subjected to physical excitation. As the parts vibrate, the distance between them may vary and variations in the electrical properties of the component can occur. As a simple example, if the plates of a tuning capacitor vibrate, the distance between the plates may vary and, as a result, there will be a corresponding variation of the capacitance. If the capacitor forms part of the tuned circuit of an oscillator, the frequency of oscillation will vary correspondingly. In other words, the vibration of the capacitor plates with respect to one another gives rise to an interference signal which produces frequency modulation of the oscillator signal. The production of an interference signal as a result of mechanical vibration is known as ‘microphony’.
This article considers microphony in valves. The vibration of the electrode structure of the valve can cause variations not only in the inter-electrode capacitances, but also in the anode current and mutual conductance. The vibration may be a result of shocks — for example, equipment in cars or aircraft, and equipment containing push buttons or stiff switches will be subjected to this type of excitation — or the result of continuous excitation from the motors of gramophones and tape recorders, and from the loudspeakers of radio and television receivers. The loudspeaker is often the most troublesome cause of microphony since it is placed close to the valves and can transmit vibrations to the valves both mechanically (through the cabinet and chassis) and acoustically (through the air). Besides causing sound interference, the excitation of the loudspeaker can also affect the valves in the picture channel or timebase circuits of a television receiver and produce interference on the picture. In television receivers, picture microphony is usually more troublesome than sound microphony.
In spite of the very narrow tolerances used in the manufacture of valves, it is impossible to avoid slight differences between valves of the same type. These differences do not have any significant effect on the electrical characteristics of the valve but may affect the microphony considerably. Consequently, certain methods of testing valves can be carried out only on a statistical basis and any modification to a valve to reduce its microphony can be checked only by testing a large number of valves.