Measurement noise and signal processing

 

5.1.1 Inductive coupling

The primary mechanism by which external devices such as mains cables and equipment, fluorescent lighting and circuits operating at audio or radio frequencies generate noise is through inductive coupling. If signal-carrying cables are close to such external cables or equipment, a significant mutual inductance M can exist between them, as shown in Figure 5.1(a), and this can generate a series mode noise voltage of several millivolts given by V_n = MI, where I is the rate of change of current in the mains circuit.

5.1.2 Capacitive (electrostatic) coupling

Capacitive coupling, also known as electrostatic coupling, can also occur between the signal wires in a measurement circuit and a nearby mains-carrying conductor. The magnitude of the capacitance between each signal wire and the mains conductor is represented by the quantities C₁ and C₂ in Figure 5.1(b). In addition to these capa[1]citances, a capacitance can also exist between the signal wires and earth, represented by C₃ and C₄ in the figure. It can be shown (Cook, 1979) that the series mode noise voltage Vn is zero if the coupling capacitances are perfectly balanced, i.e. if C₁ = C₂ and C₃ = C₄. However, exact balance is unlikely in practice, since the signal wires are not perfectly straight, causing the distances and thus the capacitances to the mains cable and to earth to vary. Thus, some series mode noise voltage induced by capacitive coupling usually exists.

5.1.3 Noise due to multiple earths

As far as possible, measurement signal circuits are isolated from earth. However, leakage paths often exist between measurement circuit signal wires and earth at both

the source (sensor) end of the circuit and also the load (measuring instrument) end. This does not cause a problem as long as the earth potential at both ends is the same. However, it is common to find that other machinery and equipment carrying large currents is connected to the same earth plane. This can cause the potential to vary between different points on the earth plane. This situation, which is known as multiple earths, can cause a series mode noise voltage in the measurement circuit.

5.1.4 Noise in the form of voltage transients

When motors and other electrical equipment (both a.c. and d.c.) are switched on and off, large changes of power consumption suddenly occur in the electricity supply system. This can cause voltage transients (‘spikes’) in measurement circuits connected to the same power supply. Such noise voltages are of large magnitude but short time duration. Corona discharge can also cause voltage transients on the mains power supply. This occurs when the air in the vicinity of high voltage d.c. circuits becomes ionized and discharges to earth at random times.