Sources of measurement noise
Sources of measurement noise
Errors are created in measurement systems when electrical signals from measurement sensors and transducers are corrupted by induced noise. This induced noise arises both within the measurement circuit itself and also during the transmission of measurement signals to remote points. The aim when designing measurement systems is always to reduce such induced noise voltage levels as far as possible. However, it is usually not possible to eliminate all such noise, and signal processing has to be applied to deal with any noise that remains. Noise voltages can exist either in serial mode or common mode forms. Serial mode noise voltages act in series with the output voltage from a measurement sensor or transducer, which can cause very significant errors in the output measurement signal. The extent to which series mode noise corrupts measurement signals is measured by a quantity known as the signal-to-noise ratio. This is defined as:
Where Vs is the mean voltage level of the signal and Vn is the mean voltage level of the noise. In the case of a.c. noise voltages, the root-mean squared value is used as the mean. Common mode noise voltages are less serious, because they cause the potential of both sides of a signal circuit to be raised by the same level, and thus the level of the output measurement signal is unchanged. However, common mode voltages do have to be considered carefully, since they can be converted into series mode voltages in certain circumstances. Noise can be generated from sources both external and internal to the measurement system. Induced noise from external sources arises in measurement systems for a number of reasons that include their proximity to mains-powered equipment and cables (causing noise at the mains frequency), proximity to fluorescent lighting circuits (causing noise at twice the mains frequency), proximity to equipment operating at audio and radio frequencies (causing noise at corresponding frequency), switching of nearby d.c. and a.c. circuits, and corona discharge (both of the latter causing induced spikes and transients). Internal noise includes thermoelectric potentials, shot noise and potentials due to electrochemical action.
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 Vn = MI, where I is the rate of change of current in the mains circuit.
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 capacitances, 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.
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.
Fig. 5.1 Noise induced by coupling: (a) inductive coupling; (b) capacitive electrostatic) coupling.
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.