Measurement noise and signal processing

5.2 Techniques for reducing measurement noise

Prevention is always better than cure, and much can be done to reduce the level of measurement noise by taking appropriate steps when designing the measurement system.

5.2.1 Location and design of signal wires

Both the mutual inductance and capacitance between signal wires and other cables are inversely proportional to the square of the distance between the wires and the cable. Thus, noise due to inductive and capacitive coupling can be minimized by ensuring that signal wires are positioned as far away as possible from such noise sources. A minimum separation of 0.3 m is essential, and a separation of at least 1 m is preferable. Noise due to inductive coupling is also substantially reduced if each pair of signal wires is twisted together along its length. This design is known as a twisted pair, and is illustrated in Figure 5.2. In the first loop, wire A is closest to the noise source and has a voltage V₁ induced in it, whilst wire B has an induced noise voltage V₂. For loop 2, wire B is closest to the noise source and has an induced voltage V₁ whilst wire A has an induced voltage V₂. Thus the total voltage induced in wire A is V₁ + V₂ and in wire B it is V₂ + V₁ over these two loops. This pattern continues for all the loops and hence the two wires have an identical voltage induced in them.

 5.2.2 Earthing Noise due to multiple earths can be avoided by good earthing practices. In particular, this means keeping earths for signal wires and earths for high-current equipment entirely separate. Recommended practice is to install four completely isolated earth circuits as follows:

Power earth: provides a path for fault currents due to power faults.

Logic earth: provides a common line for all logic circuit potentials.

Analogue earth (ground): provides a common reference for all analogue signals.

Safety earth: connected to all metal parts of equipment to protect personnel should power lines come into contact with metal enclosures.

5.2.3 Shielding

Shielding consists of enclosing the signal wires in an earthed, metal shield that is itself isolated electrically from the signal wires. The shield should be earthed at only one point, preferably the signal source end. A shield consisting of braided metal eliminates 85% of noise due to capacitive coupling whilst a lapped metal foil shield eliminates noise almost entirely. The wires inside such a shield are normally formed as a twisted pair so that protection is also provided against induced noise due to nearby elec[1]tromagnetic fields. Metal conduit is also sometimes used to provide shielding from capacitve-coupled noise, but the necessary supports for the conduit provide multiple earth points and lead to the problem of earth loops.

5.2.4 Other techniques

The phase-locked loop is often used as a signal-processing element to clean up poor quality signals. Although this is primarily a circuit for measuring the frequency of a signal, as described in Chapter 7; it is also useful for noise removal because its output waveform is a pure (i.e. perfectly clean) square wave at the same frequency as the input signal, irrespective of the amount of noise, modulation or distortion on the input signal.

Lock-in amplifiers (see section 5.5.10) are also commonly used to extract d.c. or slowly varying measurement signals from noise. The input measurement signal is modulated into a square-wave a.c. signal whose amplitude varies with the level of the input signal. This is normally achieved by either a relay or a field effect transistor. As a relay is subject to wear; the transistor is better. An alternative method is to use an analogue multiplier. Also, in the case of optical signals, the square wave can be produced by chopping the measurement signals using a set of windows in a rotating disc. This technique is frequently used with transducers like photodiodes that often generate large quantities of noise.