Noise and Coherent Interference in Measurements
Due to the differences in the sources and means of reduction of noise vs coherent interference, we will treat them separately in subsections of this chapter. Both noise and interference provide a major limitation to the precision of measurements and the detectability of the QUM. Thus, knowledge of how to design low noise instrumentation systems is an important skill that a practicing instrumentation engineer should learn. First, let us examine the distinction between noise and coherent interference. Noise
is considered to arise in a circuit or measurement system from completely random phenomena. Any physical quantity can be ‘noisy’, but we will restrict ourselves to the consideration of noise voltages and currents in this chapter. Such noise voltages and currents will be considered to have zero means, meaning that they have no additive dc components. The unwanted dc components are best considered to be drift or offset and their reductions are more appropriately treated in a text that deals with dc amplifier design or op-amp applications. Coherent interference, as the name suggests, generally has its origins in periodic,
man-made phenomena, such as power line frequency electric and magnetic fields, radio frequency sources, such as radio and television station broadcast antennas, certain poorly shielded computer equipment, spark discharge phenomena, such as automotive ignitions and motor brushes and commutators, and inductive switching transients, such as SCR motor speed controls, etc. As you will see, minimization of coherent interference is often ‘arty’ and may involve changing the grounding circuits for a system, shielding with magnetic and/or electric conducting materials, filtering, the use of isolation transformers, etc. Minimizing the impact of incoherent noise in a measurement system often involves
a prudent choice of low noise components, certain basic electronic design principles and filtering. The incoherent noise that will be considered is random noise from within the measurement system. Coherent noise usually enters a system from without.