Discussions on Noise and Electromagnetic Interference

Quality Transformer and Electronics first developed the Noise Cut Transformer (NCT) as an experimental product with the goal of providing high levels of noise attenuation for sensitive equipment in the Semiconductor Capital Equipment industry. However, we are seeing further demand and applications for these units throughout the world where power quality is an issue. Many electronic systems incorporate sensitive components that require a reduction in the noise that can interfere with a tool's operation. Quality Transformer and Electronics' Noise Cut Transformers provide a solution to this problem. Our Noise Cut Transformers combat transient and common mode noise—noise coming from a plethora of sources. Our Noise Cut Transformers significantly reduce such noise and attenuate the spikes to negligible values, providing protection against these electrical disturbances.

Electromagnetic Interference

The increase of electronic devices of varying frequency ranges held within close proximity to one another has led to an increase in stray interference, or electrical noise (EMI). The types and sources of this electrical noise vary. For general purposes, we can call the noise found affecting circuits electromagnetic interference, which is the noise that affects an electrical circuit from either electromagnetic induction or electromagnetic radiation emitted from an external source—usually other electrical equipment on the line. The disturbance may interrupt or degrade the performance of the circuit. The interfering effect from a disturbing signal on an electronic circuit depends on three things: 1) the magnitude and type of interfering signal, 2) the attenuation of the signal between source and receiver, and 3) the susceptibility of the receiver. Furthermore, depending on the impedance, magnetic or electric fields can also affect the circuit. For instance, time-varying electric fields greatly affect high-impedance circuits, while time-varying magnetic fields greatly affect low-impedance circuits.

An important topic that also needs addressing is the method by which noise can get onto a circuit as this will affect the type of solution required. Noise can get onto a circuit three ways: 1) conduction, 2) near-field induction, and/or 3) radiation (far field).

Conduction is the interference created when equipment with common impedance is located between the interfering source and the interfered-with equipment. This is due to multiple pieces of equipment sharing a common power supply and a common ground path. Effectively, the resulting common impedance couples isolated circuits. For reasons of economy or space, it can become necessary for multiple pieces of equipment to share one power supply.

Near-field induction is either, or both, inductive and capacitive coupling between circuits and is commonly referred to as common mode interference. For simplicity, we will treat inductive coupling as mutual impedance, and capacitive coupling as mutual admittance. Inductive coupling, or mutual impedance, is the result of two conductors, usually wires, configured in a way that the change in current flow through one wire induces a voltage across the edges of the other wire(s). This voltage across the other wire(s) is noise that can disrupt or interfere with the effectiveness of the equipment down the line of the circuit. Capacitive coupling, or mutual admittance, is the result of energy transfer within a circuit caused by capacitance between circuit noes. This energy transformer creates noise that can also effect electrical equipment. Generally, in most instances—instances involving large circuits with multiple pieces of equipment, the circuits are far too complex to specifically calculate the mutual inductance. However, an educated estimation is usually all that is needed to calculate an accurate measurement of the mutual inductance.

Far fields have frequencies of 3 MHz or greater, where the wavelength has decreased and the induction field is predominant out to 15 meters. The far field is commonly known as transverse mode interference. At these frequencies, we are usually concerning ourselves with sources outside of the equipment of interest. For example, broadcast or radio transmitters are usually transverse mode interference sources.

Ultimately, the distance between the source and receive and upon the frequency of the source are the most common and significant sources of the electromagnetic interferences that we are trying to mitigate.

Neutralizing Electromagnetic Interference, EMI, with a Noise Cut Transformer

The Noise Cut Transformer is designed to provide high levels of noise attenuation—essentially, it will help “clean” the power feeding into your tool. Specific design and manufacturing techniques developed at QT&E through research and development will help attenuate this noise. Some aspects of the design and manufacturing techniques are designed to attenuate the noise, while other parts are designed to ensure noise is not added to the circuit after attenuation.

Design and components used in the Noise Cut Transformer will, first stop noise from entering and disrupting the operation of your electrical equipment—essentially protecting your own equipment. Secondly, they stop your electrical equipment from putting EMI/RFI noise onto the power lines—protecting equipment down the line.

The FCC, IEC and other regulatory agencies have rules and regulations concerning the amount of electrical noise your equipment will be allowed to place onto the AC Power line. These limits must be met before your equipment will be allowed to be connected to the AC Power line. The European limitations are much more stringent than the North American limits.

Grounding Techniques

Ground is any conducting material used to connect electrical circuits to a ground. Such a ground could be a metal chassis, a rack of equipment, a ground strip on a printed wire board, or the structural metal of a building. The circuit ground does not need to be at the same potential as earth ground. Grounding the transformer ensures the proper system operation and the prevention of coupling noise through the ground network.

The connection of electronic equipment to earth-ground can be made in two ways: the multipoint, or grid system, and the single-point, or tree system. The multipoint ground system requires an equipotential ground plane or grid for the system (i.e., the structural steel of a building, ground grid embedded in concrete, etc.) where electronic equipment can be connected to this ground at multiple points with lead lengths being as short as possible. “In a single-point ground system, all pieces of equipment are referenced to a single earth ground through a tree network in which there is only one unique path to ground from any point” (Buus, 1970). It is rare to find a system that is completely multipoint or single-point, as neither method by itself is cost effective. In most instances, a hybrid of the two methods is used to create the most cost effective solution.