Electromagnetic interference (EMI) is a disturbance caused by an electromagnetic field which impedes the proper performance of an electrical device. EMI can come from man-made or natural sources such as the sun or the Earth’s magnetic fields. But most of the EMI which causes trouble for precision research applications is caused by stray magnetic or electrical fields generated by machinery or electrical equipment.
EMI can adversely affect any application which examines electrical properties of samples. Common applications which measure electrical properties are semiconductor probe stations, electrophysiology testing, wifi testing, and cell phone testing. These applications measure very small levels of electrical current, so any external source of electricity will introduce error into the measurements. If the EMI is variable, it can also degrade the repeatability of these measurements.
Another category of sensitive applications are those that utilize electromagnetic fields in their sensing mechanisms. The most common analytical techniques in this category are MRI/fMRI and electron microscopy – both scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Because the travel and behavior of electrons is integral to these sensing techniques, any stray field presents a potential issue. EMI fields generally manifest themselves in the appearance of striations throughout the images and sawtooth patterns along feature edges (see below).
Electromagnetic radiation can be difficult to diagnose, as EMI can often manifest itself in ways that are hard to detect. Users can run tests for weeks or months without realizing that EMI is degrading the accuracy of measurements. Once an issue has been detected, it can be difficult to distinguish EMI from other noise sources. Often users will need to go through a process of troubleshooting before they can correctly diagnose an EMI issue.
The sources of EMI can often be distant from the point of installation. Because EMI is caused by an electromagnetic field, it can stretch across rooms and through buildings via radiative coupling. Sometimes merely decoupling the instrument mechanically won’t solve the issue.
As with other noise sources, the simplest fixes are the best and should be pursued first. The source of the EMI should be turned off, if possible. If not, the source should be moved away from the instrument or the instrument should be placed somewhere distant from the source of offending EMI. If those measures fail or are impractical, the source of EMI should be isolated from the surrounding environment. This can be accomplished by properly grounding the machinery, constructing a Faraday cage around it, or placing it in a room constructed of conductive material.
If the source-specific remedies fail, then the EMI will have to be addressed at the instrument itself. First, you should make sure that the instrument is well-grounded. Using a piece of conductive wire, connect the instrument to a ground that is coupled to the Earth. Most labs should have ground attachments available. Next, construct a Faraday cage around the instrument. Faraday cages are five- or six-sided boxes constructed of conductive material. These effectively dissipate stray magnetic fields in the environment.
If the above measures prove to be insufficient, an active EMI cancellation system should be employed. EMI cancellation systems sense electromagnetic fields at the level of the instrument and generate fields which cancel out the troublesome fields. These are the highest performance solution for EMI issues currently available.