About EMI Isolation
Electromagnetic interference (EMI) can be difficult to isolate for a variety of reasons. EMI can travel via conductive radiation, via cables and power cords, or electromagnetic radiation which can pass through walls and other structures. For a sensitive instrument affected by EMI, the first step is to make sure the system is properly grounded. This can dissipate troublesome static buildup from the system.
A Faraday cage, at its most basic, is a box made of conductive material. The Faraday cage essentially acts as a hollow conductor. When confronted with an external electrical field, the electrical charges within the cage’s conducting material will rearrange themselves, resulting in zero field in the interior. Thus, a Faraday cage shields its contents from electromagnetic radiation in the surrounding area and provides an isolated volume in which an electrically sensitive system can be placed. The cage should be well-grounded in order to dissipate the accumulated exterior.
Faraday cages are relatively simple to construct. All you will need is a six-sided box made of conductive material. Many people will construct cages out of steel plate or wiring. Unfortunately, these materials produce cages which are heavy, hard to work with, and not very aesthetically pleasing.
Herzan takes a different approach. We’ve gone to great lengths to specify and test flexible conductive mesh material. We use multiple layers of this conductive fabric to ensure that our cages have excellent conductive properties. The cages will only be effective if they are well sealed, so we use conductive Velcro to ensure a tight seal on all doors and access points. We also employ other special conductive material to wrap cables which enter the enclosure to avoid parasitic EMI entering the system via inductive radiation. All cages are supplied with a grounding wire to ensure the charge on the cage’s exterior is properly dissipated.
These practices result in a faraday cage which is lightweight, portable, and easy to work with, but doesn’t compromise isolation performance.
Sometimes it isn’t practical to employ a Faraday cage for large-scale instrumentation like electron microscopes or magnetic resonance imaging (MRI) machines. Additionally, the sensitive frequency range for these instruments is beyond the scope of what a standard faraday cage can isolate (0 – 60 Hz EMI noise). Fortunately, solutions such as the Spicer magnetic field cancellation system have become effective in solving these problems through its combination of sensors, multicore cabling, and control mechanisms.
The performance provided by the Spicer magnetic field cancellation system is a result of it dynamically creating nearly equal and opposite field changes, improving the imaging quality of the microscope being supported. The standard Spicer control system comprises of a magnetic field control unit, one or two magnetic field sensors, and three orthogonal axis multicore cables. These components are installed into the room where the EMI noise is present and where the affected instrument will be used. Three power amplifiers in the control unit drive currents through the cables to create a field of the opposite sign to the change in ambient field. The magnetic field sensor measures the resulting field and real time negative feedback reduces the ambient field by the loop gain of the system. The system is dynamic, automatically responding to field changes within 100 μs.