Electromagnetic Interference (EMI)
February 12, 2020
Electromagnetic interference (EMI) is caused by the electromagnetic disturbance of one electrical device to another; it is also called radio frequency interference (RFI) or RF noise.
EMI can be caused by a communication system, such as a CB radio, but is more often caused by electronic devices. The interfering device can be virtually anything, from an electric drill to a garage door opener to fluorescent light ballasts. EMI can even be caused by electric power lines.
EMI can have a significant effect on a vibration testing system when there is electrical noise in the surrounding environment. Long cables, poorly shielded cables and components, and certain amplifiers can act as an antenna, pick up EMI, and introduce it into the testing system.
Detecting and Minimizing the Effects of EMI
The simplest way to detect EMI in a vibration control system is to move the cabling. If you move the cabling, and the input’s signal shows variation, then EMI is the most likely culprit.
In a typical lab, electrical noise can be generated by fluorescent lights, electric motors, pulse width modulators, or a variety of other systems. Most cables have shielding to protect the circuitry, but it can wear down over time.
If replacing the cables or moving the cables into a protected location away from high voltage wires, light, and other RF noise sources doesn’t resolve the issue, then connecting jumpers on the front panel of the VR9500 can minimize the effects of EMI on the system.
The VR9500 vibration controller has jumpers on the front input panel to address EMI. In most cases, the jumpers are not connected when a VR9500 is shipped from the factory.
When connected, the jumpers mechanically bond the shield/ground side of the input channels directly to the ground reference (see the next lesson for a discussion of electrical ground.) This bond bypasses some of the protective circuitry which could act as an amplifier for EMI and convert it into a measurable voltage. By bypassing the circuitry, the jumpers reduce or eliminate the effects of EMI on the system.
For detailed instructions on connecting the VR9500 jumpers, ask the VR support team for the document “VR9500 Front Panel Bypass Jumpers,” or read our Noise Troubleshooting Tips
There are two downsides to connecting the VR9500 jumpers: (1) it bypasses the 200V transient peak protection circuitry built into the VR9500 and (2) differential mode cannot be used.