Boom Box

Electromagnetic interference, or EMI, is a high frequency, low voltage signal on the power lines. Its source may be conducted, meaning it started out on the power lines, or radiated, meaning it started out in the air and was coupled into the power lines.

If EMI is radiated, it could also couple into data cables. This is one reason most data cables are shielded. Shielding protects the high frequency noise from entering the cable and disrupting data flow, assuming it is properly connected.

High frequencies, such as found in electrical noise, travel through power supplies and couple onto the DC voltage and signal lines inside the device. So, data may be corrupted from the source side, or during its journey to another device.

Just about all electronic devices have EMI filters built into their power supplies. This is a requirement from the Federal Communications Commission. So how does EMI get into a device if it has a filter?

Quite often, the answer to that is through the ground. We think of ground as some vast sink for unwanted electrical garbage. Yet, at high frequencies, ground is simply one more conduction path for noise.

We mentioned earlier how the ground is the digital reference for the device. Noise can travel along the ground and affect the circuits and components inside.

Or, if data cable shields are not properly terminated, noise can enter through them.

The first step to reduce data flow problems due to EMI is to ensure all system components and devices are properly grounded. This includes all data cables. If necessary, isolated ground systems should be installed.

If problems still exist after this on stand-alone equipment, then consider installing a transformer-based power conditioner. Because it uses a transformer, the phase conductors will be re-referenced to ground on the secondary. Place this transformer device as close to the equipment as possible. Plug-compatible units work well for this.

If problems still exist with distributed systems, consideration should be given to the use of fiber-optic data transmission. Since fiber-optic cables are non conductive, no EMI coupling will occur.

Links to Related Topics

Wiring and Grounding
Voltage Sags
Current Distortion