Ever wonder why your laptop computer, handheld data device, or even pocket calculator has that FCC compliance label on the back? It reads something like this, "This unit complies with Part 15, FCC Rules." What does the Federal Communication Commission have to do with your $15 calculator?
It all comes down to switching devices. Whenever you switch or oscillate a circuit fast enough (10 kHz and higher), it becomes a transmitter of EMI (Electromagnetic Interference) now known as RFI (Radio Frequency Interference). Any handheld device that uses diodes and transistors, which turn on and off at such a high rate, radiate electromagnetic waves.
Here is a good way to demonstrate this fact and check your TV remote control at the same time. Take an AM pocket radio and tune it to a dead part of the AM band (where you don't pick up any radio station, but you do hear a small hissing sound). Now, put it next to your TV's remote control and press one of the buttons on the remote. The digital pulse train you hear is the diodes switching current to the infrared LED on the remote control. As you press different buttons, you get different pulsing sounds. By the way, a $5 AM pocket radio makes for a pretty valuable troubleshooting tool. I always carry one in my troubleshooting kit.
With the advent of high-speed switching power supplies used in just about every electronic piece of equipment on the market today, you can see how this phenomenon becomes a serious problem. Unwanted noise in a process or control system can cause major upsets, costing hundreds of thousands of dollars in lost production and wasted product.
A large enough RFI signal can cause misfiring of SCRs in drives and actually destroy the equipment itself. Now you know why it's important to keep those doors closed on the drives while they're operating. The metal acts as a shield against unwanted interference. Try to imagine what would happen if you triggered your 5W handy-talkie next to an open drive cabinet!
Okay, RFI is a fact of life. But what can you do to minimize its impact on your equipment? It's a matter of degrees. How much noise determines how much we have to do. In many cases, you just need a twisted pair of wires to solve the problem. When this just isn't enough, consider the following methods:
Shielded cabling. Shielding helps break down the coupling effect between the noise and the signal. Coax cables are a good example of this. Often, you'll find sprayed copper shielding or a thin film of aluminum foil on the inside of the plastic covers of a PLC used for shielding the electronics inside the PLC.
Balanced circuits. By using a twisted pair, you don't keep the noise out, but you do couple the noise into both conductors (signal and return), which then becomes subtracted as common-mode noise. The balanced circuit creates a high impedance for the coupled noise.
RF chokes. As a high impedance, you can effectively use these chokes with data and power cables. Look at some of your portable equipment line cords. You'll most likely see a split core choke installed close to the line side input of that equipment.
Fiber optic cable. For severe noise contamination areas, nothing beats a fiber optic setup. Although there's a higher initial cost, fiber optic cable is immune to all electrical noise, period!
If you think you have operational problems as a result of RFI, use some common sense troubleshooting techniques. First, walk around your facility. Do you see anything new or changed? Did someone forget to put covers back on a transformer or close a door to a piece of equipment? Then, take out your pocket AM radio and start listening for static in the environment as you walk around. The stronger the static, the closer you are to the source. Good troubleshooting!
Turkel is a senior instructor, ATMS Technical Training Co., Owings Mills, Md.
Sidebar: RFI -- Then and Now
You may think radio frequency interference is a relatively new problem, but think back. Do you remember your family's car from the 1960s? Sometimes, when you pressed down on the accelerator you would get this whining sound on the radio's AM stations. To get rid of this problem, you'd have to go out and get a choke for the distributor to filter out the noise from the switching of the points on the distributor. The same problem happens today. In fact, auto manufactures have a lot to worry about nowadays. Just think about those ABS brakes that use switching devices and a trunk-mounted cellular phone in the same proximity!