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Computer Interference FAQ

Q: I want to use a computer in my shack. I have heard that computers can cause interference to my station receiver. Aren't there laws against computers interfering with hams?

A:  Unfortunately, nearly any computer system has the potential for causing interference. Computers are classified as unintentional radiators under Part 15 of FCC regulations. Part 15 limits the amount of interference that will be caused by a computer system, but the regulations protect a neighboring home from having television interference when you operate your computer, for example. They are not usually sufficient to ensure that your computer will not interfere with sensitive reception in the same room.

Q: I have heard that there are different requirements for industrial and home computers. Are the industrial computers the best?

A: Not from an EMI point of view! The FCC has two classes of computing devices -- Class B for home use and Class A for industrial environments. The Class B devices are quite a bit less noisy than their industrial counterparts. Make sure you use only Class B devices.

Q: How do I know which is which?

A: If the computer is being offered for retail sale at your local computer store or being advertised in a magazine for the general public, you can be almost certain that the computer is a Class B device. However, some of the "treasures" available at flea markets may have been salvaged from an industrial computer, so stay away from unknown bargains if you want to have an RF-quiet computer system.

Q: How is this noise generated by a computer?

A: There are several things in a computer that can generate noise. All computers use digital signals -- square waves rich in harmonics. These signals can be generated by the several oscillators found in most computer systems. Signals from the oscillators can interfere with the signals we want to receive. In addition to the oscillators, all computer circuits sub-divide these oscillators into signals that are sub-multiples of the oscillator frequencies. Additional digital noise can be generated by the video monitor circuits. Computers also use switching power supplies. "Switchers" can also be prolific generators of RF noise. The monitor has a separate power supply, plus sweep and high-voltage circuits that can also generate noise. When you put them all together, a computer system can generate RF signals from below the HF band well into VHF!

Q: How do all of these signals get out of the computer system and into my station receiver?

A: Most of them are radiated, and then picked up by your antenna. This suggests one easy cure -- put as much distance between the computer system and your receiving antenna as you are able to. Interference that is intolerable with an indoor antenna might go away completely if you erect a good, outdoor antenna. In addition to reducing the amount of signal picked up from the computer, this has the benefit of increasing the amount of desired signal. Most computer systems have shielding to minimize the radiated signals. The shielding can be external, provided by a good metal case and shielded cables, or internal, provided by ground planes or metal partitions on critical areas on the printed circuit boards. This shielding is not perfect, however; a surprising amount of energy can be radiated from the slots and seams that are part of most shielding systems. All other things being equal, a computer with a metal case is usually much quieter than one with a plastic case. The expression "Components don't radiate, wires do" is well-known in the EMC field. The wires and cables used to interconnect the parts of your computer system are much better antennas than are the printed-circuit boards used inside, so most radiation takes place from the wires. The video, keyboard, mouse and printer cables are prime suspects. The interference could also be conducted -- transmitted directly by wires connected to both the computer and your receiver. If your computer is plugged into the same AC circuit as is your receiver, you are asking for trouble. If so, you may be able to use an AC-line filter to filter the computer, your receiver or both.

Q: Do all computers cause the same amount of interference?

A: No. There are significant differences in the amount of interference generated by different computers. The amount of noise is determined by the speed of the digital devices, the design of the printed-circuit boards, the quality of the shielding used in the computer, the shielding of the computer cables and the configuration combination used. The interference potential of a computer system often changes each time you change peripheral cards, printers, cables, keyboard or monitor.

Q: Wow. It sounds like it's impossible to figure out what computer system is best. What should I do?

A:  First, talk to other hams with new computer systems. If they have found a new model that is interference-free for them, chances are you can make it work, too. Make sure you are comparing apples and oranges, though; if you want to use the computer to operate on HF packet, don't ask only 2-meter operators. The interference of a computer system often varies considerably with frequency; a computer that is noisy on 14 MHz might be squeaky clean on 144 MHz, or vice versa.

Q: I want to buy a new computer with the latest features, but none of the locals hams have one, so I can't ask them if they have a quiet one. Do you think the dealer can help?

A: The dealer probably doesn't know much about Amateur Radio (perhaps there is some "Elmering" opportunity here!), but if you are willing to do most of the work, you might be able to find a cooperative dealer. (If a dealer doesn't want to work with you when you are standing in the store with a pocketful of money, this may tell you something about the level of support you will get if you have problems later!) You want to identify a computer that is relatively radio quiet. Bring along a battery- operated receiver that tunes the frequencies you want to use. (There are several portable short-wave receivers that cover HF and VHF bands.) You will be looking for two things: first, you want to determine which computer systems are the most quiet, overall. To do this, turn off all the computers in the store. (If you show up at the dealer when there is a store full of customers, you probably won't be able to do this.) Turn on the radio and note the amount of ambient noise level on several different bands, using either an "S" meter, or an evaluation of the amount of sound coming from the radio. Now, select a computer system and turn it on. Note how much the noise goes up. If you are interested in a few specific frequencies, check those first. You may be able to live with some broadband noise, but would find a birdie right on top of your favorite repeater or net to be intolerable. You also want to get an idea of the amount of broadband noise. Tune to your favorite bands and note the amount that the noise goes up when you turn each computer on and off. All other things being equal, you want to select the most quiet computer. Do your initial testing with the peripherals turned off and their cables unplugged. Turn on the peripherals one at a time; you may find that the bulk of the noise is coming from the printer or monitor, for example.

Q: I found a relatively quiet computer and purchased it. But my old machine that I want to use as a packet terminal is much noisier. How can I get the gremlins out of both of them?

A: The cures for both are similar, although we are going to check a few things on the old machine that will not be necessary on the new one. First, simplify the computer system as much as possible. Determine if the computer is noisy when turned on with no keyboard, mouse or monitor. If it is, listen to the sound, preferably using an SSB or CW receiver. If the noise is a broadband hash, decreasing slowly from band to band, it is probably the power supply.

Q: That is what I heard. Where should I begin?

A:  There are three techniques useful to cure any interference -- ferrites, filters and shielding. Ferrites are the easiest to use. (A partial list of ferrite suppliers appears at the end of this article.) To make a ferrite common-mode choke, wrap 5-10 turns of a conductor onto an FT-240-43 ferrite core. (The "240" indicates that the outer diameter of the core is 2.4"; the "43" designates the material. Other materials may be useful, but 43 is a good all-around material.) Do not substitute snap-on beads or other unknown materials unless you are understand how to use ferrites. Unfortunately, it may be necessary to remove one end of cables with large connectors. (The "snap-on" types of ferrites can be effective against VHF noise, but it takes a lot of them to do much for the lower end of HF.) To cure broad-band noise, or any other noise that is being radiated by the power leads, first install a common-mode choke on the AC-line cord, right at the computer. If it doesn't cure the noise, try an AC-line filter next. That should take care of most cases of power-supply noise.

Q: I don't have an AC-line filter; can I use a surge suppresser?

A: Although a few of the surge suppressers also contain EMI filtering, a surge suppresser is not an EMI filter! Even those that do have EMI components are not usually as effective as a standalone EMI filter.

Q: That took care of the broadband noise, but I still hear lots and lots of carriers up and down the bands. When I hooked up the monitor and keyboard, they seem to change every time I make the computer do something.

A: You are hearing digital signals, harmonics of the various data signals inside the computer. It sounds like the computer itself is doing some radiating. Start by removing the case. There are dangerous voltages inside, so this should be done only by qualified people! This could also affect the warranty on your system, so check with the dealer first. Inspect the computer for missing or loose shields, disconnected ground straps, etc. Many a computer noise problem has been created when some work was done on the computer. That little ground connector or the bolts holding back of the computer to the case may have helped reduce noise. It is also possible that one or more connectors may have developed poor contacts.

Q: I did fix a few problems and it helped some, but I still hear noise. What is next?

A:  Temporarily operate the computer without peripherals, monitor or keyboard. If you still have problems, the computer itself is still radiating. The only way this could be cured is by redesigning the computer or its shielding. This is not always practical. If the computer has a metal case, EMI tape can be used to seal any slots or seams in the case (be sure you don't cover ventilation holes, however!). If the case is plastic, a drastic step is to use EMI spray on the inside of the case. EMI spray is a conductive paint that can add shielding.  But, postal regulations make spray paint hard to ship, so you may have difficulty finding it. Several words of warning are in order! Changing shielding is not a simple matter. Adding metal shielding to a design often changes the thermal characteristics. Components that operated safely in open air can overheat when surrounded by metal. EMI tapes can also come loose, causing short circuits. EMI spray is paint! If the surface being painted is not clean, the paint can flake, sending metal flakes all over the place. This will cure the EMI problem by killing the computer.

Q: I'm convinced; I am going to leave the shielding alone. The computer box still makes a bit of noise, but I can live with it. However, hooking up the monitor and keyboard each add about an S unit to the noise level. What should I do?

A:  Repeat the process described above. But be careful! The monitor has some particularly dangerous voltages! Unfortunately, the monitor and keyboard both have large unshielded areas (the cutouts for the CRT and the keys). Adding shielding to these devices is sometimes impossible. But before you replace them, try adding ferrite chokes to the wires. For all peripheral devices, you need to try a ferrite on each end of each cable. Use only computer cables made from shielded wire. If you are buying a cable, ask the dealer if it is shielded. Most of the cables sold by Radio Shack are shielded. If you do run across an unshielded cable, you may be able to add a shield. The shield can sometimes be carefully worked off an old piece of RG-8 size coax. One of the ARRL Lab staff recently found a nice surprise -- we purchased a braided grounding strap at a flea market and found that it was hollow. This will make a nice shield for a noisy cable. If you add shielding to cable, try grounding to the chassis it an one end, the other end and both ends; it is often difficult to predict which will work best.


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