Technical Bulletin
| Telephone
Interference Technical Bulletin |
Telephone RFI can be cured in even the most irritating cases! The following information will provide a good understanding of telephone RFI and offer practical, proven procedures you can use to eliminate it. |
| About K-COM Filters K-COM filters are electrically effective, mechanically sturdy devices designed for rejection of radio frequency interference to telephones and telephone equipment. Any number of K-COM filters may be used to block RF with no adverse effect on telephone equipment performance. K-COM manufactures a full line of plug-in modular filters. Modular filters offer the fastest, most convenient method of fighting telephone RFI. But in instances of severe interference, filters only at the telephone may not sufficiently reduce RFI. For this reason, K-COM also manufactures filters in a hard wired configuration that lets you insert additional interference rejection in telephone wiring where modular connectors are not used. |
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| Interference Overview Telephone radio frequency interference (RFI) is the demodulation (deriving of information) of radio signals by telephones. Modern telephones with solid state components exaggerate the problem due to their low tolerance to even the slightest levels of RF (radio frequencies). When radio transmitting sources are nearby, wires connected to telephones can act as an antenna, thus inviting the flow of RF current. This current flows directly into the telephone where demodulation will combine interference audio with desired telephone communications audio. RFI can also cause improper operation of telephone equipment including false triggering of answering machines, corruption of fax and modem data, and irregular function of special features on electronic telephones. RFI may have a greater effect on some telephones than others within the same house. This is due to variations in the amount of RF current reaching individual telephones and differences in telephone designs. The following K-COM interference-fighting program is based on reducing RF current on telephone wiring before it reaches telephone equipment. A combination of system diagnosis/improvement and K-COM filters will help you solve the interference. Be a "frequent listener" by checking affected telephones each time you change anything in the system. You could be pleasantly surprised to notice improvements at any time. |
| Faulty Telephones As a result of component failure or poor design, it is possible for a telephone to be the cause of interference. Some telephones continue to operate even with a malfunction that causes interference to other phones in the home! If you have two or more telephones, conduct the following test to find out if telephones connected to your system are adding interference. Tested phones should be kept in the "on hook" (hung up) mode. Select a telephone site at which interference can be heard (we will refer to this as the control point). Unplug all the other telephones and any equipment connected to the telephone line such as answering machines, modems, spike protectors, etc. Listening at the control point, reconnect one device. If interference increases by any noticeable amount, that device is the source or at least a contributor to interference. Unplug that device and reconnect another. Use the same method to check all equipment. To complete the test, move the original control point telephone to another phone jack. Using one of your other phones at the control point, listen for any interference variance. Through process of elimination, you have now determined what effect, if any, your equipment has on the telephone circuit. |
Identify System Components Home telephone systems start at the Protector Box installed on the exterior where the telephone company "drop" reaches the house. This device protects home telephone wiring from lightning. From here, wiring goes inside the house to the Service Entry (also referred to as an Interface or Connector Block). Wiring leaves from this point to telephone jacks.
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| System Condition Telephone system configurations vary widely from one house to another. However, this principle applies all the time at every site: if the wiring and the ground do not meet established specifications - that system is overly sensitive to RFI. This explains why interference can be worse at some houses than others exposed to the same nearby radio transmitting source. So, it makes sense to verify the general condition of an affected telephone system. Good practice suggests that you begin with an examination of the telephone wiring. Even experienced telephone personnel might overlook these three conditions that contribute to RFI: (1) Poor Ground; (2) System Deterioration; and (3) Installation Errors. |
| System Ground For maximum interference rejection, make sure your system ground is of the highest possible quality. Proper telephone installations establish system ground either at the Protector Box or the Service Entry. You should be able to see a heavy gauge ground wire that goes to either the power company ground or to a known-to-be-conductive cold water pipe. Connections to any other source of ground should be considered suspicious. Telephone companies will generally correct an improper ground at their expense. Good grounding goes a long way toward shielding your telephone wiring from intruding RF. An upgrade in system ground at some sites can eliminate telephone RFI. At other sites, a better ground can reduce interference noticeably, requiring less work and/or fewer filters. |
| System Deterioration Home telephone systems can be plagued with many problems and still keep operating! When system deterioration is slow, users may notice nothing in telephone performance to be concerned about until a nearby RF field is present. Poor electrical or mechanical conditions in telephone wiring add greatly to a home's susceptibility to RFI. Assuming you have already determined good condition of system ground, look for signs of deterioration. These include corroded wires, poor mechanical or electrical connections, moisture, breakdown of insulation on wires and cables, and stretched or damaged cables. Examine as much of your system as you can from the Protector Box to the Service Entry to every visible cable run to every phone jack. Make corrections as necessary. |
| Installation Errors When your home was wired for telephones, what if the installer took shortcuts, did inferior work or overlooked something? Before you say "impossible", remember that many problems can exist in telephone systems with no noticeable effect until a nearby RF source appears. Here are key errors to look for as you examine the system: A common oversight in Loop Series installations is interruption of continuous ground. Telephone cable leaving the Service Entry includes the "pair" and the important ground wire. Correct wiring brings the cable to the first phone jack in the system, connects pair to the jack and then continues pair and ground to following jacks. We have seen instances where the installer continued the pair from jack to jack but did not continue the ground wire. As you examine each telephone jack, make sure the ground conductor continues to the next jack. Wiring will stop at the last jack in the system. Beware of speaker wire, thermostat wire or any wire type that isn't actual telephone wire. Unqualified personnel often add new telephone jacks without regard to the proper type of wire (see Poor Quality Wire). Or, they may have used telephone wire but failed to choose a matched pair of wires within the cable. Staples driven through telephone cables can create real havoc. Staples can cause wires to cross, resulting in unbalance which upsets the system's designed-in interference rejection. If an installer's badly aimed staple severed one of the wires comprising the pair, the telephone fed by that cable wouldn't work and he would have fixed the error. But, if it only severed the ground wire or caused crossed wires, there's a good chance it went unnoticed. Make a careful visual inspection for such damage. |
| Unused Conductors and
Cables To upgrade your system's RF shielding, ground all unused wires inside active cables. Unused wires are commonly left hanging at the Service Entry. Simply connect them to the system ground. If your home is wired for Parallel Distribution and there are extra phone jacks you aren't using, disconnect the cables feeding them at the Service Entry. Cables disconnected in this manner need not be grounded since they are no longer part of the system. |
| K-COM Filter Applications When RFI persists on a telephone system with a clean bill of health, use K-COM filters to chop intruding RF down to size.
K-COM developed the RF-2 to provide a convenient method of electrically breaking up the line. Use it at the Protector Box, the Service Entry, the beginning and end of cable runs and inside phone jacks. Refer to Figures 3 & 4 for RF-2 installation instructions. |
Step One Begin
with a Model RF-1 modular filter inserted at the line cord input of the telephone. It's
important to locate the filter near the telephone because long line cords can intercept 14
- 30 Mhz RF. (If you're working with a wall phone or older non- modular equipment, install
an RF-2 behind the wall mounting plate or inside the phone jack). Check results. |
Step Two Figures
1,2 & 4
indicate possible sites for filter insertion; however, filters are rarely needed at all
sites. Requirements at your location determine which sites are applicable. Generally, the
best site for installation of a second filter, if required, is at the Service Entry.
Install an RF-2 in the wiring just ahead of the connector block (in other words, before
the cable(s) going to phone jacks). Check Results.The reason for filter placement at the Service Entry is to "shut out" any RF currents picked up by phone company wiring outside of the house. Same applies to filter insertion at the Protector Box. Filters may sometimes be needed at both sites. From here forward, your system wiring and the intensity of the RF will determine appropriate action. |
| Loop Series Filtering Install an RF-2 inside a phone jack (preferably the first jack in the system). Note Figures 3 & 4 to make sure you install the RF-2 so it filters the wiring, not just the wires feeding the phone jack. Check results. Add filters at additional jacks as needed. You should detect significant RFI reductions as you go along. If you have placed RF-2 filters in every jack and traces of interference remain, return to the Service Entry and add an additional RF-2 (in series with the one installed previously). Only the most difficult RFI cases require doubling up on filters, but you have that option anywhere in the system. |
| Parallel Distribution
Filtering An RF-1 at the telephone in combination with an RF-2 at the Service Entry (as described above) will usually solve RFI. If further treatment is required, return to the Service Entry and install an additional RF-2 at the beginning of the individual cable feeding the phone being treated. Check results. The old expression "One bad apple can spoil the bunch" is sometimes applicable to Parallel Distribution wiring. One or more of the cables may be intercepting greater amounts of RF current than others. Because all the cables terminate at the Service Entry, filtering just one may not solve the interference. The best troubleshooting approach to take when you have many cables feeding phone jacks is to "simplify the problem." To do this, disconnect all but one cable at the Service Entry. With the combination of filters described above, you should be able to remove all traces of interference on the single active cable. Then, one at a time, reconnect cables, adding filters as required. Massive RF levels on an exceptionally long cable can create a worst-case condition. It may become necessary to physically cut a long cable in one or more places for addition of extra RF-2 filters. Consider doing this only after you have filtered all the other cables and have experimented with doubling up on filters at various points. If a hard cut is required, be sure to maintain integrity of the ground wire in the cable. |
| Coiled Cords The preceding text is devoted to helping you "cool down" RF current in system wiring before it reaches the telephone. Little, if any, RF current should now be reaching the line input of your telephone equipment. The coiled cord between the base and the handset on telephones is another potential interference entry point. Coiled cords are available in a variety of lengths, many of which are nearly the electrical equivalent of resonant antennas at radio frequencies 14 - 30 Mhz. This interference can sometimes be confirmed when movement and/or stretching of the coiled cord causes changes in the amount of interference you hear. You can also try replacing the cord with another of a different length. This may or may not provide a solution, but if you note a change in the interference, you can be certain RF is entering through the cord.
Some styles of telephones contain electronics in the handset (Slimline, Trimline, amplified phones, etc.). With these phones, install the coiled cord filter at the handset. |
| Cordless Telephones The most common causes of RFI on cordless phones are: (1) RF on the incoming phone line; (2) RF on the power supply line cord; and (3) RF interference with the phone's radio circuitry. To determine whether RF is entering through the phone line, disconnect the telephone line cord from the base of the phone. Use the intercom feature to activate the phone so you can listen. If interference is gone you have confirmed RF on the phone line and can eliminate it using the same methods described herein for regular telephones. If interference persists with the telephone line cord disconnected, one of the remaining causes is probably at fault. RF on the power cord can sometimes be identified when loudness of the interference varies as you move the position of the cord. Try moving the base unit to another location. In some cases, moving the base to a different AC outlet will solve the problem. In the alternative, the use of a ferrite choke on the power cord is recommended. RF interference to the phone's radio circuitry is most likely the result of a design or manufacturing deficiency. Try moving the base unit to another location where a reduction in the RF energy field could make a difference. If this fails, you may need to replace the phone with another that is less susceptible to RF. Due to variances in manufacturing, it is possible for one phone to be less RF-sensitive than another of the exact make and model! Cordless phones operating in the 900 MHz range appear to be less interference-prone than those at 40 MHz. |
| Modem Protection Protection From RFI: Radio Frequency Interference can corrupt Fax/Modem data transfer when interfering audio combines with desired signals on the telephone circuit. Connect a K-COM RF-1 Single Line Filter to the telco input of the modem. If additional filtering is required, try a second K-COM filter in series with the first. For severe interference, refer to Telephone Interference Technical Bulletin. Protection From Lightning: Modems are subject to damage from electrical storms. The consequences of a direct lightning strike are obvious. But many modem failures are caused by Electrostatic Discharge (ESD). A high voltage potential from a lightning strike can be induced on telephone lines, even when the strike occurs beyond the immediate vicinity. Most every form of induced energy travels in the common-mode where K-COM Filters are designed to reduce current flow. Thus, K-COM Filters provide spike attenuation. Users also report their modems were saved because K-COM Filters opened the telephone circuit in fuse-like fashion. In these instances, voltage spikes destroyed the filters but never reached the modems. |
Two Line Telephone Systems Two line
telephone systems are treated similar to single lines. Use RF-2
filters for each line. Use RF-1 Two Line filter at the
telephone set. |
Business Telephone Systems Proper filter selection for the input
(telephone line) side of the KSU depends on wiring configuration. When RJ-11 jack supplies
single phone line(s), use RF-1 Single Line filter. When RJ-14
jack supplies two lines, use RF-1 Two Line filter. If system is
fed by RJ-21X block, use RF-2 Hard Wired filter on each incoming
line. K-COM RF-1 Two Line filters are compatible with the
output side of electronic key phone systems since individual stations (voice terminals)
are typically fed with two lines from the KSU. |
| Poor Quality Wire In recent years K-COM has noted a disturbing trend in the manufacture and installation of telephone cable. Some manufacturers are ignoring the long-established criteria for telephone conductors; that is, the "twisted pair". Each pair of conductors inside a quality telephone cable will be twisted together in an aggressive manner that is easy to spot. Two-pair or four-pair cables commonly found in residential installations should have twisted pairs which then twist around the remaining pairs through the length of the cable. The twist is a simple, yet effective method of avoiding a wide variety of interference. Using the principle of phase cancellation, the twisted pair allows telephone wire to be installed in or near electrical or magnetic fields. With this form of "shielding" the wire is often installed parallel to electrical circuits with no ill effects. Be extremely suspicious of the newer flat multiconductor cables in telephone wiring. There is no interference protection. Inexpensive round cables sold as telephone wire may have only a gradual twist, twisting four or more conductors as a group. For all practical purposes these are straight-line wires, no more suitable for telephone applications than thermostat or audio wire. Not only are these highly susceptible to interference from external sources, but they can be self-trashing when two or more pairs are used (each circuit can interfere with the other!). We bring this to your attention for two important reasons:
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| Summary Some of the conditions and treatments just described apply only when interference is extreme. Most RFI cases can be solved with just a few filters and very little effort. Breaking up the line with brute force RF filtering applied as instructed absolutely must result in RFI reductions. If you were unable to accomplish reductions, a faulty wiring condition is likely in your system. Here's a hint: if filters have no effect; if interference is equally loud on every telephone in the house; and if the interference sounds like a raw AC hum, the condition is not RFI but "Foreign EMF" which cannot be corrected with filters. The source of this may have been mentioned earlier under System Deterioration and Installation Errors. Telephone company assistance may be required to locate and correct this condition. K-COM recommends that persons inexperienced in working with electric circuitry avoid contact with telephone wiring. Telephone wiring in the U.S. carries 24 to 48 VDC and a 20 Hz 90 VAC potential during incoming rings. |
| Additional Reading To learn more about contemporary solutions to interference problems, refer to the following:
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| K-COM, P.O. Box
82, Randolph, Ohio 44265 USA Toll Free 877-242-4540 · Phone 330-325-2110 · FAX 330-325-2525 |
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