INFORMATION BULLETIN Federal Communications Commission Field Operations Bureau _________________________________________________________________ Telephone Interference ---------------------- This document has been prepared to assist you in understanding why interference to your telephone system occurs. Recommended solutions for you and your authorized telephone service technician are also provided. Bulletin FO-10 September 1986 WHY TELEPHONE INTERFERENCE OCCURS Telephone technology today uses circuitry which, if left unprotected, will respond to the radio frequency (rf) signals from nearby radio transmitters such as Amateur, Citizens Band and AM/FM broadcast stations. When the telephone circuitry responds to the rf signal you hear the interference on your telephone. The rf signal can be entering at the telephone instrument or on the inside or outside wiring. Cordless telephones are also susceptible to rf signals. Cordless phones are low-power transmitters using radio frequencies. As with any radio transmitter, they can receive interference from other nearby transmitters. Interference can also occur if your neighbor's cordless phone is using the same radio frequency as yours. Since the FCC does not offer interference protection to cordless telephones, you should contact your dealer or manufacturer for assistance when interference occurs. RECOMMENDED SOLUTIONS The installation of a modular filter is suggested as a first step. Modular filters are available from most telephone stores. It can be easily installed by you if your telephone has a modular jack. You may wish to verify if the filter can be returned for a refund if it does not eliminate the interference. The effectiveness of filtering may vary according to the type of telephone you are using. Also, modifying certain types of telephones, as suggested below, may be impractical or expensive. When it is, consider changing to another brand or model with better interference protection. If you have taken the above steps, and the interference is still present, the telephone instrument or wiring will probably have to be filtered. YOU SHOULD NOT INTERNALLY MODIFY THE INSTRUMENT YOURSELF NOR ATTEMPT FILTERING OUTSIDE PHONE LINES. ONLY AUTHORIZED SERVICE TECHNICIANS MAY INTERNALLY MODIFY TELEPHONE INSTRUMENTS. ONLY TELEPHONE COMPANY PERSONNEL MAY FILTER OUTSIDE TELEPHONE LINES. This rule is designed to ensure that phones continue to comply with FCC registration standards. Interference picked up by the inside wiring can be corrected by installing SHIELDED inside wiring. If you have subscribed to inside line maintenance, the local telephone company can install shielded inside wiring for you. If you have NOT subscribed to inside line maintenance, they can still perform this service for a fee. SERVICE TECHNICIAN SECTION If your customer has tried the modular filter and shielded their inside wiring and the interference is still present, we recommend you filter the inside of the telephone instrument. Interference in older rotary dial phones (without special features) can usually be resolved by bypassing the carbon microphone. Install a 0.001 mfd ceramic disc capacitor in the back of the mouthpiece in the handset. Where possible, solder the capacitor directly to the microphone contact fingers with the shortest possible leads. Phone instruments with special features such as memory, automatic redial, speakers, push-button dialling, and sound amplification, contain components which are sensitive to rf signals. Shielding and bypassing of these components are necessary to isolate the affected circuit(s). Refer to the design specifications. Bell System technicians may refer to the Bell Systems Practices Plant Series Manual Section 500-150-100 for necessary modifications. Another possible solution would be to install ferrite cores. These are donut-shaped devices through which the phone cord can be wound. Ferrite cores are available in various sizes. Use one with a hole large enough to permit passing the phone cord through it two or three times. To install a ferrite core, first disconnect the phone cord between handset and telephone base. Loop the phone cord through the core two or three times. Tape or fasten the core as close as possible to the handset. Reconnect the phone cord. Two ferrite cores on the handset might be necessary. If so, place one near the handset and another near the telephone instrument. If the interference continues after filtering the phones and/or installing inside shielded wiring, the interference is probably entering the system through the outside wiring. The local telephone company service department should be contacted for assistance. Only telephone company personnel may filter outside telephone lines. Two devices which may be used for eliminating outside interference are: -a 40 BA capacitor installed at the service entrance protector, and -a 1542A inductor installed at the connector block. Your next step would be to have the telephone instrument filtered. If you own your phone, contact the dealer or manufacturer for assistance. If you lease your phone, contact the local phone company's service department. You may wish to share the Service Technician Information section of this document with the technician assigned to assist you. Even though you may be experiencing interference to other home electronic entertainment equipment, such as televisions, stereos, or VCRs, the telephone should be filtered. If the rf signal is entering the telephone system, the interference can only be eliminated at the point of entry. Filtering or shielding of the phone instrument also offers future protection to your equipment from other rf signals. You may provide to the FCC the name and/or address of the owner/operator of the radio equipment involved when other equipment is receiving interference. The FCC will communicate with that person to determine if they can assist you in eliminating the interference. FCC office addresses are listed at the end of this document. CAUTION: Only authorized service technicians may internally modify telephone instruments. Only telephone company personnel may filter outside telephone lines. END Newsgroups: rec.radio.amateur.misc From: ehare@arrl.org (Ed Hare (KA1CV)) Subject: Re: Telephone RFI Date: Mon, 5 Dec 1994 14:04:58 GMT Brian Kantor (brian@nothing.ucsd.edu) wrote: : TELEPHONE INTERFERENCE SURVEY The FCC Telephone-Interference Survey -- a Followup When the report of the FCC survey arrived here in the Lab, I was pleased! It contains useful data about telephone interference. As I read the survey, I noticed that out of the 108 transmitting stations involved, 27 were amateur. I was surprised that not one of the 27 hams had called the ARRL RFI desk to say "The FCC van just pulled up!" I was also surprised when I read the FCC conclusion that telephone filters "can't be relied on to eliminate telephone interference." The FCC report indicates that filters were only effective in about one third of the cases. My experience and the experiences of telephone-filter manufacturers are that filters work in a much higher percentage of cases. Here is some additional information that may help improve the success rate of filters. There are many variables that affect telephone filtering. The most important is frequency. Most telephone-EMI filters are optimized for specific frequency ranges. A filter designed for the HF band is not effective on the AM-broadcast band, for example. Separate filters are often required for telephone lines and handsets. A handset cord can pick up an RF signal from an HF or VHF station. Some telephones require the use of a handset filter. In stubborn cases of RFI, it is often necessary to use multiple filters, primarily to break up resonance in telephone wiring. This is not always a practical solution, especially with a neighbor's installation. You (or the FCC) should not modify a neighbor's telephone wiring; this is a job for qualified repair personnel. Some telephones are so susceptible to interference that it takes a multiple-solution approach to solve the problem. Properly selected and installed filters solve many problems. The survey report included a list of filters that were effective most of the time under the test conditions used in the survey. Some filter manufacturers offer detailed installation notes and personal assistance. "Bulletproof" telephones can eliminate interference. The FCC reported that they were 96% effective. The reports received by the Lab support that conclusion. A bulletproof phone is an excellent troubleshooting tool. They have one major drawback -- they lack all of the bells and whistles that are important to some consumers. Telephone interference can be cured! Often, a combination of immune telephones, multiple filters and troubleshooting techniques is required to have a complete solution. The ARRL has several publications available to help. The book, Radio Frequency Interference: How to Find It and Fix It has a chapter on telephone interference. In addition, a Technical Information Service information package ("EMI/RFI - Telephone") is available for an SASE with two units of first-class postage. If, after you have read our literature you could use some expert help, contact the ARRL RFI desk or your section Technical Coordinator. - - Ed Hare, KA1CV, ARRL Laboratory Supervisor -- Ed Hare, KA1CV, ARRL Laboratory, 225 Main, Newington, CT 06111 203-594-0200 ehare@arrl.org