Q) What is the most likely cause of interference to a non-cordless telephone from a nearby transmitter?
A) The telephone is inadvertently acting as a radio receiver
The FCC published an interesting bulletin (FO-10) in 1986 regarding telephone interference. Technology has come a long way since then. However, the article is still interesting and relevant. The installed base of simple hard wired telephones is still very large.
Federal Communications Commission
Field Operations Bureau
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.
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.
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,
-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.