Solving Telepone RF Interference - Helpful
Hints And Suggestions
With the increasing popularity
of complex and computer-aided home telephone units manufactured by numerous
companies worldwide comes a corresponding increase in both the quantity
and severity of telephone interference caused by local radio broadcast
transmitters. The new telephones, both domestic and imported, are the most
susceptible and delicate ever built, and few manufacturers of the devices
have given much consideration or effort to designs which include resistance
to RFI and other forms of electro-magnetic interference.
worse is the fact that ground terminal
connections for telephones are rarely available at the location of
an extension telephone, and telephone wiring is all unshielded and exposed.
Telephone interference is
caused by radio signals produced in the irivnediate vicinity that are intercepted
by the mass of telephone wiring in the home and outside on telephone
company elevated wiring. Signals enter the phone on different conductors,
working their way through phone circuits and causing current flow, resulting
in voice distortion or noise.
Most telephone interference
can be remedied by the simple installation of a telephone filter that plugs
simply into the rear of the phone. These devices are designed to
insert a choking effect, or loss at radio frequencies, into the phone wiring.
They have no effect on the telephone operation. About the only
realistic way to resolve phone interference short of making internal circuitry
changes to the phone is by choking the RF signals before they enter.
There are two ports of entry
that interfering signals can enter a phone unit. The first, and most common,
is through the house wiring and into the telephone set directly as
mentioned above. The second is through the handset cord (cord attaching
the handset to the phone body). In nearly all cases a telephone line
filter will be part of the solution to eliminate the interference.
But in cases where the line filter is not completely effective another
small filter device in the handset lead may be needed to bring back quiet
enjoyment of the unit. An easy way to judge for yourself if the handset
cord is suspect is to make a short handset cord about 6-12 inches long
with the cord and tools available at most radio parts stores. If
the interference is not present when the short cord is used to connect
the handset with the phone body then the handset cord will probably have
to have its own filter installed. If the phone has a speaker then
simply disconnect the handset cord and run the test with the internal speaker
(and a line filter installed).
If a line filter and handset
filter are both installed and interference persists, then it's time to
recognize that the.telephone itself is inherently hyper-sensitive
to external electromagnetic fields.
Possible cures are replacement of the phone with a different
type or brand, or internal circuitry modifications done by a local technician.
Generally in our experience the worst offenders of telephone
interference susceptibility are AT&T and Panasonic
manufactured units. The best performers are built by Radio Shack
If you go shopping for phone
filters obtain a unit with at least 30db measured attenuation in the HF
range (3 to 30 MHz). If the filter manufacturer doesn't publish
his figures, shop elsewhere. And get one that is designed to prevent
BOTH comon mode and differential mode interference.
For the most part telephone
interference is the easiest type of interference to deal with, but sometimes
it can be insidious. Don't be afraid to experiment with different
combinations of filters, phone locations, or lead lengths to seek a final