
How to get rid of Radio Frequency
Interference (RFI) in modem connections
to telephone systems.

RFI is a common problem when you use a
direct connect modem and live close to
a strong radio signal transmitter. The
problem itself is the picking up of the
radio signal (the music or talk) from
the phone lines and having it interfere
with the normal operation. The wire of
the phone line itself acts as an 
antenna and the modem acts as a
detector. The phone itself could also
pick up some RFI.

To eliminate the problem you must
prevent the radio signal from entering
the telephone/modem setup. This is done
with an RFI filter. A RFI filter can
be bought from most any Radio Shack or
electronics store, or you can build
your own with a minimum of parts.

A store bought RFI filter is usally
a small metal box with 4 terminals on
it. (Also known as a Power Line RFI
filter, used to prevent RFI comming
in/out a power circuit.)
This box is connected between your
phone line and the telephone/modem.


        *********
>-------@       @-------->
Phone   *       *     phone/modem
Line    * RFI   *
        * Filter*
>-------@       @-------->
        *********	            |
           Ground case.


You can build your own RFI filter from
2 inexpensive .001 disk capacitors.
These too, are avalible from electronic
stores. You can also add Ferrite Beads
if your RFI is very bad. The beads are
small parts of an iron substance that
help block radio signals and can also
be bought at the electronic store.


>----------+--------===------->
Phone      |       Bead    phone/modem
Line       |
           O  .001 Cap.
           |
           |
           X----->Good Earth Gr	 nd.
           |
           |
           O
           |
           |
>----------+--------===-------->


You can see that both sides of the
phone line are connected through a
.001 disk capacitor to a good earth
ground and that the Beads are slipped
on a bare piece of wire in the phone
line. This should help eliminate
most if not all of the radio signal
comming in over the phone line.

Please note that this information
is provided for reference only and that
RFI problems should be reported to
your local pho	  company.
-Chrisdos

 [76703,717]
