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1989-06-29
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This describes the construction of a passive filter intended to
suppress Radio Frequency (R.F.) interference.
* * * * CAUTION * * * *
Assembly of this, and any other electronic devices should be
attempted only by qualified persons bearing the experience and
know-how of standard electronic construction techniques. No
warranty is given, either expressed or implied, as to the
suitability of this design for any particular application. The
user assumes all responsibility for any subsequent damages to
persons or equipment. This design is given freely; no charges or
fees of any kind may be exchanged for the trade of these files.
All files must be transferred intact and in full.
* * *
You should have the following files within FONEFLTR.ARC:
FONEFLTR.ASC: is the schematic diagram of the filter in ASCII
format. If you do not have AutoCAD, OrCAD, or
desktop publishing software capable of reading
FONEFLTR.PLT, you will most likely read this file.
FONEFLTR.DWG: the schematic diagram file for AutoCAD. AutoCAD
release 9.0 was used to create this copy. It is
believed that this should also be readable by
earlier versions of AutoCAD.
FONEFLTR.PLT: an AutoCAD plot file of the schematic, readable by
PageMaker, and other desktop publishing software.
This file was created using the ADI plot format.
FONEFLTR.SCH: the schematic file created with OrCAD version
3.11. Earlier versions of OrCAD will not accept
this format. All parts within the drawing are
contained within the DEVICE.LIB as shipped from
OrCAD Systems.
FONEFLTR.TXT: this file.
APPLICATIONS
The filter circuit represented in the various FONEFLTR schematics
is intended to suppress R.F. energy in standard Ma Bell dial
circuits. If you can hear radio stations in your dial telephone,
this filter should solve the problem. This filter will NOT fix
"static noise" problems. If you hear "crackly noises" in your
phone, this filter will not be of much help. The "static noise"
problems are always created by intermittent connections, either
with your equipment, or with the phone company, but in no case
resolved with a filter. If you live very close to a radio station
(specifically A.M.) things get more difficult quickly, and this
circuit may not be sufficient.
Printed October 23, 1988
(C) 1988 Martac Engineering Page 1
FONEFLTR.TXT Elimination of Radio-Frequency Interference
CIRCUIT DESCRIPTION
The filter circuit is arranged in a balanced "O" configuration,
with a shunt input capacitor, two series inductors, and a shunt
output capacitor. The capacitors present a low impedance path to
R.F. signals, while the inductors present a high impedance path
to R.F. signals. Thus, R.F. is blocked by the inductors, and
shorted by the capacitors.
For convenience, the circuit is usually connected via standard
modular phone jacks. Note that polarity is intentionally crossed
in the filter diagram. This is to maintain proper Touch Tone
operation with many phones. Neglecting to cross the polarity as
shown, will prevent some tone telephones from dialing out. If
your Touch Tone telephone won't "dial out", or "break dial tone",
the cause may be flipped polarity of the wires.
* * *
CONSTRUCTION TIPS
Most of the parts may be obtained locally. The most difficult
parts to find will be the inductors (also known as "chokes").
Make sure you're getting "1.0 mili-Henry" (also 1.0 mH)
inductors, NOT "1.0 micro-Henry" (also 1.0 uH)! The capacitors
should be rated at no less than 200 volts (200 WVDC). The
preferred composition of the capacitors is "Metalized Polyester
Film". Non-polarized capacitors should be used, and Electrolytic
or Tantalum capacitors should NEVER be used in this type of
circuit. Usually, the most expensive parts will be the box that
the circuit goes in, and the modular jacks. Exactly how the jacks
get mounted on the box is not terribly important. However, it is
important that the circuit be protected in some kind of
enclosure(1); connections should be physically and electrically
secure and free of short-circuits. Metal boxes are preferred over
plastic, since metal helps to shield the components against
inductive pickup. Two modular jacks may be mounted on one side of
the box, and the filter may be wired inside, directly to the
screw terminals on the jacks (if available) as they extend inside
the box.
--------------------
1. Generally, all circuits constructed should conform to "Good
Engineering Practices", providing adequate physical support for
all components as well as electrical integrity. Construction
techniques using perfboard, or mounted solder lug strips can all
be acceptable means of wiring.
"Wire-Ball" construction is NEVER acceptable. Component leads
that have been "tack-soldered" together, and components pressed
into a wad, eventually result in circuit failure. Potential
intermittent joints, electrical shorts and the like are marks of
the neophyte. Insist on quality!
Printed October 23, 1988
(C) 1988 Martac Engineering Page 2
FONEFLTR.TXT Elimination of Radio-Frequency Interference
All parts can be purchased through:
MOUSER ELECTRONICS
1-800-34-MOUSER
Mouser Electronics is a good, (usually inexpensive) source for
most common electronic needs. They have catalogs free for the
asking...
Suggested Mouser part numbers for components:
reference description Mouser part number
========= ======================== ==================
L1, L2 1.0 mH inductor ME434-1120-103K
C1, C2 1.0 uFd 250WVDC non- 23MA510
polarized capacitor
Address further questions or comments to myself on:
Channel 1 Communications BBS:
Main access line: 617-354-8873
(please leave messages in the Main Conference - 0)
written by: Marty Acuff
Printed October 23, 1988
(C) 1988 Martac Engineering Page 3