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