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- Newsgroups: comp.sys.ibm.pc.hardware
- Path: sparky!uunet!gatech!usenet.ins.cwru.edu!agate!linus!alliant!mydual!olson
- From: olson@mydual.uucp (Kirtland H. Olson)
- Subject: Re: EMF interfering with modem communications--Any remedies?
- Message-ID: <1992Nov6.142750.20740@mydual.uucp>
- Keywords: modem, communications, RFI, EMI
- Reply-To: olson%mydual.uucp@alliant.com
- Organization: The Harvard Group, 01451-0667
- References: <1992Nov5.213042.10314@midway.uchicago.edu>
- Date: Fri, 6 Nov 1992 14:27:50 GMT
- Lines: 78
-
- In article <1992Nov5.213042.10314@midway.uchicago.edu> spcberto@psyche.spc.uchicago.edu (Robert Osterlund) writes:
- >Last weekend, we moved into a new home close by a local radio station. I
-
- >The phone
- >lines are quite noisy, and if you listen closely you can make out the voices
- >and music of the radio station's broadcast. Some times are worse than
- >others. The phone company says there is nothing it can do.
- >
-
- >Bob Osterlund
- >berto@clio.spc.uchicago.edu
- >SSPPCC--University of Chicago
- >
-
- You can solve this problem, but expect to learn more than you ever
- wanted to know about RF, balanced circuits, unbalanced circuits,
- nonlinear behavior of devices, and "grounds>."
-
- If none of what follows makes any sense to you, save it and find someone
- who understands RF. Give it to them and ask for help.
-
- Here's a quick explanation of what's happening. The radio frequency
- signal is so large (in amplitude) that small nonlinearities in telephone
- components act as rectifiers (detectors) and create the audio that you
- hear. If the noise and program info are generated on the phone system
- outside your property, you will need help from the phone company. But
- you will have to work with them because they will not know what to do or
- what to look for.
-
- Use a real AT&T phone--an old rotary dial type is best--connected
- directly at the entry point to your house to listen to the line with all
- the internal wiring completely disconnected. If the noise is still
- there, you need to get the outside stuff done first.
-
- If the noise is not present, connect the internal wiring and check
- again. If the noise does not reappear, an RF filter at the entry point
- should cure the problem.
-
- If the noise reappears, remove devices from the line (phones,
- answering machines, modems) and test after each is removed. If the
- problem persists with all devices removed, look for joints or
- connections in the wiring.
-
- Every joint is suspect because copper oxide/copper makes a diode good
- enough to detect a strong signal.
-
- If one device or joint accounts for the problem, it needs to be repaired
- or replaced. If the problem fades slowly as devices are removed, you
- need to reduce the RF voltage on the wiring.
-
- You will need RF filters to reduce the voltage at the devices. You may
- need to reduce the voltage between the two phone wires, between the
- phone wires and the neutral of your power system, or both.
-
- To get practical information on filters, I suggest the Radio Amateur's
- Handbook or other publications for amateurs or technicians. You might
- also check the amateur radio newsgroups and the HAM radio shareware
- files. You can buy parts at HAM fleamarkets.
-
- One useful component is the lossy core, a toroidal or cylindrical
- ferrite that dissipates RF energy, turning it to a low level of heat.
- You may have seen such cores on monitor cables, covered with plastic, or
- on the PC Jr keyboard cable. You use these by wrapping the wire through
- the center and around the outside several times.
-
- Solving problems in circuits big enough to walk around in (like a house)
- does not reliably respond to "buy X and plug it in." Near a radio
- station, the voltage between two ends of a wire can be large enough to
- affect electronics so measuring at one end does not suffice. But if you
- work methodically and test carefully, your problem can be reduced.
-
- Good luck.
-
-
-
-
- --
- Kirtland H. Olson olson%mydual.uucp@alliant.com
-