Mtn Charley
join:2002-02-17
Zyante
| Grounding unused IW pairs
Has anyone tried grounding their unused IW pairs to help eliminate RFI or other DSL interrupters? If so, has anyone had any success? It is a common practice in ribbon cables to have every other conductor ground. Seems logical that grounding unused pairs in a four pair may help. |
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nunya
SEE ROCK CITY 475 MILES
Premium,MVM
join:2000-12-23
O Fallon, MO
clubs: | I do it any time I appear to be getting rfi. I get mixed results. It's worked better than filters in some instances.
Sometimes it works wonders, other time it doesn't help at all.
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Call your CLEC after a tornado. |
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n8crwlr
Premium
join:2000-12-10
Cleveland, OH
clubs: 
| reply to Mtn Charley
I have done it also in the past with mixed results. I have also used RFI filters that work fine in the NID, and others that worked only at the phone. I have also seen just a modem on the line converting RFI to audio onto the line and the filter would not work anywhere on the line with the modem attached. There is not a one fix all in these cases, and a friend of mine has a T.V. that picks up the transmission from a local tower and he had them check the levels in his yard, they ended up rotating the tower slightly....
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DSLR SETI join today at www.broadbandreports.com/forum/seti |
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RadioDoc
Sortofadog
Premium,ExMod 2000-03
join:2000-05-11
Chicago, IL
 ·AT&T Midwest
| reply to Mtn Charley
RFI suppression is an art, not a science. The same thing may not work twice, even in the same location. All it takes is a change in the environment.
One of the oddest things I've run into is corroded rain gutters and downspouts reradiating radio signals on different frequencies than the originals. I had to actually RDF (radio direction find) to the house with the bad gutters in order to fix the problem (which wasn't my client's responsibility but we did it anyway).
Grounding unused pairs can actually increase the amount of RF in a cable since you then have an antenna. As long as both ends of a balanced line (which is what the phone loops are supposed to be) are not grounded there should be no RF current except the "common mode" signals, which cancel out in the equipment since they are out of phase on the loop.
It is always better to try to find the cause of the RF injection or detection than to just treat the symptoms. If it never gets converted to audio it won't bother POTS, but might mess with DSL. If/when you get a lot of RF in a loop, look for a partial short to ground, or equipment at the customer location which may be causing it (like that modem you mention).
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"Nothing fuels a good flirtation like need and anger and desperation"-Aimee Mann |
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Mtn Charley
join:2002-02-17
Zyante
| [QUOTE= RadioDoc 
. As long as both ends of a balanced line (which is what the phone loops are supposed to be) are not grounded there should be no RF current except the "common mode" signals
[/QUOTE]
The difference in "common mode" and "normal mode" always escapes me. I want to say that "common mode" is on both inputs of an opamp, so it never shows on the output, but that is probably wrong. Remember doing 100% testing for noise rejection of both types on some NASA stuff but that was an awful long time ago. |
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tr6scott
join:2002-03-23
Oxford, MI
·AT&T Midwest
| reply to Mtn Charley
said by RadioDoc :
RFI suppression is an art, not a science.
Maybe a name change is in order... How about RadioVooDoo, You did mean the Black Arts?
But I would suggest RadioSherlock, Gutters as wave guides, who would have thunk it.  |
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RadioDoc
Sortofadog
Premium,ExMod 2000-03
join:2000-05-11
Chicago, IL
·AT&T Midwest
| reply to Mtn Charley
Actually you have it right.
"Common mode" refers to equal in-phase signals on both sides of a balanced line. It is one of the design basics of unshielded twisted pair...the twists keep the common mode signals equal. The input to a perfect op-amp only reacts to the difference in voltage between it's two input pins (inverting and noninverting). Theoretically you could put 1000 volts of common mode RF across them and it would be ignored since it would be equal and in phase. Reality intrudes into Electronic Utopia though since the inputs do have voltage limits and are not perfectly linear or identical.
Transformers, which are what existed before op amps of any flavor, are what balanced (audio) lines were originally designed to work into. Those definitely only respond to the differences between their two input wires. With proper equipment on each end you can get 90-100 dB signal-to-noise on a reasonable unloaded phone pair used for a 15 KHz audio circuit. We used to do it all the time feeding program audio to transmitter sites, where the RF is 1,000 times more intense than at any customer location you might encounter.
As for the name change. ROFL. I have a client who says I need a black pointy hat with gold stars on it, and a wand.
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"Nothing fuels a good flirtation like need and anger and desperation"-Aimee Mann |
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claco
join:2002-09-29
Tallmadge, OH
·AT&T DSL Service
| reply to tr6scott
said by tr6scott :
said by RadioDoc :
RFI suppression is an art, not a science.
Maybe a name change is in order... How about RadioVooDoo, You did mean the Black Arts?
My favorite is H.P.F.M.
Hocus Pocus Flippin (or other variation] Magic |
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Mtn Charley
join:2002-02-17
Zyante
| reply to RadioDoc
said by RadioDoc :
RFI suppression is an art, not a science.
In another of my many endeavors, I worked for a company that used RF to weld 50mil plastic bags for anerobic chambers. (biological and gene research). I had worked on radar in the past but could not believe the strange things that RF would do in that work space. It is definitely closer to VooDoo than science. I'm sure that some Phd could come up with 15 or 20 simultanious equations to explain it. |
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Jan Janowski
join:2000-06-18
Skokie, IL
·AT&T Midwest
| reply to RadioDoc
said by RadioDoc :
RFI suppression is an art, not a science.
AMEN DOC!!! Personal experiences:
1. About 20 years ago, Receivers downtown got heavy Intermod (mix products) that got worse after rainstorms....
They found a rusty cresent wrench lying on metalwork at top of Handcock. Dis-simelar metals acting as diodes, and length of cresent wrench acting as antenna....
2. Very regular flashing on TV set (Ch 2 & 5) nextdoor. Neighbor came over, thinking I was interferring with his TV (I'm a Ham Radio Operator, too).... He saw me sitting watching TV, and no flashing.... Traced to snow in broken outside Christmas lights, that had a flasher in the string...
3. Neighbor (different one) hearing strange whining in AM radio. TV on Channel 2 has intermittant herringbone. No simelar occurrances in surrounding homes. Traced to oscillating incandescent lamp.
4. AM Radio station heard at low level off and on, with no radio turned on..... Sound eminating from heating ducts. added ground braid and screwed sheet metal screws in ductwork. Problem went away.
5. AM Radio heard in cheapie telephones... Not heard in other phones not needing "Wall Wort" power supplies. Looped phone line and power lead through Ferrite bead.. Fixed.
6. "Donald Duck" sound heard on stereo (Single Sideband transmissions being demodulated in audio amplifier). .001/100V caps on speaker wire connections at back of stereo. Ferrite filters on same, and AC power to Stereo. High Pass Filter on FM antenna on Stereo. Problem 99.44% gone.
7. Cable TV causing herringbone interferrence in neighbor's TV set. Terristrial TV signals on same channel & TV received fine, just cable feeds bad. Different TV in other rooms or this room does not have problem. TV is early 1980's vintage. Unable to fix: Vintage receiver in TV was not designed to reject many adjacent channel signals at strong amplitudes. Get new TV.
8. Wierd whining or whish-ing heard on cheapie. AM radio during suppertime. Traced to interferrence from Microwave oven.... (Only when Microwave is on) Other radios, TV's un-affected by this. Ferrite filter un-successful in removing noise. Found I could eliminate problem by reversing the AC Plug (both prongs same size).
9. Inside an Ham Radio Transmitter, a Power modification was done, and the increase in RF power was affecting the audio circuits, causing RF feedback. Increased power supply decoupling without success. Eventually added .001 and .1 bypass caps on all Op-Amps in audio circuit, and changed power into this shielded circuit from a plastic Molex connector, to .001 feedthrough-VHF style caps drilled/soldered through metal shielding, and added ferrite beads on either side of the caps. Audio never sounded so clean!
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Looking for 1939 Indian Motocycle |
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MikeC
Premium
join:2001-09-24
Des Plaines, IL | reply to Mtn Charley
Good info. Could this be added to the FAQ? |
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