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DSL uses some of the same frequencies as the AM radio band, so you can often use a portable AM Radio as a detection device to track down the source.
Tune your AM radio to a quiet spot in-between stations. There must be no radio station signal for this to work. You might have better luck finding one below 1200 KHz.
Now you can "Go Sniffing" for an interfering signal.
A few of the more likely causes might be:
A two wire outlet that has reverse polarity.
An incandescent light bulb
A fluorescent light assembly
Flashing lights, such as Christmas decorations
A light dimmer... even when the lights are off.
A microwave oven
A timer motor for lights or watering the lawn
A motor (a fan in a bathroom, furnace, oven, or ceiling)
A transmitter (Ham or CB)
A darkness sensing outdoor light, even at a place down next door
If you believe you've found the source of interference, check your DSL and see if the problem is still there. If it is, turn the suspected device off and see if the problem goes away. If the problem is still there, keep going! You might have two sources of interference to deal with.
You will probably need to spend quite some time scanning until you find the source of your woes.
Here's how to fix the problems listed above. Re-scan after every fix.
If it is a two wire wall plug... REVERSE THE PLUG!!! On over half the interfering devices I've found, reversing the polarity of the plug resolves the RFI.
If it is an incandescent light bulb, REPLACE THE BULB.
If it is a florescent lamp assembly, first try new lamps, and then check the wires to the ballast (white is neutral, black is 'hot'). If that fails, replace the fixture.
If it is a flashing light string, obviously, REPLACE THE LIGHT STRING!
If it is a dimmer, check wiring on dimmer... If that tests okay, REPLACE THE DIMMER.
IF it is a microwave oven, TAKE THE OVEN IN FOR AN EMISSIONS TEST (if your store or repair facility doesn't understand what one is, go elsewhere!)
If it is a timer motor, REPLACE THE TIMER.
If it is a a TV with a three-prong cord, MOVE the television. You might also want to try an RFI/EMI AC Power filter box.
If it is a DC Motor, the coils are probably arcing. If it is an AC motor, it might be the AC start capacitor.
If it is a Ham or CB transmitter, use some diplomacy. Laws and theory are in the Ham's favor because of their FCC license. Approach the person and ask if he/she add a low pass filter to his/hers transmitter. That is about all the FCC will request on your behalf. RFI from transmissions will usually be within a maximum of 500 feet, so don't travel a mile looking for an RFI problem.
Should all the above fail, there are few other things you might try.
Visit Radio Shack and get a "Clip-on RFI Filter". It is a Ferrite core split in half, and you clip it on to the wire that's acting like a receiver. In 'mild' cases, tin foil around the wire will work. You'll also want to ground this wire to an AC outlet cover hold-down screw, or heat radiator. What you are trying to do is provide the interfering signal a convenient path to ground and eliminate it, without doing the same thing to the DSL signal.
Warning Will Robinson!!! This is a Black Magic Art....
What works today, may not work tomorrow....
Sometimes, a small amount of attenuation of the desired (Received) signal is "acceptable" in order to eliminate the offending signal. Since we don't have access to diagnostics, this part is definitely based on skill, experience, and luck. Just make certain that by winning the battle, you don't surrender the war.
I had the same problem with my treadmill and adsl. The RFI interference from the treadmill was being transmitted up the power wires around the house and passing the interference in to the telephone lines. I resolved the problem by plugging the treadmill in to a Belkin PureAV surge protector. Apparently, its "Power Filter Technology (PFT) isolates contamination to prevent AC and EMI/RFI power distortions". Whatever, it resolved the problem perfectly. Obviously, you would need the correct (voltage) version of this product for your country. http://www.amazon.co.uk/Belkin-8-Socket-Isolator-Protector-Composite/dp/B0002TSU9W
When the FAQ says "DSL uses the same frequencies as the higher end of the AM radio band" it is in error in that DSL uses the lower part of the AM band (138-1107 kHz, 224 downstream bins (32-255) ). So I would tend to find a quiet frequency at the lower end. But note that you would be looking for broadband noise-- which would be expected to hit frequencies throughout the band. So finding a "quiet" channel is most important in the procedure. Also, change -beteween" to "between".
A low pass filter on a ham or cb transmitter will not help. DSL is lower than those frequencies. Low pass filters are for TV interference. The ham or cb transmitter issue is due to fundamental overload. CB is limited to 5 watts, and ham radio to 1500 watts. If it is CB, the FCC will be able to do something if the transmitter is running more than 5 watts (this is pretty common). It will be a task to get the FCC to take action. If it is a ham transmitter running up to 1500 watts, then you will need to find a DSL filter that is specifically designed to attenuate frequencies above 1107 kHz. Such filters are not included on DSL modems because they would add to the cost and are seldom needed. However, it's still your responsibility to filter out the legal signals that are interfering with your modem.