Help needed to identify source of interference.

A little background:-

My property has three BT lines. Each line was ADSLMax enabled in June 2006. The three lines all use Speedtouch 536 routers connected to an ADSL faceplate on the master sockets and there are no extensions from these sockets.

The three lines have similar characteristics:- attenuation 50 to 52dB and an IP profile of 4.5 megs.

On Jan 13th I noticed that all three lines resynched at exactly the same time and dropped to a 3 meg profile. I use DMT tool to monitor the lines and it quickly became apparent that the SNR was fluctuating by 6dB in a regular repeated pattern. The SNR is 'normal' for exactly 3 mins 30 secs and then falls by 6dB for exactly 2 mins 25 secs. Initially this was happening for only a couple of hours a day but now it is 24/7.

See graph:- [url] www.filehigh.com/viewimg.php?f=31885&i=333731 [url]

From reading this forum I would guess that this is caused by RF interference which I think must be external to the property as it persists when ALL electrical items other than 1 PC and 1 router are turned off.

The phone lines run from a cabinet at the end of the road about half a mile away and are overhead passing numerous houses. About 300 metres further away from me in the opposite direction to the cabinet is a Sewage Works which is also supplied by the overhead cable.

My questions:-

1) Can the source of the interference be anywhere along the path of the overhead cable or does it have to be close to the house?

2) Can anyone think what type of equipment would generate this type of regular interference so that I can track it down? (I've tried the medium wave radio trick but no success.)

Many thanks

Paul

OK, firstly are your neighbours seeing the same?

Once you've found out if the problem is external to your property (as it surely is - with all other electrical items off), you need to start thinking laterally....

Assuming the events aren't exactly at intervals of 3:30 and 2:25, but just appear that way, as DMT is logging at 5 second intervals.
Looking at the period of the square-wave, it's (as near as makes no odds) 6 minutes.
I.E. Whatever is causing the problem switches on every 6 minutes (or ten times an hour), and runs for about 2.5 minutes.
Maybe start looking at pumps, compressors etc.
Might be worth chatting to the guys at the sewage works (something along the lines "do you have any pumps, or a new pump, maybe a discharge pump, that cycles in & out every five or six minutes" - they may think you're nuts, but you might just get an answer).

When the problem first started (you mentioned it only started for a couple of hours a day), what time of day was it?

Also, maybe look at anything that could be triggered by some kind of cyclic timer (maybe a security system switches on a particular camera every six minutes).

Is there a pub, or shop, nearby that has external condensing units (the outdoor part of a refrigeration or air conditioning system)?

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1) Can the source of the interference be anywhere along the path of the overhead cable or does it have to be close to the house?

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Anywhere along the route

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The phone lines run from a cabinet at the end of the road about half a mile away and are overhead passing numerous houses. About 300 metres further away from me in the opposite direction to the cabinet is a Sewage Works which is also supplied by the overhead cable.

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Also interesting is where both you and the sewage works get your electricity supply

Thanks for the reply.

The first time that I noticed the problem was 11am. It then occurred intermittently over the next couple of weeks at various times of the day but not every day.

Unfortunately there are only three other houses at my end of the street and they don't use broadband and I don't think they have security cameras. The houses closer to the cabinet are old folks bungalows and I don't think that it's likely that they would be able to help. There are no pubs or shops nearby.

I think I ought to try to investigate the Sewage works it seems the more obvious source.

I assume that the sewage works gets the same electricity supply as me. It's also carried by overhead cables at the opposite side of the single track road to the telephone cables.

Maybe I should get hold of something that can clean up the mains supply in case the interference is being carried by that?

Edited by deleted (Mon 05-Feb-07 20:55:54)

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Maybe I should get hold of something that can clean up the mains supply in case the interference is being carried by that?

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Almost certainly a waste of money. You assume that the interference comes from the sewage works on the mains, into your house, onto your router and thence onto the ADSL line. More likely, if it starts in the sewage works, it comes up the mains and is coupled in the air to your phone line. A mains filter will be ineffective.

Could be a pump inside the works which is operated by a float valve. What you need to use is a portable MW radio with a ferrite rod aerial. Tune the radio to a quiet part of the band where there are no strong stations, then by putting the radio near the phone and power cables, you can detect interference. As you get closer to the source, you can use the radio as a direction finder.