The Shop > Electronics & IC Programing
Any Telephone Engineers on the forum ?
awemawson:
Out of interest I put my (ex BT) Tektronix Time Domain Reflectometer on my internal wiring this morning. Proved total cable length from Master Socket to internal Panasonic Exchange is 67 metres, with minor reflections from each of the Krone junctions on the way - largest being the first which is 26 metres, the next one being a further 10 metres down line. I did try re-making the punch down connections on the first Krone but it made no detectable difference.
I always find it fascinating that reflections from an 'open line' have the same polarity as the out going pulse, so hump upwards on the graph, whereas if you loop the line by putting a link across the far end, the reflected pulse goes negative (downwards)
This particular instrument allows you to calibrate it for cable characteristics by using a known length of the type of cable that you are measuring, so I cut an accurate 10 metre length of the same cable, and set it up so it measured correctly by setting the propagation velocity for the cable in use.
https://en.wikipedia.org/wiki/Time-domain_reflectometer#/media/File:Partial_transmittance.gif
I also did a 'loop resistance' measurement giving 12.2 ohms for the 67 x 2 metres of wire, which works out at 91 ohms per kilometre. As it's apparently nominally 84 ohms per kilometre there's nothing too horrendous I think.
trevoratxtal:
I am sorry I am unable to help on this matter.
But would like to thank all input from folk as it has been most informative.
Very useful posts.
awemawson:
Well interesting results today :ddb:
Before Mr BT arrived, by chance I was running a logging program, reading noise margin figures from the Home Hub 5a, and charting them. Then the 'phone rang, one of those nuisance calls that this time did me a favour :lol:
The noise margins went all over the place and the Hub disconnected and restarted! When it was back up, I called my number from my mobile, and sure enough I could reproduce the effect :thumbup:
So bright and early, Mr BT came, put his magic gizmo on the line, pressed a few buttons and diagnosed "Rectified Loop Fault' - most likely a damp connector block somewhere. Well the outside line comes from a pole in the road, to our gutter board, down the wall and appears on the other side of the lounge where the Master Socket is - but the Master socket has internal type wire, whereas the outside line disappears into the wall in external type cable. So there must be a joint box under the floor. 'There's your fault' he says. Hang on says I - prove it to me, ladder up to the gutter board, cut the cable and test again BOTH ways - into the house and back to the Exchange. He does this and guess where the fault is? Yes - on the line to the Exchange (Phew !!!!)
So off he trots to the Street Cabinet (which is over a mile away) does a few things, runs his tests again - ALL CLEAR. So what did he do? Used a different pair from the Street Cabinet to the Exchange, and used a different pair from our gutter board to the Cabinet (luckily it's as two pair)
The ONLY bit of cable unchanged it that under our lounge floor AS IT's OK :lol:
Hub up and running, seems immune to incoming calls and connected at 22 Mbps :clap:
Early days of course - only been up an hour or so :scratch:
Graph attached: 1st disturbance was the nuisance call, 2nd disturbance was me calling on my mobile.
John Swift:
that's a good result
nothing to pay and it now works !!!
John
Will_D:
Great result there Andrew. Always pays to be persistent and get them to do the job thoroughly!
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