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Author Topic: Help with TDR traces  (Read 6280 times)

les-70

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Help with TDR traces
« on: June 14, 2012, 07:03:19 PM »

  I have borrowed a 301C TDR and would be really pleased to hear from anyone with TDR experience.

 I attach photo's of the TDR traces with the marker at few places and one montage of stats from the router

 As a guide to thinking about the results the estimates of cable are
 to top of pole ~45m (after pole underground)
 to CAB about 499m
 to exchange about 1100m
 Nearby cable is copper but the area does have a fair bit of aluminium so rest of cable
 is not known

  The trace has 3m leads to the master socket and think I see a number of
 feature start points.
 at 50m on the pole, a -ve then +ve dip then hump
 at 241m on way to CAB a -ve dip
 at 356m on way to CAB a +ve hump
 at 503m at or near CAB a weak -ve dip -- maybe at the cab
 at 1412 +ve hump  -- further than expected to exchange
 
  Some of these ups and downs may be linked.

  With regard to the 50m feature an engineer, when up the pole
 in front of my house last week cursed a lot when,
 on opening the junction box, a large volume of water landed on him.
 

  I really need help on what mischief the 301C balance control can do, I set it up to minimise the initial 10-20m of up and down.

  I am in the process of studying the traces against any examples I can find and trying not to jump to conclusions.   Any advice the traces will be most gratefully received.

If anyone can answer --- might an engineer think the features worth trying to remove?   
I suspect that separate from the TDR the response would be working within limits!

   Thanks in advance

 

 
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les-70

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Re: Help with TDR traces
« Reply #1 on: June 14, 2012, 07:07:48 PM »

Trace with few more position markers
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burakkucat

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Re: Help with TDR traces
« Reply #2 on: June 15, 2012, 03:21:09 AM »

Les -- The correct use of the balance control is to minimise the effect of the transmitted "ping" appearing on the (received) trace (when one is interested in near-end "pongs").

If you would care to let me know your e-mail address via a PM, I have a copy of the User Instructions for the Tester 301C that I could send to you.

For expert opinion on those traces you really need to catch Black Sheep's eye, as Walter, Bald_Eagle1, Asbokid and I are but novices in that art . . .  :-\
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GunJack

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Re: Help with TDR traces
« Reply #3 on: June 15, 2012, 01:43:12 PM »

My TDR theory is somewhat rusty, but what you're probably seeing there is the significant points of resistance on the overall line between you and the exchange. Between you and the cab there will probably be a junction box (possibly underground) which you're seeing around 240m, and then the exchange at 1400m (but if you were expecting 1100m, this could be down to either the cable route being longer than expected, or indeed Au cable which should show as being longer than the equivalent copper line due to the properties of the metals involved). Pretty much any blip is down to a difference caused by, e.g. a junction box, faulty cable, etc. where the signal is significantly slowed/degraded/etc. over what you would expect from a single continuous run of the same material.
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les-70

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Re: Help with TDR traces
« Reply #4 on: June 15, 2012, 02:26:41 PM »

 I suspect that you are correct.  I have been experimenting with the gain and have added 76 m of cable prior to the master socket to move the launch signal from the region of interest.  Increasing the gain seems to add under and overshoots to features and not just magnify the trace.  It seems to me that you need experience in the best use of the balance and gain.

 The first attached (badly photographed) trace is a lower gain and with the 76m meters of cable.   The cursor is placed about where the top of the pole should be. The main things are a rise at about 360m and again at 1532m.  Allowing for the 76m they are 284m and 1454m from me.  Perhaps a poor joint and the exchange.

In the second attached photo the gain is high and more appears!  An adjustment of the balance can get you back to previous pictures or elsewhere!! but with generally the same wiggles.
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guest

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Re: Help with TDR traces
« Reply #5 on: June 15, 2012, 03:07:17 PM »

The oscillation is a discontinuity in the cable - as Gunjack said its likely to be junctions.

By "discontinuity" I mean that the impedance of the cable is not the same as it is at the junction box. Any sudden change in impedance on a transmission line will cause a reflection back down the line and that's what you're seeing. The impedance change will be frequency dependent and hence so will the reflection. Depending on the jointing method I'd expect to see more reflections at higher frequencies.

Heh this has made me remember having to prove the characteristic impedance of a transmission line from first principles. The guy who taught me how to do that had a brother who scored 113 goals for Celtic. Damn odd the way things associate in the brain mmm :D
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Black Sheep

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Re: Help with TDR traces
« Reply #6 on: June 15, 2012, 06:45:43 PM »

Just seen this thread.

Trying to decipher a photographed TDR trace is pretty awkward, to say the least. What I would say though, is you are correct about getting the gain correct.

With the gain pumped up, you can have a circuit look like the backdrop of 'The Alps', even though there is nothing wrong with the circuit. As has been mooted, cable poundage changes also play a great part in what the TDR is looking at. As I haven't used one of those devices for donkeys years,(HAWK and JDSU for at least 8yrs now), I forget how the screen-shots progress, but I would view the screens with the auto-applied dB gain, then decrease the gain to see how it looks then. A prevalent HR will still be visible even on a low gain.

There really is a lot to consider when looking at traces, especially on miniscule type faults. If its a full-blown short circuit, or a cut cable .... then happy days, TDR will home in on it in a breath. But developing faults are a different story, depending on the severity the fault. 

I've mentioned this before but just for you ....... I would personally lose the 76mtrs of cable you have applied (a very odd ammount might I add ??  :)), and connect straight into the test socket. Ideally, the connection would be onto the bare wires, but rules & regs prevent this  ;). Now, with the TDR running I would ring your landline number and watch the trace for increases/decreases in peaks/troughs, As the ringing voltage is approx 90-120Vac, it puts more stress on the circuit than the nominal 50Vdc it would be under when you have your TDR connected.

If there are any faults brewing, there's a probable 80% chance this trick will highlight where they are. You have to have good eyes to scan the full range, but I've found many a HR using this trick that all the other testers miss ..... PQT/Eclipse/DSL Close-out etc etc ....

In a nutshell though it is as with most things, experience and knowing your own meters nuances that allow a good decision to be made on TDR traces. Believe me, if you're going to request traffic-lights and a 'dig' from our contractors (ie- a lot of money), then you dont want the outcome to reveal a damned poundage change, whereas you thought it was a HR fault on the TDR. Been done many, many times ........ but not by me. ;) ;) ;D ;D   
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les-70

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Re: Help with TDR traces
« Reply #7 on: June 21, 2012, 03:55:34 PM »

  Thanks for the reply Blacksheep, I have been thinking over my TDR trace and from experiments can confirm the wisdom of your comments.  I attach an anotated TDR and Hlog plot.

 I conclude that I have 3 main TDR features. Feature A at or near the pole.  Feature B about half way to the CAB and Feature C believed to be the exchange.  None of these are influenced by the phone ringing.

 More to my surprise the neighbours either side of me both have almost exactly the same features. This suggest to me that the features are probably cable type changes and not a special feature of the joints on my cable. 

I am not sure about feature A but as it is nearby its amplitude is exagerated compared to the later features. It may be the change from drop wire into a fat multi-strand cable.  On the neighbours traces Feature B moves consistent with their respective difference from the CAB, one about 40m nearer one about 40m further.  Feature C (I assume the exchange) on the other hand is about 150m nearer on the side of me nearer the exchange and about 40m nearer! in distance the other side.  I would have expected 44m nearer for one and about 44m MORE for the other! It may be significant that they are on BT broadband and I am TalkTalk.  I know my DSLAM was moved to a separate part of the exchange building last July. That is around when my line sync deteriorated and my interest in what might be different started.

 It may or may not be incidental that the dips in my Hlog could be caused by feature C.  That has a length of 120m and reflections looking like either a bridged tap or like a lower impedance cable section (I assume it must be the later). The ~120m length of the feature would I think give Hlog dips at about tones 116, 232 and 348 depending on the cable VOP (see http://www.jdsu.com/ProductLiterature/loopanalysis_an_tfs_tm_ae.pdf). In the Hlog plot attached an around 116m dip may be masked by the start of the profile but the other dips are more like tones 250 and 375 and only consistent if the cable VOP is about 0.6 which is I think low. An explaination is possible depending on the cable VOP. I am however suspicious of what has happened at the exchange. Maybe the DSLAM move added a 120m or 60m feature that you can't see on the TDR. Unfortunately my neighbors don't have stats from the their modem/routers and I don't like to trouble them with more tests using my modem/router.

 I am surprised at the consistency of the features between neighbours and puzzled by the differing exchange distances. I am sure that there is definitely nothing that I am likely to get changed, but would welcome views or corrections to my interpretation.

 At least if anything happens in the future, I may by comparison with the current traces be able spot it.
 
« Last Edit: June 21, 2012, 04:11:26 PM by les-70 »
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burakkucat

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Re: Help with TDR traces
« Reply #8 on: June 21, 2012, 04:54:03 PM »

Hi Les,

I'll just make the observation that you do have the gain rather high (at 24 dB) and that may be why you are seeing "features" that are quite inconsequential. I use the rule of thumb: "the higher the gain, the less significance should be attached to the deflection(s) observed in the trace". With my line, if I look very hard, I can see where the CW1308 internal cable is joined to the (two pair) drop-cable via a BT80B-RF3, the joint at the pole-top DP and also the crimped joint between the D-side & E-side cables in the PCP. However hard I try, I cannot see any evidence for the HDF in the exchange. I wonder what the 301C trace would have looked like with the gain <= 12 dB?

Certainly your Hlog graph shows that there is some form of imperfection. As to the extent?  ???

Personally, I would regard the data you have just presented as the "base line" and use it for comparison purposes, in the future.
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les-70

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Re: Help with TDR traces
« Reply #9 on: June 21, 2012, 06:19:58 PM »

 That is an easy question to answer and I have attached 12db and 18db traces.  The change points in the traces are in the same places and amplitudes change by rougly with the factor of 4 expected with the 6db intervals. The 12db trace is little diffferent in character but there is still a jump in the same places. 

I went up and down all the pairs in all the cable that I have and ended up with effectively about 200m of cable.  The TDR on that 200m at 12db and 18db barely showed the joins and gave an open circuit amplitude of about about 3 times that of feature B.  i.e. feature B is I think therefore quite significant at about 30% of open circuit.  Feature A which looks bigger is about 15% of the open circuit amplitude at its distance.  I have not tried to estimate the size of feature C and I am not sure what to expect at the exchange?

 I tried  180m of cable with a central part 20m composed of doubled up pairs.  i.e. two pairs in parallel for 20m gave a down then up response of about 50% of open circuit.  I think this illustrates what a cable section of half the impedance could do?

  I fully agree with your final point though --- a useful reference for the future!
 
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burakkucat

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Re: Help with TDR traces
« Reply #10 on: June 21, 2012, 06:50:42 PM »

 :hmm:  Hmm, interesting experiment.

Perhaps Black Sheep could please enlighten us as to what feature one would expect to see on a TDR trace for a typical exchange?
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Black Sheep

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Re: Help with TDR traces
« Reply #11 on: June 21, 2012, 06:58:38 PM »

Apologies for not having the time to read and fully digest les's posts just yet, but to answer B*Cats question, you will see a 'dis' (ie: a peak) at the Exchange measuring back from the EU's premises, or the PCP etc.

The 'dis' will be visible where the DSLAM equipment is within the Exchange. In other words, if it's a large Exchange the trace will continue through the MDF and up the 'Tie-pairs' to where the DSLAM is located. This could be a few hundred metres away on the 7th floor of the building, for example.
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burakkucat

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Re: Help with TDR traces
« Reply #12 on: June 21, 2012, 07:04:39 PM »

Quote
<snip> to answer B*Cats question, you will see a 'dis' (ie: a peak) at the Exchange measuring back from the EU's premises, or the PCP etc.

Much appreciated! Thank you.

Now I have a subsidiary question, please. This hypothetical line does not have a broadband service running over it, it is pure telephony. What could we expect to see (in favourable conditions)?
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Black Sheep

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Re: Help with TDR traces
« Reply #13 on: June 21, 2012, 07:11:03 PM »

Ha ha ..... it's been that long since I've had to perform such a measurement against such an 'antiquated' circuit (what ?? ... you're certain they dont have Broadband ??  ;D) ..... that I can not genuinely answer the question.

Maybe the other engineer on here has had recent experience of this, and can clarify, however my poor memory recalls that it shows itself as just a continuous flat-line, as in no peaks.  :)
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burakkucat

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Re: Help with TDR traces
« Reply #14 on: June 21, 2012, 09:36:49 PM »

Thank you for an honest reply.

(Yes, absolutely no broadband. It is the circuit providing the service to Great Aunt Maud's house, where the telephone -- a two-tone blue 706 from late 1959 -- resides on a glass topped table in the hall.)
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