This is good use for a scope.

[Jamie]

http://www.youtube.com/watch?feature=endscreen&NR=1&v=Il_eju4D_TM

Although I have done this many times myself, I thought this guy
did very well in this demo.

Years ago I had coax going up to the roof and it was in the walls
to make things look pretty on the out side.. I had a DC short and
I wasn't going to tear out all the walls to find it. I shot a
pulse up it, did a rough calculation and found it on the second
floor. It turns out the electric company had put in new wire on the
outside and a strap fastener poked into the wire. I got with in
foot where it was.

But over the years of doing this I've always question this practice.
Can we rely on velocity being a constant? I know where I work currently
we make many different communication cables and one the factors is
chemistry change in the dialectic with age, especially with foams. This
also effects the impedance.

I know recently I made an inquiry here to see if any actually uses that
method. I was going to implement it in a cable debugger tool but then I
realized the velocity isn't a constant due to inconsistent geometry.


Just something to think about I guess.


Jamie

[Phil Hobbs]

Time domain reflectometry, it's called, and it's been used for yonks in
both electrical and optical versions.

See e.g. this nice Agilent app note: http://tinyurl.com/bhd8yol

A Tektronix 11800 series scope with an SD-24 plugin is the bee's knees
for TDR.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510 USA
+1 845 480 2058

hobbs at electrooptical dot net
http://electrooptical.net

[John Larkin]

There are also lots of small LCD handheld TDRs with slower risetimes and
kilometers of range, for checking cable TV and phone wires and such.

Velocity should be pretty much constant for any known cable type.


--

John Larkin Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

[Jamie]

We have HP TDR's at work in the lab, both optical and electrical.
But at home, I do not have one, although I have been offered one that
is no longer being used in the lab, maybe I should add that to my
collection here at home :)

Jamie

[tm]

"John Larkin" <jjla...@highNOTlandTHIStechnologyPART.com> wrote in message
news:ijrip8drjoov9jg2o...@4ax.com...

What's really neat and I find amazing is sending an optical pulse down and
back 50+ miles through a glass fiber. It is amazing how clear that glass is.

With some fiber, the twist in the bundles adds measurable length to the
physical cable that one must account for when looking for a fault.

tm

[Jamie]

You would think that however, we've tested products that have been in
storage (samples) from years ago and found the Vfactor to change, due
to the dialectic aging. It does not happen to all of the compounds used,
just a couple that had been used for coax cables and such.. With foam,
the Z changes and some grades will actually shrink in size..

It gets real bad when you check your cable and all you have is wire
in the center with flatten braid and nothing but soup between.

Jamie

[Jamie]

Really? I can only assume you are referring to fiber that is still
sitting on the roll with no body bindings on it?

You're talking 264K feet of fiber or, half that for reflection time,
but still. 132k ?

we run like 1 mile lengths bunched with a bundle of them and a TDR
shows losses.

Jamie

[tm]

"Jamie" <jamie_ka1lpa_not_v...@charter.net> wrote in message
news:YFemt.12512$7h5....@newsfe07.iad...

No, I am saying it is a 50 mile long, installed cable with 216 single mode
fibers. The OTDR pulse travels 50 miles out and 50 miles back and you can
see every splice in the return waveform.

The fiber cable is installed in 2 inch buried conduits with access pull
boxes every 1000 feet or so.

Regards,
tm

[Jeff Liebermann]

On Sun, 19 May 2013 20:34:07 -0400, Jamie
<jamie_ka1lpa_not_v...@charter.net> wrote:

>http://www.youtube.com/watch?feature=endscreen&NR=1&v=Il_eju4D_TM
>
>Although I have done this many times myself, I thought this guy
>did very well in this demo.

This might help interpret the display.
<http://www.epanorama.net/circuits/tdr.html>
See the section "Looking at the TDR Signals" near the bottom.

>Years ago I had coax going up to the roof and it was in the walls
>to make things look pretty on the out side.. I had a DC short and
>I wasn't going to tear out all the walls to find it. I shot a
>pulse up it, did a rough calculation and found it on the second
>floor. It turns out the electric company had put in new wire on the
>outside and a strap fastener poked into the wire. I got with in
>foot where it was.

My favorite screwup is using T-25 staples to secure a run of RG58/u or
CAT5 cable, and puncturing the jacket at some point along the way. A
visual inspection is no fun. So, I use a TDR. I can usually "nail"
the length to within a foot or two.

Incidentally, I've built several TDR pulse generators along the lines
of the schematic at the top of the page. No need to drag along a
function generator. Incidentally, the scope calibration output is
usually worthless due to high output impedance, slow risetime, and no
control over frequency.

>But over the years of doing this I've always question this practice.
>Can we rely on velocity being a constant?

Constant from cable to cable or constant along the length of the coax
cable? As far as measuring cable lengths, characteristic impedance,
and locating shorts, it's quite constant along a length of cable.
However, different cables have different dielectrics, properties,
configurations, and therefore different velocity factors (VF).
<http://www.repeater-builder.com/antenna/wa2ise-coaxial-cable.html>
If I'm not certain about the type of coax (many are not labeled), then
I take a known length, measure the propagation delay, and calculate
the VF. That also gives me a clue as to what flavor of coax I dragged
home from the hamfest.

Twisted pair and network cable has more variation in VF. The cheap
junk varies all over the place. Capacitance can be anything between
13.5 and 17pf and be considered "good". The nominal VF of 0.7 for
CAT5 will vary (not sure how much).

>I know where I work currently
>we make many different communication cables and one the factors is
>chemistry change in the dialectic with age, especially with foams. This
>also effects the impedance.

You may be thinking of laboratory tests and phased matched cables. A
TDR is not normally a precision instrument. The range of error caused
by age, water absorption, contamination, and people walking on the
coax, is not going to make much difference to the TDR. Two decimal
place accuracy is great and even one decimal place will work.

>I know recently I made an inquiry here to see if any actually uses that
>method. I was going to implement it in a cable debugger tool but then I
>realized the velocity isn't a constant due to inconsistent geometry.

It's in every network cable certifier and some testers on the market.
<http://www.flukenetworks.com/datacom-cabling/copper-testing/dtx-cableanalyzer-series>
It's also in every OTDR (optical TDR) for testing fiber. Anything
that claims to measure cable length probably uses some flavor of TDR.
If you have room for more feature bloat, by all means, please do cram
in a TDR. It's very handy.

You can also do it in the frequency domain. Just add an FFT and GHz
bandwidth:
<http://www.jdsu.com/productliterature/fdrdefine.pdf>
<http://www.mohr-engineering.com/documents/TDR_vs_FDR_Distance_to_Fault-A.pdf>
<http://www.coe.utah.edu/~cfurse/Publications/FDR%20.pdf>
etc...

> Just something to think about I guess.

I'm thinking about dinner. Gone...

--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

[John Larkin]

In Cat5 that doesn't have individual pair shielding, like UTP or F/UTP, each
pair usually has a different twist, to reduce crosstalk. That makes each pair
have a different velocity, numbers like 45 ns skew per 100 m, ballpark 10%.
Sometimes even individually-shielded-pair cables have different twists.

https://dl.dropboxusercontent.com/u/53724080/Parts/Hitachi_Pairs.JPG

--

John Larkin Highland Technology Inc

[dave]

On 05/19/2013 05:26 PM, Phil Hobbs wrote:

>
> Time domain reflectometry, it's called, and it's been used for yonks in
> both electrical and optical versions.
>
> See e.g. this nice Agilent app note: http://tinyurl.com/bhd8yol
>
> A Tektronix 11800 series scope with an SD-24 plugin is the bee's knees
> for TDR.
>
> Cheers
>
> Phil Hobbs
>

This one is for mere mortals such as meself.

[dave]

http://www.triplett.com/shop/tdr/

[Jeff Liebermann]

On Sun, 19 May 2013 20:07:21 -0700, dave <rick...@earthlink.net>
wrote:

>> This one is for mere mortals such as meself.
>http://www.triplett.com/shop/tdr/

$160 to $200. The problem with it is that you don't have the benefit
of seeing what's happening which requires a scope. All you get is a
cable length number. No clue if it's intermittent, leaking, shorted
or open, misterminated, etc. It won't find missing T-connectors in
10base2 (cheapernet), or crushed cables.

Accuracy is rather lousy:
� 2 feet Cables up to 10 ft,
� 5 feet Cables 10 to 200 ft,
� 3% - Cables longer than 200 ft
I can usuallly do better than that with a decent scope.

[Michael A. Terrell]

Jamie wrote:
>
> You would think that however, we've tested products that have been in
> storage (samples) from years ago and found the Vfactor to change, due
> to the dialectic aging.

That sounds like Jersey Specialty Company crap wire. The worst crap
wire that I've ever run into.

[Michael A. Terrell]

tm wrote:
>
> What's really neat and I find amazing is sending an optical pulse down and
> back 50+ miles through a glass fiber. It is amazing how clear that glass is.
>
> With some fiber, the twist in the bundles adds measurable length to the
> physical cable that one must account for when looking for a fault.

CATV trunk & feeder lines have a lot of expansion loops, which at the
the actual VS physical length.

[Phil Hobbs]

Nah, they're pretty cheap if you're patient--I got an 11801C with SD-24
and SD-26 plugins included for $1600 last year. That's about 3 or 4
cents on the dollar. I've been paying about $100 for SD-24s lately,
which is less than 2 cents on the dollar.

I've got a metric pantload of plugins, I think 18 in all, including my
latest prize, an SD-48 30 GHz OE converter that I paid $375 for. I
still want one of the passthrough sampling heads, but otherwise I'm
pretty well done except for replacements.

Did I mention that I love those things? ;)

I actually have a project coming up that will need most of the
capability of the 11800s -- a coherent lidar system for looking at
single particles down to 0.15 micron diameter, moving at up to 3 km/s.
It's a follow-on to the ISICL sensor from 20 years ago. I obviously
won't be able to see individual particles with a sampling scope, because
they're nonrepetitive, but I'll need it to tune up the sensor and the
back end. Fun stuff.

[Dave Platt]

In article <_pemt.9653$v46....@newsfe13.iad>,

Jamie <jamie_ka1lpa_not_v...@charter.net> wrote:

> We have HP TDR's at work in the lab, both optical and electrical.
>But at home, I do not have one, although I have been offered one that
>is no longer being used in the lab, maybe I should add that to my
>collection here at home :)

Oh, I certainly think you should!

I've done this sort of TDR with a scope-and-pulse-generator lashup to
help characterize some of the antenna lines at our city EOC... a crude
setup but it worked very nicely for the job. Then, a couple of years
ago, I was fortunate enough to pick up a Tek 7S12 plugin with the
necessary sampler heads for a very nice price (and won an S53 trigger
recognizer head on eBay also at a nice price). Makes a very nice
combination... being able to see impedance bumps a fraction of an inch
apart is quite an experience!


--
Dave Platt <dpl...@radagast.org> AE6EO
Friends of Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

[Jamie]

We have HV bridge fault analyzers that seem to work very well for
locating shorts and opens.. But looking for a way to do it with out HV
is always a plus.

In any case, it can find the defect with in 4-5 feet on lengths up in
the 10's of thousands of feet! The method is archaic, but it works.
There is a 10 turn pot that you set to null the meter. This meter is in
the bridge circuit. You note the reading, then reverse the test leads
and do this again.. This gives you the % of the real length to find it.

If you are dealing with an open, the process is the same, except now
you need to apply HV to form a bridge.

Jamie

[miso]

> No, I am saying it is a 50 mile long, installed cable with 216 single
> mode fibers. The OTDR pulse travels 50 miles out and 50 miles back and
> you can see every splice in the return waveform.
>
> The fiber cable is installed in 2 inch buried conduits with access pull
> boxes every 1000 feet or so.
>
> Regards,
> tm
>
>
>

There are fiber optic perimeter detection systems using TDR. They can
sense when a buried cable is driven over or a cable on a fence is jiggled.

[lang...@fonz.dk]

I believe it read somewhere that the railways use (or work on) using a
fiber optic
cable along the rail to detect where trains are on the track by
measuring the distance to where the fiber is bend by the weight of the
train

-Lasse

[josephkk]

If you have heard of Sprint the telecom carrier? That is a division of
Southern Pacific Railroad leasing some of their excess FO capacity.

?-)

[Jeff Liebermann]

On Wed, 22 May 2013 17:47:20 -0700, josephkk
<joseph_...@sbcglobal.net> wrote:

>If you have heard of Sprint the telecom carrier? That is a division of
>Southern Pacific Railroad leasing some of their excess FO capacity.

The Southern Pacific Railroad connection with SPRINT was only for a
few years, roughly between 1972 and 1982, when SPC merged with GTE.
See the history at:
<http://en.wikipedia.org/wiki/Sprint_Nextel#Southern_Pacific_Communications_.26_Sprint>

[miso]

On 5/22/2013 5:53 PM, Jeff Liebermann wrote:
> On Wed, 22 May 2013 17:47:20 -0700, josephkk
> <joseph_...@sbcglobal.net> wrote:
>
>> If you have heard of Sprint the telecom carrier? That is a division of
>> Southern Pacific Railroad leasing some of their excess FO capacity.
>
> The Southern Pacific Railroad connection with SPRINT was only for a
> few years, roughly between 1972 and 1982, when SPC merged with GTE.
> See the history at:
> <http://en.wikipedia.org/wiki/Sprint_Nextel#Southern_Pacific_Communications_.26_Sprint>
>

Running fiber optic cable along railroad right of way makes sense since
the railroads seem to have cart blanche on their right of way. I don't
know if they have an additional fiber optic sensor related to the tracks.

The power companies have been running fiber in ground wires for years,
but it is probably dark.

It would be interesting to know just how much dark fiber there is and
who owns it, but I suspect such a document doesn't exist.

[josephkk]

From my field experience, a lot of it is 80 to 90% dark. There are some
cases where 50 to 75% is lit though.

?-)

[Sylvia Else]

On 20/05/2013 10:34 AM, Jamie wrote:

> But over the years of doing this I've always question this practice.
> Can we rely on velocity being a constant? I know where I work currently
> we make many different communication cables and one the factors is
> chemistry change in the dialectic with age, especially with foams. This
> also effects the impedance.
>
> I know recently I made an inquiry here to see if any actually uses that
> method. I was going to implement it in a cable debugger tool but then I
> realized the velocity isn't a constant due to inconsistent geometry.

I can't see why geometry would have much effect on the overall
propagation delay. If one's concerned about the wave velocity, one need
only perform the experiment from both ends. That gives the total
propagation time for the cable, and thus the proportion of the cable
traversed to the fault, provided, of course, that there's only one.

Sylvia.

[Jamie]

It has a lot to do with it.

Power cables made with materials that are soft tend to not hold their
wall thickness as they are twisted and bunched. Sending a fast raise
pulse down the cable results in random readings from one length to
another.
If it were cables like molded twins or coax, you could then use that
method, however, even molded twin laying on top of each other on a spool
can cause that reading to be random.

Of course we are talking about lengths of 1000 feet or more.

After I thought of the idea originally, I then remembered that we
tried experimenting with a TDR on these types of cables and it wasn't
consistent enough to use. It works great on small signal twisted pairs
and such.. Those types of cables must maintain their physical structure
though out.

So it boils down to using a wheatstone bridge with HV if opens are to
be found.

Jamie

[Charlie E.]

Many years ago, when I was working on toll roads, they were trying to
make a truck scale using fibers embedded in a rubber strip in the
road. After spending quite a few bucks (including a test installation
on the Autobahn) they finally figured out that the environment was too
variable to get reliable measurements. Temperature, water, snow,
rubber aging, they all changed the response significantly...

Charlie