TDR with multiple shorts

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I am clearly out of my depth and the question is practical: what would  
TDR show if there is a pair of wires shorted in multiple places, say 5,  
each spaced by 1m? Will there be sufficient energy to detect the echo of  
the last of the shorts?

    Best regards, Piotr

Re: TDR with multiple shorts
On Sun, 26 Jul 2020 15:36:07 +0200, Piotr Wyderski

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You will see the first short, as a short, and nothing else. There's no
voltage downstream of the first short.

TDR is often imagined as a graph of impedance vs distance, but that's
just the ideal/imaginary case. In real life, every feature along the
line, including normal losses, changes both the outgoing and return
steps.



--  

John Larkin         Highland Technology, Inc

Science teaches us to doubt.

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Re: TDR with multiple shorts
snipped-for-privacy@highlandsniptechnology.com wrote:

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Thank you, John, this answers my question (and kills a weird idea).

    Best regards, Piotr

Re: TDR with multiple shorts
Piotr Wyderski wrote:

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Well, if a full short, that's right.  If a "weak" short, ie. it has  
significant resistance, then you might be able to see beyond it.
If you have multiple partial shorts, or multiple impedance changes all  
spaced equidistant, then you will have a mess that would take a computer to  
deconvolve, with reflections traveling back and forth between the  
discontinuities.  So, that would severely complicate what you propose.

Jon

Re: TDR with multiple shorts
wrote:

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Some things can't be deconvolved. Imaging TDRing a coax that has a 60
dB attenuator somewhere along the line. The attenuator is invisible,
but sure affects anything that you think you see downstream.

The other issue with deconvolving reflections and such is noise.



--  

John Larkin         Highland Technology, Inc

Science teaches us to doubt.

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Re: TDR with multiple shorts
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The telcom TDRs can detect bridge taps and load coils in copper, somehow.

Re: TDR with multiple shorts
Cydrome Leader wrote:
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It would be nice to massage TDR or VNA S11 data to yield a
clean impedance vs. distance plot, if the discontinuities
aren't too wild. I don't see off-hand how to achieve this,
but somebody somewhere must have done it already?

Jeroen Belleman

Re: TDR with multiple shorts
On Wed, 29 Jul 2020 10:40:43 +0200, Jeroen Belleman

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Geologists have done a lot of work in untangling TDR data.



--  

John Larkin         Highland Technology, Inc

Science teaches us to doubt.

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Re: TDR with multiple shorts
On Wed, 29 Jul 2020 10:40:43 +0200, Jeroen Belleman

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My old 11802 does just that. The impedance at each cursor dot is
displayed.  

Re: TDR with multiple shorts
On 2020-07-29 20:00, John Larkin wrote:
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Maybe, but does the displayed value still make sense after
a preceding reflection?

Jeroen Belleman

Re: TDR with multiple shorts
On Wed, 29 Jul 2020 22:11:03 +0200, Jeroen Belleman

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In general, no. It is useful on a PC board, but it becomes obvious
that trace losses and vias and things smear out the resolution as you
go farther out the trace.


Re: TDR with multiple shorts
On 2020-07-29 04:40, Jeroen Belleman wrote:
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As John said, the deconvolution isn't unique, because a  
perfectly-matched attenuator doesn't cause any reflection but does knock  
down the returns from anything following it.

It's much easier in optical fibre, because Rayleigh scatter produces a  
continuous return signal from all distances.  The slope of that signal  
tells you the loss per unit length, and any step discontinuities mean  
that there's a matched loss, e.g. someone from a TLA tapping into it by  
bending the fibre.

Optical TDR is a cool technology--you can map a whole multitap network,  
which you can't in coax on account of not having the continuous Rayleigh  
return.

Cheers

Phil Hobbs

--  
Dr Philip C D Hobbs
Principal Consultant
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Re: TDR with multiple shorts
On 29/07/20 23:32, Phil Hobbs wrote:
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Does anybody male photon counting OTDRs for use in the field?
If so, how is the detector cooled?

I considered that ~1980 in conjunction with BT Martlesham
Heath, but couldn't find acceptable cooling technology.

I also wondered if there were any coding schemes that
could increase range. Given a peak-power limited transmitter,
they might allow there to be more power in the fibre at
any one time. The best code I found was a Barker code, but
with a max length of 13 (IIRC), it wasn't worth the effort.

Re: TDR with multiple shorts
On 2020-07-30 02:07, Tom Gardner wrote:
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<https://arxiv.org/pdf/1001.0694.pdf has a pretty good discussion of  
the tradeoffs circa 2009.

In the early '80s, working at 1.3 um, you would probably have been stuck  
with an S-1 photocathode, right?  They're famously bad for dark counts  
even at low temperature--you have to get them down to 77K or thereabouts  
to get decent sensitivity.

Cheers

Phil Hobbs


--  
Dr Philip C D Hobbs
Principal Consultant
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Re: TDR with multiple shorts
On 30/07/20 11:23, Phil Hobbs wrote:
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Thanks, and yes, the temperature was a bit too low to be practical!

Re: TDR with multiple shorts
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Can the data circuit optical TDRs see things like splitters/prism/taps?  
How do they show up? If so does the direction you test them matter? Years  
ago I was talking with some datacenter people about how the tapping  
warrants work in facilities with lots of dodgy customers. The answer was  
it's pretty easy as all optical circuits already have passive optical taps  
installed. If a customer asks or notices, they're told it's for testing,  
which is legit and true. When the feds showed up the warrant, they just  
plug into the "test" ports. Even with a nice clean fiber connection,  
you're still at the mercy of the optics at each end, and somehow those go  
bad or just wear out. I've never gotten a solid answer as to why this  
happens either. Does the laser start to fade out? Does the receiver become  
less sensitive? It didn't seem to be an issue with older stuff like  
gigabit ethernet. 8Gb, 10Gb, 16Gb? They all seem to fail in weird ways.


Re: TDR with multiple shorts
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Yes.  

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As a localized loss without a significant reflection.  

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Yes. Returns from the branches will in general arrive in a different order seen from the other end.  

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The lasers have a limited lifetime.  

Cheers

Phil Hobbs

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