TDR with multiple shorts...

[Piotr Wyderski]

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

 

[server]

On Sun, 26 Jul 2020 15:36:07 +0200, Piotr Wyderski
<peter.pan@neverland.mil> wrote:

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

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.

Claude Bernard

 

[Piotr Wyderski]

jlarkin@highlandsniptechnology.com wrote:

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.

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

Best regards, Piotr

 

[Jon Elson]

Piotr Wyderski wrote:

jlarkin@highlandsniptechnology.com wrote:

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.

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

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

 

[server]

On Sun, 26 Jul 2020 12:28:23 -0500, Jon Elson <elson@pico-systems.com>
wrote:

Piotr Wyderski wrote:

jlarkin@highlandsniptechnology.com wrote:

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.

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

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.

Claude Bernard

 

[Cydrome Leader]

Jon Elson <elson@pico-systems.com> wrote:

Piotr Wyderski wrote:

jlarkin@highlandsniptechnology.com wrote:

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.

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

The telcom TDRs can detect bridge taps and load coils in copper, somehow.

 

[Jeroen Belleman]

Cydrome Leader wrote:

Jon Elson <elson@pico-systems.com> wrote:
Piotr Wyderski wrote:

jlarkin@highlandsniptechnology.com wrote:

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

The telcom TDRs can detect bridge taps and load coils in copper, somehow.

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

 

[server]

On Wed, 29 Jul 2020 10:40:43 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

Cydrome Leader wrote:
Jon Elson <elson@pico-systems.com> wrote:
Piotr Wyderski wrote:

jlarkin@highlandsniptechnology.com wrote:

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

The telcom TDRs can detect bridge taps and load coils in copper, somehow.


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

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



--

John Larkin Highland Technology, Inc

Science teaches us to doubt.

Claude Bernard

 

[John Larkin]

On Wed, 29 Jul 2020 10:40:43 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

Cydrome Leader wrote:
Jon Elson <elson@pico-systems.com> wrote:
Piotr Wyderski wrote:

jlarkin@highlandsniptechnology.com wrote:

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

The telcom TDRs can detect bridge taps and load coils in copper, somehow.


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.

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

 

[Jeroen Belleman]

On 2020-07-29 20:00, John Larkin wrote:

On Wed, 29 Jul 2020 10:40:43 +0200, Jeroen Belleman
jeroen@nospam.please> wrote:

Cydrome Leader wrote:
Jon Elson <elson@pico-systems.com> wrote:
Piotr Wyderski wrote:

jlarkin@highlandsniptechnology.com wrote:

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

The telcom TDRs can detect bridge taps and load coils in copper, somehow.


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.

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

Maybe, but does the displayed value still make sense after
a preceding reflection?

Jeroen Belleman

 

[John Larkin]

On Wed, 29 Jul 2020 22:11:03 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

On 2020-07-29 20:00, John Larkin wrote:
On Wed, 29 Jul 2020 10:40:43 +0200, Jeroen Belleman
jeroen@nospam.please> wrote:

Cydrome Leader wrote:
Jon Elson <elson@pico-systems.com> wrote:
Piotr Wyderski wrote:

jlarkin@highlandsniptechnology.com wrote:

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

The telcom TDRs can detect bridge taps and load coils in copper, somehow.


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.

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


Maybe, but does the displayed value still make sense after
a preceding reflection?

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.

 

[Phil Hobbs]

On 2020-07-29 04:40, Jeroen Belleman wrote:

Cydrome Leader wrote:
Jon Elson <elson@pico-systems.com> wrote:
Piotr Wyderski wrote:

jlarkin@highlandsniptechnology.com wrote:

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

The telcom TDRs can detect bridge taps and load coils in copper, somehow.


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

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
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Tom Gardner]

On 29/07/20 23:32, Phil Hobbs wrote:

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.

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.

 

[Phil Hobbs]

On 2020-07-30 02:07, Tom Gardner wrote:

On 29/07/20 23:32, Phil Hobbs wrote:
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.

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.

<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
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Tom Gardner]

On 30/07/20 11:23, Phil Hobbs wrote:

On 2020-07-30 02:07, Tom Gardner wrote:
On 29/07/20 23:32, Phil Hobbs wrote:
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.

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.

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.

Thanks, and yes, the temperature was a bit too low to be practical!

 

[Cydrome Leader]

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2020-07-29 04:40, Jeroen Belleman wrote:
Cydrome Leader wrote:
Jon Elson <elson@pico-systems.com> wrote:
Piotr Wyderski wrote:

jlarkin@highlandsniptechnology.com wrote:

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

The telcom TDRs can detect bridge taps and load coils in copper, somehow.


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

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.

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.

 

[server]

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.

Can the data circuit optical TDRs see things like splitters/prism/taps?

Yes.

How do they show up?
As a localized loss without a significant reflection.

>If so does the direction you test them matter?

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

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.

The lasers have a limited lifetime.

Cheers

Phil Hobbs