On 11/10/2010 11:56 AM, Barry wrote:
wrote in message
...
Yeah but in the real world you don't usually need to match a commercial
TDR to find a fault.

Back in the days of ethernet over RG-58 I was able to find lots of
poorly
attached connectors and cables crushed by the electricians that
installed
them (there were no network engineers and installers in those days)
with
a 100 MHz 'scope and a pulse generator made from a 555 timer and a 50
Ohm
line driver.

Can you detect where RG-58 was kinked and then straightened out with the
damaged area being less than 1/2 inch? Likewise can you detect an
abraded shield leaving a 1/4 inch hole in the shielding?

An LM555 has a rise and fall time of 100 nanoseconds. I have no idea of
how much your line driver sharpens the edges of the pulse, but a good
line driver should provide rise and fall times of 5 nanoseconds. The
original TEK 1502 at 140 picoseconds can resolve to about 2 centimeters.
With a risetime of 5 nS, your resolution will be about 70 times worse
(140 cm). Add to this the 100 MHz limitation of the scope (an additional
10 nS), and it becomes far worse.

Yes, you can easily detect shorts and open circuits with your setup, but
you will not be able to detect the 1 to 2 inches or so where the center
conductor has migrated in the foamed dielectric from hanging the cable
over a building edge.

The accuracy of the measurement for fault finding doesn't need to be
much
better than a few feet to be able to find the fault visually once you
know
about where it is and faults at the end are immediately obvious.

If the task is to find {gross} faults, a 'scope and simple pulse
generator works
just fine.

If the task is to certify 6 inches of hard line to GHz to some
mil-spec, you
probably want something more sophisticated.

The kind of fault Spamhog originally described, a 75 ohm cable dropping
to less than 50 ohms over two inches and going back to 75 ohms, will not
be detected by your setup.

73, Barry WA4VZQ



Are you telling all the rest of us that may have inferior equipment that
we shouldn't even try to measure things?

Just asking.

And we aren't all dumb enough not to understand what gives us resolution.

tom
K0TAR

Jim,

I think you need to go back and read Spamhog's original question. He was
trying to determine whether the center conductor of a piece of coax had
migrated away from center. He knew where this might have happened - 10
feet from the end, and the migration would have occurred over less than
two inches. So the question of locating where the problem might be is
moot. What is needed is a measurement of the cable impedance in this
region.

First, let us get an estimate of what the impedance of the damaged
section might be. Spamhog was using RG-6 cable with a foamed
polyethylene dielectric. Its velocity factor is 0.85 making its relative
permittivity 1.384. The center conductor is 1 mm, and the normal
diameter of the center insulator is 4.7 mm. The thickness of the
insulator is 1.85 mm. We need to know the impedance if the center
conductor had migrated 0.925 mm toward the jacket.

For a quick estimate, use the formula for off-center coax
(http://www.microwaves101.com/encyclo...offcenter.cfm). This
gives an impedance of 69.8 ohms in this section compared to 78.9 ohms in
the non-distorted coax. A TDR displays the reflection coefficient
from -1 (short) to +1 (open). Here the reflection coefficient is -0.06.
So the TDR trace will drop from the center line by 6% for 200
picoseconds.

If your 100 MHz scope has a typical Gaussian response, its rise time is
at least 3.5 nanoseconds. Do you really think that your oscilloscope
trace will clearly show the 200 picosecond dip? Even with the
wide-screen magnifier that KB7QHC suggested, I think you will have great
difficulty seeing this.

73, Barry WA4VZQ

"tom" wrote in message
. net...
Are you telling all the rest of us that may have inferior equipment
that we shouldn't even try to measure things?

No. In fact, playing with a scope and a pulse generator is quite
educational.

And we aren't all dumb enough not to understand what gives us
resolution.

tom
K0TAR

Trying to measure the thickness of a single sheet of paper with a ruler
graduated in eighth's of an inch is analogous. About all you can say
with certainty is that the paper is much thinner than 1/8 inch.

My last post gave a nominal value for the reflection coefficient of coax
with a migrated center conductor like N1PR postulated. At 2 GHz, the
2-inch section will be about 0.4 wavelength. Will you get detectible
reflections? Yes.

73, Barry WA4VZQ

Barry wrote:
Jim,

I think you need to go back and read Spamhog's original question. He was
trying to determine whether the center conductor of a piece of coax had
migrated away from center. He knew where this might have happened - 10
feet from the end, and the migration would have occurred over less than
two inches. So the question of locating where the problem might be is
moot. What is needed is a measurement of the cable impedance in this
region.

First, let us get an estimate of what the impedance of the damaged
section might be. Spamhog was using RG-6 cable with a foamed
polyethylene dielectric. Its velocity factor is 0.85 making its relative
permittivity 1.384. The center conductor is 1 mm, and the normal
diameter of the center insulator is 4.7 mm. The thickness of the
insulator is 1.85 mm. We need to know the impedance if the center
conductor had migrated 0.925 mm toward the jacket.

For a quick estimate, use the formula for off-center coax
(http://www.microwaves101.com/encyclo...offcenter.cfm). This
gives an impedance of 69.8 ohms in this section compared to 78.9 ohms in
the non-distorted coax. A TDR displays the reflection coefficient
from -1 (short) to +1 (open). Here the reflection coefficient is -0.06.
So the TDR trace will drop from the center line by 6% for 200
picoseconds.

If your 100 MHz scope has a typical Gaussian response, its rise time is
at least 3.5 nanoseconds. Do you really think that your oscilloscope
trace will clearly show the 200 picosecond dip? Even with the
wide-screen magnifier that KB7QHC suggested, I think you will have great
difficulty seeing this.

73, Barry WA4VZQ

The bandwidth of the 'scope will make the trace have a ripple intead of
the nice, sharp bump you would get from a faster 'scope.

Spamhog's original statement was that the cable was crushed to half diameter
for about 2 inches as I recall.

I was able to see ripples in the display for cables with less crush then
that which were on the order of 1/4 wide.

The bottom line is the faster the 'scope you use and the faster the rise
time of the applied pulse, the better the meaurement.

And since this is a hobby and not building man rated space craft, I would
say try whatever you can get your hands on for free and see what happens.

Or spend eternity arguing whether or not it is possible to do.

--
Jim Pennino

Remove .spam.sux to reply.

In article ,
"Barry" wrote:

"tom" wrote in message
. net...
Are you telling all the rest of us that may have inferior equipment
that we shouldn't even try to measure things?

No. In fact, playing with a scope and a pulse generator is quite
educational.

And we aren't all dumb enough not to understand what gives us
resolution.

tom
K0TAR

Trying to measure the thickness of a single sheet of paper with a ruler
graduated in eighth's of an inch is analogous. About all you can say
with certainty is that the paper is much thinner than 1/8 inch.

My last post gave a nominal value for the reflection coefficient of coax
with a migrated center conductor like N1PR postulated. At 2 GHz, the
2-inch section will be about 0.4 wavelength. Will you get detectible
reflections? Yes.

73, Barry WA4VZQ

Well not exactly, IF, one were to take a "Stack" of said paper, that was
exactly 1/8 of an Inch thick, then count the number of sheets, and
divide the 1/8 inch by the number of sheets, One could get a VERY close
approximation of the thickness of a single sheet...... Duh...

"you" wrote in message
...
In article ,
"Barry" wrote:
Trying to measure the thickness of a single sheet of paper with a
ruler
graduated in eighth's of an inch is analogous. About all you can say
with certainty is that the paper is much thinner than 1/8 inch.

Well not exactly, IF, one were to take a "Stack" of said paper, that
was
exactly 1/8 of an Inch thick, then count the number of sheets, and
divide the 1/8 inch by the number of sheets, One could get a VERY close
approximation of the thickness of a single sheet...... Duh...

Note that I said the thickness of a _single_ sheet. Your method would
give an approximation of the average thickness of the sheets used to make
up the stack. But if you have ever worked with a Fourdriner machine that
is not controlled well, i.e. the thickness varies with time due to any
number of variables, you would need to know the thickness of a number of
individual sheets to determine which variable is causing problems.
Typical variables that would cause the thickness of a sheet to vary
include, but are not limited to, head box level, "wire" speed, pulp
"consistency", felt pressure, calendar pressure, the type of wood used,
the lignin removal process (Kraft, caustic, or solvent), Crown Control
pressure, drying roll steam pressure, and at least a dozen more. For
example, sloshing or waves in the head box would cause thickness
variations in both the machine and cross direction, while "wire" speed
would only cause thickness variations in the machine direction. So there
are real reasons for measuring the thickness of a single sheet.

On Thu, 11 Nov 2010 18:17:16 -0500, "Barry" wrote:

But if you have ever worked with a Fourdriner machine that
is not controlled well, i.e. the thickness varies with time due to any
number of variables,

I have. That and a year's worth of studying paper process chemistry
for precision (sic) measurement of K and Kappa.

When it gets down to what you describe as "not controlled well" that
is more the definition of a Paper Mill that is destined for bankruptcy
before the end of one week - if not a weekend. I've seen the
production floor flood with product when the process encounters a
bottle neck. The production pipeline is enormous with a lot of
intertia.

Measuring a number of sheets achieves sufficient accuracy.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Thu, 11 Nov 2010 18:17:16 -0500, "Barry" wrote:

But if you have ever worked with a Fourdriner machine that
is not controlled well, i.e. the thickness varies with time due to any
number of variables,

I have. That and a year's worth of studying paper process chemistry
for precision (sic) measurement of K and Kappa.

It has been about 30 years ago, but I remember that while Kappa is
related to the amount of lignin remaining in the pulp after cooking,
every wet end process had its own peculiar measurement. I worked on the
online measurement of Kappa for several solvent pulping processes that
were to produce dissolving pulp for Eastman's cellulose ester process.
Once the vendor that supplied our pulp got wind that we were researching
solvent pulping, they quickly dropped their price and raised their
quality enough that we halted work on our own pulping processes. One
process we were considering involved the use of supercritical CO2. We
never got beyond early pilot plant work on this.

I fully understand the {sic} in your statement.

When it gets down to what you describe as "not controlled well" that
is more the definition of a Paper Mill that is destined for bankruptcy
before the end of one week - if not a weekend. I've seen the
production floor flood with product when the process encounters a
bottle neck. The production pipeline is enormous with a lot of
intertia.

I have toured quite a few paper mills over the years with both Kraft and
caustic pulping. I never really got to see one of the large continuous
digesters though. We had a small caustic plant that used hardwoods in
town with us. The odors were interesting - mercaptans by them and the
occasional butyric acid spill by us.

I always wonder how long the cellulose acetate business will last. While
cigarette smoking is declining in the US, increasing demand from China
seems to more than make up for the loss.

Going back to the original TDR discussion, probably the most interesting
use I ever put one through was in diagnosing thermocouple problems in our
coal gasifier. I saw that the platinum-rhodium thermocouples had water
in them. We finally got a metallurgist to do an "autopsy" on one of the
thermocouples pulled out during a shutdown. When we broke open the
thermocouple, water ran out. We saw a buildup of salts from the
Saureisen cement at exactly the place that I predicted. Water was
diffusing in through a silicon carbide protection tube and an Inconel
sheath.
Of course the gasifier operated at high temperature and pressure. A
Nastran simulation of the thermowell showed that the temperature at that
point was low enough for condensation to occur. Some extra insulation on
the external flange solved that problem!

73, Barry WA4VZQ

How the heck did we get off topic this far? :-)

On 11/11/2010 8:01 PM, Barry wrote:
"Richard wrote in message
snip

73, Barry WA4VZQ

How the heck did we get off topic this far? :-)



Keep it up. It's interesting.

tom
K0TAR

On Thu, 11 Nov 2010 20:07:31 -0600, tom wrote:

On 11/11/2010 8:01 PM, Barry wrote:
"Richard wrote in message
snip

73, Barry WA4VZQ

How the heck did we get off topic this far? :-)



Keep it up. It's interesting.

If that means the lore of paper processing, I was peripherally
involved in trying to characterize Black Liquor
(http://en.wikipedia.org/wiki/Black_liquor). Gladly I was at the
extreme periphery (I am sure my sense of smell was debilitated in
those years).

However, returning to things RF, I also was tasked with calibrating a
wood moisture meter. It used an HF RF source as part of a Z meter
were Z was correlated to moisture content. The probe was a fixed ring
surrounding point probes much like a Kelvin Bridge. It has always
seemed paradoxical that steam is used to dry wood.

Now, finding calibrated wood was no easy task. And if we found it, we
would have to first validate it (sort of a circular form of Sysphus'
task). My best guess at that work set to us was that we gun-decked
it.

It was some years later that that task came around again when I was
measuring K and Kappa as I averred. Here came the requirment for
"Bone Dry" paper. Try as you might to dry paper bone dry (absolutely
no water content), that as soon as it comes out of the oven it is
almost back up to several percent water content (15% to 20% would be
the end point).

Ironies compound in that I now live in a community where 100 years ago
our cedar wood mills produced nearly a Billion shingles in a year.

73's
Richard Clark, KB7QHC