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'[EE] Measuring characteristic impedance of coax ca'
2011\11\29@083520 by Bob Ammerman

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I have a customer who wants to measure the characteristic impedance of a length of coax. Actually, we know what it is supposed to be according to the manufacturer, but they want to verify it.

Test equipment available: high accuracy multimeters (5-1/2 digit?), cheap analog oscilloscope, relatively low frequency signal generator (sine, square, triangle) (top end is maybe 1Mhz), frequency counter, bench supply.

Willing to buy high-precision R's or C's if needed.

Accuracy required: hopefully a few percent, 10% is some help.

I am really a digital guy, so this is a bit out of what I do.

Any ideas?

-- Bob Ammerman
RAm Systems

2011\11\29@085158 by Harold Hallikainen

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> I have a customer who wants to measure the characteristic impedance of a
> length of coax. Actually, we know what it is supposed to be according to
> the
> manufacturer, but they want to verify it.
>
> Test equipment available: high accuracy multimeters (5-1/2 digit?), cheap
> analog oscilloscope, relatively low frequency signal generator (sine,
> square, triangle) (top end is maybe 1Mhz), frequency counter, bench
> supply.
>
> Willing to buy high-precision R's or C's if needed.
>
> Accuracy required: hopefully a few percent, 10% is some help.
>
> I am really a digital guy, so this is a bit out of what I do.
>
> Any ideas?

How about this?

Drive the coax with a square wave through a resistor that is the same as
the suspected characteristic impedance (the driving resistance is not
critical here). Watch the coax input with a scope. Vary the resistance at
the far end of the coax until the scope shows a square wave. With anything
other than the characteristic impedance terminating the coax, you should
see the reflection. If the driving impedance is the same as the
characteristic impedance, you should see only one reflection. In that
case, the square wave will either have a step up or down before settling
at the proper voltage depending on whether the terminating impedance is
low or high.

Good luck!

Harold



-- FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
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2011\11\29@092535 by alan.b.pearce

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> How about this?
>
> Drive the coax with a square wave through a resistor that is the same as the
> suspected characteristic impedance (the driving resistance is not critical here).
> Watch the coax input with a scope. Vary the resistance at the far end of the coax
> until the scope shows a square wave. With anything other than the characteristic
> impedance terminating the coax, you should see the reflection. If the driving
> impedance is the same as the characteristic impedance, you should see only one
> reflection. In that case, the square wave will either have a step up or down before
> settling at the proper voltage depending on whether the terminating impedance is low
> or high.

Flyback of a 'scope sweep waveform can often be a good source of fast edge.

You will also need to make sure the total source impedance presented to the cable is the nominal cable impedance.

Assuming that the cable already has connectors on it then standard BNC type termination (either end or feedthough) will do for the far end.

I take it they are just wanting to verify that it is within manufacturers specification (rather than exact to the last 0.1 ohm etc).


-- Scanned by iCritical.

2011\11\29@105653 by David VanHorn

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> Drive the coax with a square wave through a resistor that is the same as
> the suspected characteristic impedance (the driving resistance is not
> critical here).

A very good approach, the poor man's TDR.

If the cable is already terminated to connectors, then it's easy to
buy precision terminators to match the cable

2011\11\29@111016 by Sean Breheny

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An alternative method would be to measure the capacitance per foot
with both ends open and the inductance per foot with the far end
shorted (measure at one end, short the other). The characteristic
impedance is then the square root of L/C. The accuracy of this will be
limited by the losses in the line.

Your multimeter may be able to measure C. I don't know if you have
access to anything to measure L.

Sean


On Tue, Nov 29, 2011 at 10:56 AM, David VanHorn <spam_OUTmicrobrixTakeThisOuTspamgmail.com> wrote:
>> Drive the coax with a square wave through a resistor that is the same as
>> the suspected characteristic impedance (the driving resistance is not
>> critical here).
>
> A very good approach, the poor man's TDR.
>
> If the cable is already terminated to connectors, then it's easy to
> buy precision terminators to match the cable.
>

2011\11\29@112350 by Kerry Wentworth

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Harold Hallikainen wrote:
{Quote hidden}

Kerry

2011\11\29@164402 by Richard Prosser

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To get the sort of accuracy you're asking for, you really need a
network analyser of some sort. The impedance will vary with frequency
- especially from audio frequencies through to VHF etc.  The main
component of this is skin effect, where the inductance changes
slightly as the centre conductor moves into its RF region. The outer
conductor (screen) impedance is also likely to change somewhat. There
is also an effect known as "structural return loss"
which is due to manufacturing variations in the cable.  Where these
variations correspond to a 1/2 wavelength the cable impedance can
change markedly as the cable starts to operate as a filter. Obviously
manufacturers try to minimise this but it can be s real problem with
the cheaper grades of coax.

For an overall appraisal the TDR method is probably going to be the
easiest to implement with the cable terminated in it's nominal
impedance. But you may get different results when using it at RF.

What sort of frequency rang is involved?  - is it used for data (i.e a
TDR method may be more appropriate anyway) or RF, in which case some
sort of network analyser is really needed. If it's to be used over a
narrow frequency band you may be able to get an idea with a bridge
and a signal generator but it's not going to be easy.

RP



On 30 November 2011 05:22, Kerry Wentworth <.....kwentworthKILLspamspam@spam@skunkworksnh.com> wrote:
{Quote hidden}

>

2011\11\29@220316 by Roy

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This pdf file may be of some help- is for constructing a precision SWR meter
but also show a way of determining impedance.

If this link doesn't work search for constructing a precision swr meter

www.google.co.nz/url?sa=t&rct=j&q=constructing%20a%20precision%20swr%
20meter&source=web&cd=1&sqi=2&ved=0CBsQFjAA&url=http%3A%2F%2Fhttp://www.parc.org.za
%2Fattachments%2Fprojects%2FSwrmeter%2520Project.pdf&ei=1ZTVTtWII6asiAeXysWk
Dw&usg=AFQjCNFsPTwM4HJnp6ddfyPlDxAMYZ50-A

Roy

{Original Message removed}

2011\11\29@224859 by Sean Breheny

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I just tried my method of measuring L and C and indeed it does not
work very well (at least not when measuring with audio frequencies).

I have two 4.5 foot lengths of cable, one RG-58C/U (50 ohms) and the
other RG-59B/U (75 ohms), both terminated at both ends with BNC
connectors.

For the 50 ohm cable, I measured C=145.2pF and L=1.05uH at 10kHz,
using a bench top high quality digital LCR meter (Danbridge CT-20).
The C indicated a high Q while the L had such a poor Q that the meter
auto-detected it as a resistance with parasitic series inductance. The
far end was shorted with a shorted female BNC connector for the L
measurement.

For the 75 ohm cable, I measured C=96.8pF and L=1.3uH.

sqrt(L/C)= 85 ohms for the 50 ohm cable and 116 ohms for the 75 ohm
cable. Correct direction (75 ohm is larger), but not even the correct
ratio of impedances.

The problem is the L measurement. The theoretical C per foot for
RG-58C/U is 30.8 pF/ft, which would be 138.6pF for this 4.5 foot
length. My measurement is only 5% off. For the RG-59B/U, the value
should be 20.5pF/ft or 92.3pF for this cable. I was once again about
5% high. This is no doubt partially due to the BNC connectors and
parasitics in the leads from the test jig to the BNC connector.

I tried to obtain a value for the inductance external to the cable by
connecting my BNC "short" directly to the leads going to the test jig
and I got 0.6uH, which gives you an idea of the magnitude of the
parasitics involved here. Subtracting 0.6uH from each of the two above
L values and re-computing Zo for each gives 55 ohms for the 50 ohm
cable and 85 ohms for the 75 ohm cable. Much closer but still very
hand-wavy when I have only one (partially) significant digit in my L
measurement after subtracting-out the stray inductance.

I bet that I could do this much more accurately with my 1MHz HP4271A
LCR meter, but I have not yet gotten around to making a test jig for
it so I am not even sure if it works properly :(

Sean


On Tue, Nov 29, 2011 at 4:43 PM, Richard Prosser <rhprosserspamKILLspamgmail.com> wrote:
{Quote hidden}

>> -

2011\11\30@014341 by Brooke Clarke

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Hi Bob:

It turns out that the characteristic impedance of a transmission line is not frequency independent, although most people think that's the case.
Below some frequency the imedance is anything but constant, and a number of the proposed measurement methods might be testing the cable at too low a frequency.
For real data and more info on this see: Transmission Line Zo vs. Frequency
http://www.prc68.com/I/Zo.shtml

Have Fun,

Brooke Clarke
http://www.PRC68.com


EraseMEpiclist-requestspam_OUTspamTakeThisOuTmit.edu wrote:
> [EE] Measuring characteristic impedance of coax cabl

2011\11\30@055606 by peter green

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> How about this?
>
> Drive the coax with a square wave through a resistor that is the same as
> the suspected characteristic impedance (the driving resistance is not
> critical here). Watch the coax input with a scope. Vary the resistance at
> the far end of the coax until the scope shows a square wave. With anything
> other than the characteristic impedance terminating the coax, you should
> see the reflection. If the driving impedance is the same as the
> characteristic impedance, you should see only one reflection. In that
> case, the square wave will either have a step up or down before settling
> at the proper voltage depending on whether the terminating impedance is
> low or high.
One issue with this method is that to do it with cheap equipment you need a long peice of cable. IIRC wave propogation speed in coax is of the order of 2x10^8 meters per second. So if your cable is a meter long your signal generator and scope will need to resolve a reflection arround 10ns after the original transition. OTOH if you have 100 meters of cable to test then your signal generator and scope will only need to resolve a reflection arround 1us after the original transition

2011\11\30@101649 by Bob Ammerman

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Thanks for all the ideas. Unfortunately (or actually fortunately) my customer found out that another company in the building has a network analyzer and is happy to test the cable.

-- Bob Ammerman
RAm Systems

2011\11\30@204613 by Justin Richards

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Is there an implied frequency when discussing characteristic impedance.
When we need to analyze cables with respect to their RF
characteristics we end up with a large table S parameters that have
been recorded over a range of frequencies.

So when a cable is quoted at 75 ohms, that must be for a given
frequency.  If so what is it.

Justi

2011\11\30@212106 by Harold Hallikainen

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Ideally characteristic impedance is independent of frequency. In reality,
it may vary some with frequency, but I don't think the variation is
substantial. The attenuation can vary substantially with frequency. I keep
thinking back to Smith charts. Also, I did some stuff with Hyperlynx
recently. It's pretty neat. It looks at traces on a board layout and
models them as transmission lines. It then shows a series string of
transmission lines representing the trace(s) and lets you look at pulse
waveforms at various points on the trace.

Harold




> Is there an implied frequency when discussing characteristic impedance.
> When we need to analyze cables with respect to their RF
> characteristics we end up with a large table S parameters that have
> been recorded over a range of frequencies.
>
> So when a cable is quoted at 75 ohms, that must be for a given
> frequency.  If so what is it.
>
> Justin
>

2011\11\30@213043 by peter green

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Justin Richards wrote:
> Is there an implied frequency when discussing characteristic impedance.
> When we need to analyze cables with respect to their RF
> characteristics we end up with a large table S parameters that have
> been recorded over a range of frequencies.
>
> So when a cable is quoted at 75 ohms, that must be for a given
> frequency.  If so what is it.
>   An ideal coaxial cable (made up of perfect conductors and perfect dielectrics in a perfect geometery) will have a constant characteristic impedance across all frequencies. I'm not sure on the details but my understanding is that if the characterstic impedance is changing significantly with frequency you are probablly above the cable's usable frequency range.


'[EE] Measuring characteristic impedance of coax ca'
2011\12\01@022643 by Richard Prosser
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On 1 December 2011 15:30, peter green <plugwashspamspam_OUTp10link.net> wrote:
> Justin Richards wrote:
>> Is there an implied frequency when discussing characteristic impedance.
>> When we need to analyze cables with respect to their RF
>> characteristics we end up with a large table S parameters that have
>> been recorded over a range of frequencies.
>>
>> So when a cable is quoted at 75 ohms, that must be for a given
>> frequency.  If so what is it.
>>
> An ideal coaxial cable (made up of perfect conductors and perfect
> dielectrics in a perfect geometery) will have a constant characteristic
> impedance across all frequencies. I'm not sure on the details but my
> understanding is that if the characterstic impedance is changing
> significantly with frequency you are probablly above the cable's usable
> frequency range.
>

Justin,

Cable characteristics are normally referred to at a standard
frequency, depending on cable likely application. 200MHz, 400MHz and
1GHz being typical but impedance should be pretty stable above a few
MHz.

Mathematically the impedance is give by Z=sqrt((R+jWL)/(G+jWC))
Since G is normally close to zero in most cases this reduces to
sqrt(L/C) at high frequencies - where WL is >> R.
(W = 2 * PI * f)
..
The inductance is not constant although it only changes slightly as
skin effect moves the current to the outside of the centre conductor.
The R term is also not constant with frequency, it increases due to
skin effect (with sqrt(frequency)) and linearly with frequency as
dielectric loss starts to dominate. There is also a loss component
proportional to frequency involving the screen, but this is normally
not significant.  Insulators like PVC have a very frequency dependent
dielectric constant also (This adds to the G term IIRC), which is why
PVC is not used for RF cable insulation - although the effect is
present with other materials as well.


Richard P

2011\12\01@041239 by alan.b.pearce

face picon face
> Justin Richards wrote:
> > Is there an implied frequency when discussing characteristic impedance.
> > When we need to analyze cables with respect to their RF
> > characteristics we end up with a large table S parameters that have
> > been recorded over a range of frequencies.
> >
> > So when a cable is quoted at 75 ohms, that must be for a given
> > frequency.  If so what is it.
> >
> An ideal coaxial cable (made up of perfect conductors and perfect dielectrics in a
> perfect geometery) will have a constant characteristic impedance across all
> frequencies. I'm not sure on the details but my understanding is that if the
> characterstic impedance is changing significantly with frequency you are probablly
> above the cable's usable frequency range.

The other option is that the source and load are imperfectly matched, and large changes in effective  impedance at the end of the cable can be seen due to the length being close to an odd integer multiple of a 1/4 wavelength at the frequency of measurement.
-- Scanned by iCritical.

2011\12\01@054225 by Yigit Turgut

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I am replying a bit late on this and couldn't read the other replies
completely but I would suggest using a time-domain-reflectometer. This
precisely fits your requirements, using a high speed capacitor with a
PIC having CTMU, people have reported that they reach a resolution
level of "less than 1ns". I am currently trying to achieve better, you
might want to look into AN1375 and en542792.pdf.

Good luck.

On Tue, Nov 29, 2011 at 3:35 PM, Bob Ammerman <@spam@picramKILLspamspamroadrunner.com> wrote:
{Quote hidden}

>

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