Metals inductance/capacitance per meter
Feb 27, 2006 at 9:53 PM Thread Starter Post #1 of 17

JJ15k

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Anybody know where I can find this? I tried google but couldn t come up with anything very interesting.
I m basically looking for the caracteristics of copper/silver whatever wires. They can be modelised by a lowpass filter and I m looking for the values I should input.
Thanks
JJ
 
Feb 27, 2006 at 10:00 PM Post #2 of 17

kramer5150

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An ASTM or machinist's handbook should have the data your after... note thou that its the raw element... not cable
 
Feb 27, 2006 at 10:03 PM Post #3 of 17

JJ15k

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Yeah I know it s the raw element, I was only looking for a rough approximation, since I am a little sceptical about some claims made and I wanted to see if they ere realistic (I m keeping an open mind...).
 
Feb 28, 2006 at 1:57 AM Post #4 of 17

Xakepa

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From what I recall capacitance have nothing to do with the material (only with the one inbetween) but with the distance and area, and inductance is a function of the shape and freq, and, again AFAIK, could be considered purely resisive in audio applications

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Feb 28, 2006 at 4:47 AM Post #5 of 17

kramer5150

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Quote:

Originally Posted by kramer5150
An ASTM or machinist's handbook should have the data your after... note thou that its the raw element... not cable


You know what.... don't quote me on this.... I have only looked up surface resistivity in an ASTM manual. There were a bunch of other electrical and thermal properties, but I can't say for certain if inductance or capacitance were listed.
???

Garrett
 
Feb 28, 2006 at 5:40 AM Post #6 of 17

amb

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These are not simple and generic characteristics that you could just "look up". The inductance and capacitance of cables are greatly influenced by the mechanical construction and the materials used (both the conductors and the insulators).
 
Feb 28, 2006 at 9:35 AM Post #7 of 17

Francis_Vaughan

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This has the danger of getting into cable wars.

Both capacitance and inductance are not intrinsic properties of materials. So you will never find such data. They are properties of a geometrical arrangement of conductors. Where the conductors need to be very close together we may employ an insulating material to maintain the mechanical arrangement, and in the case of a capacitor, to modify the dielectric constant (permittivity) of the space between the conductors.

You need to be quite careful about what it is you are trying to model. For a simple cable the metallic properties are essentially irrelevant until skin effects become important. Which (despite occasional protests from the ignorant) does not occur at anywhere near audio frequencies. And when it is important, it is very very messy to model from first principles.

If you are winding an inductor you may need to take into account the magnetic permeability of the metal - since the coil is now mostly not full of nothing (which the simple formulae assume) but full of copper or silver. This changes the final inductance. You will find permeability as a material property. Silver and copper are interesting as they are diamagnetic materials and their presence actually makes the inductance drop slightly, aluminium is paramagnetic and the inductance very slightly increases. Wind the inductor with steel wire, and the ferromagnetic properties of the material will cause all sorts of fun, but in general yield a very large increase in inductance.

If you wish to model very high frequency (RF energy) behaviour of a wire (for instance if you wanted to use it to transmit microwave energy) you would need to know the resistivity, permeability and permittivity. Then, knowing the frequency involved, and the cross section geometry of the conductor it is possible to calculate the effective resistance of the conductor. You could model the conductor as a lumped constant RLC net across a range of frequencies, but that is only an approximation. It is not an intrinsic charaterisation of either the material or the conductor.

Thin films of materials get totally weird, and here these parameters actually become complex values, not real values. But since we are not interested in specialised optics we can ignore that.
 
Feb 28, 2006 at 9:45 AM Post #8 of 17

JJ15k

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I am thankful for all the answers, but as I said, I am looking only for a ROUGH approximation, i know that models can become,if we only use maxwell equations with no approximation, but I only wanted to see wether some things were realistic.

In this case I ll try another question: has anybody tested the inductance/capacitance of a cable and would they be kind enough to post them.

PS: I have no intention of starting a flame...
 
Feb 28, 2006 at 10:19 AM Post #9 of 17

amb

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Quote:

Originally Posted by JJ15k
I am thankful for all the answers, but as I said, I am looking only for a ROUGH approximation


Well, the variables can be so great that one cannot come up with such an approximation. Let's take a couple of shielded cables from Belden as example (I use these to make my own audio interconnects so I am familar with them).

The Belden 9221 and the 8451 are roughly the same physical thickness (with jacket), the 9221 is a single center conductor cable with a coaxial braided shield, and tinned stranded copper wires. The 8451 is a two-conductor cable with foil shield and similar wire as the 9221. The insulation material are somewhat different. However, the 9221 is only 17.3pF capacitance and 0.097µH inductance per foot. Compare that to the 8451 which is 34pF and 0.17µH. If a cable that is dimensionally so similar to another yet exhibit ~100% more capacitance and inductance, imagine cables of vastly different constructions...
 
Feb 28, 2006 at 10:49 AM Post #10 of 17

JJ15k

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Well, from your measurements and my rough model I must admit that cables could have a very audible impact on the sound.
Now I know some of you have just said how transmission lines can be complex, but bear with me I m only beginning my engeneering studies.
 
Feb 28, 2006 at 12:55 PM Post #11 of 17

setmenu

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Running simulations of typical ic cable length rcl characteristics in it does not
seem that they would have any real influence on the audio band.
Unless one is using very long cable runs.
But then there are super tweeters and sacd etc that are supposed to deliver
frequency spectra above the limit of human hearing.
I have often read reports that this additional hf extension is audible as an
effect on the sound within the normal human hearing range.
Explanations seemed to tend toward the interaction of the hf with the lf
frequencies.
It would be interesting to do test where ultrasonic noise was injected into
the music signal at different spot frequencies and see whether listeners
reported a change in sound.[I guess this is already the case with distortion
spectra in any case]
If such a test did produce repeatable audible results , then perhaps cable rcl
values that would only affect these frequency spectra would also make a
difference to the hf sound/noise content of a signal.

Any thoughts or am I talking the blindingly ovbvious/rubbish?
redface.gif






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Feb 28, 2006 at 12:57 PM Post #12 of 17

JJ15k

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Well, when using the lowpass filter model with the values given by amb, there seems to be a non negligible difference in the gain at a given frequency depending on the cable. Wether it can be heard......
I ll admit I m not willing to go through all the calculations to get a precise answer...
 
Feb 28, 2006 at 1:06 PM Post #13 of 17

setmenu

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Quote:

Originally Posted by JJ15k
Well, when using the lowpass filter model with the values given by amb, there seems to be a non negligible difference in the gain at a given frequency depending on the cable. Wether it can be heard......
I ll admit I m not willing to go through all the calculations to get a precise answer...



What results did you get?
what frequency range did things start to go awry?
All I did was plug value such as those given by amb into microcap.
But then again I am no expert and could have been in error.


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