Manufacturers cable inductance specification.

[sh-eda]

Hi

I have another simple question, I hope someone might be able to help me with.
When cable manufacturers specify the cable inductance per Metre, (specifically for single pair or multi-pair type, i.e NOT coxial) are they generally specifying a complete loop inductance (i.e for 1 pair shorted at the end of the cable) or the inductance of each single wire.
i.e Do I have to double the inductance to calculate the characteristic impedance, like the cable resistance? This doesn't always seem clear to me.


Thanks

 

[FvM]

I would expect twisted pair specification referring to pairs rather than wires. But primarly I'll require a clear manufacturer specification.

As a hint, twisted pair impedance will be in a 90 to 120 ohms range for usual cable geometries.

 

[sh-eda]

Hi

Thanks for answering. That's what I thought.
I have uploaded typical cable types that I am looking at, hope you can see them.
Assuming that the inductance spec is of a loop,
For Amercable #14 awg I calculate 66.3ohms/metre.

For the Taiham, unfortunately does not directly give the inductance, but I calculated it to be around 444uH/m from the L/R ratio and the physical dimensions.
(2.5mm^2) so I calculate at 54.1ohms/m.

Thanks

 

[FvM]

For Amercable #14 awg I calculate 66.3ohms/metre

Characteristic impedance values aren't giver per metre.

I notice that the resistance specifications in the datasheets are clearly given for single ended wires (you can easily calculate), thus the L and C values can be assumed to be single ended as well, otherwise the intrinsic safety related L/R numbers would come out wrong. I guess, they are defined for single ended wires in the EXi regulations, you can check.

Specific L and C values of single ended wires must be decomposed into common and differential mode parts, differential pair impedances can be specified for both modes as well. Depending on your application problem, either both or differential impedance only may be relevant.

62 or even 54 ohm differential impedance is effectively impossible for regular twisted pairs, the doubled value sounds plausible as said.

In case of doubt, I would prefer to measure both parameters empirically.

 

[sh-eda]

(Yes the characteristic impedance/metres was a mistake)
I wish it was a bit more explicitly stated, I did notice the resistance was for a single ended wire. So I most likely need to double the inductance.
I am not sure how the capacitance/ metres can be singled ended? Surely cable capacitance would be measured between wires in the cables. Unless it is to the shield / screen.

 

[FvM]

Surely cable capacitance would be measured between wires in the cables. Unless it is to the shield / screen.

It's most likely single wire against shield + other wires, once more the safety relevant total capacitance. In a shielded TP, shield capacitance is the biggest part of it.

 

[mtwieg]

I dunno, I could probably believe a Z0 of ~60 ohms for twisted pair if it was finely stranded, had a thin and tight jacket, and was very tightly twisted. But more than likely your numbers are measuring the effective single ended Z0, not the true differential Z0.

 

[sh-eda]

I've been looking into the cable calculations, for the Taiham cable.

capacitance = pi.Er.Eo/(acosh(dist/dia))

or = pi.Er.Eo/(ln(dist/dia)) if the distance is much greater than the diameter.

and
inductance = ur/pi.acosh(dist/dia)

dist - distance apart
dia - cable diameter
Er = 2.4
Eo = 8.85419E-12
Ur = 1.25664E-06

I get a capacitance of 65pF/m which is nothing like the datasheet. I notice that if you use the other formula
pi.Er.Eo/(ln(dist/dia)) you do get 150pF/m, which I think might be wrong.
The inductance of 406nH/m which gives an impedance of around 78ohms. Does this sound more likely?
I am beginning to think that they have actually calculated the values rather than measured them.