Coax modelling question

I can model a coax cable in LT spice using the inbuilt model or a lumped collection of R C and L.

But the cable I want to model has no overall screen insulation, and somewhere between most and hardly any of it is very tightly wound onto a drum, so the outer on the drum is all 'shorted' together. The outer is steel and has the same order of resistance as the copper inner.

That means there is part of the cable which looks like, I suppose, a mass of steel with an embedded coil.

I don't have a cable to hand.

How would you go about modelling this? I suppose that for the wound section, the R reduces, the C remains the same, but I'm not sure what to think about the L.

Cheers

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Syd
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Syd Rumpo
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Transmission lines are usually modeled as a two-port. In a two-port, you get relations among V1, I1, V2 and I2 but you have no information on other variables, for instance VDB.

A------[ ]------C V1 [ box ] V2 B------[ ]------D

You can have a situation where B and D are the same physical point (some baluns did that if memory is still ok) or you can have a situation where the coax shield forms a second transmission line with another conductor (perhaps "ground").

In your case, the differential signals V2 and V1 would be the same. The shorting would have an effect on the second transmission line. You would probably get a big capacitance between B and D and the true "ground"

Pere

Reply to
o pere o

On Sep 13, 2016, Syd Rumpo wrote (in SED article ):

Transformer and lumped-element models fail here. The issue is that above a critical frequency, outside and inside currents decouple due to skin effect. In other words, above that frequency, shorting the outsides of the shields by winding the coax on a metal rod has no effect on the currents on the inside of the shield, even if you solder coax shield to rod.

Joe Gwinn

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Joseph Gwinn

The LTspice transmission line model fails even worse, because iirc it acts as a completely isolated, time-delayed source, as though it had a perfect balun attached.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

true

so the OP can use this model and add a resistor in series with the two output leads to simulate the wire resistance and then short together the input and output shield ends to simulate the fact that it is would on a roll.

m
Reply to
makolber

I don't think that works, because as Joe pointed out, the inner and outer surfaces of the shield decouple at high frequency, and unbalanced currents see a lot of inductance. A steel shield will decouple sooner than copper, because there's a factor of 1/sqrt(mu) in the equation for skin depth. (Of course if it's nonmagnetic stainless, that doesn't help.)

At low frequency, much of the ground current would flow across the turns of the coil because of the reduced resistance, but this would diminish a lot at higher frequency.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

The LT Spice txline makes a nice 1:1 dc-daylight isolation transformer.

I don't think it qualifies as fails. If they didn't do it their way, they would have to separately model the shield-to-universe impedance, which would be hard to do. Or make it a 3-wire device. Or something. As-is, you can add your own shield model, or just ground the shield pins on both ends.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

Are you talking about a spool of insulated wire inside a metal bucket?

That would have various amounts of wire-wire coupling, which will cause all sorts of nasty effects.

A sketch would help.

"somewhere between most and hardly any" makes modeling difficult.

What is this for?

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

I didn't mean that it failed altogether, just that I don't think it will work in the OP's application. I don't immediately see how you could use it to model a coax balun, even.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

It already models an ideal coax balun!

In the non-ideal case, one would connect something between the ground pins of the two ends, to simulate the common-mode impedance. That could be an inductor, or even another transmission line. And you could add Z to ground on both ends, to sim coupling of the shield to the universe. That is all far more complex than the simple Heaviside model of a coax.

I have broken a txline up into a lot of short segments, so that I can add distributed-to-ground capacitance along the line.

The LT Spice lines are fun, in that you can play games with the shield currents. I could post a fun circuit, except that I'm still trying to sell it.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

One with infinite bandwidth, which is the problem.

I haven't played with it much myself.

Cheers

Phil Hobbs

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Phil Hobbs

LTspice (and PSpice) also includes a dissipative transmission line model. I don't know if even that is suitable for modeling transformers. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

Then add a lowpass filter in the signal path, or add some common-mode impedance. The ideal txline is just a starting point.

Someone could create a better, paramaterized coax+balun model, based on the ideal line as a starting point, but it would be insanely complex. I just add the things that I figure will matter.

People (not you, but more like beginners) can learn a lot by fiddling with the txline and pulse generators and resistors in LT Spice. Blumline sorts of things.

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John Larkin         Highland Technology, Inc 

lunatic fringe electronics
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John Larkin

I suppose a loosley wound spool of insulated wire in a cylinder of mercury might approximate.

Just take some coax, strip off the outer insulation and wind it on a drum.

Yes, but that's the reality.

A long cable (a wireline) for use in boreholes. The outer is steel armour for strength and has no insulation covering it. The inner is copper and there's a dielectric, probably PTFE. It's partly on and partly off a winch drum. Instruments at the bottom of the hole send signals to equipment at the top, sometimes vice-versa.

I'm not sure what happens to the inductance of the part on the drum compared the the length in the borehole. You'll have the effect of the shorted outer, which must reduce overall inductance and resistance, and then the fact that the inner is making a big coil, with possibly many hundreds of turns on a 1 or 2 metre diameter.

So does the inductance increase or decrease with winding?

Cheers

--
Syd
Reply to
Syd Rumpo

Differential mode inductance will hardly change, since there's no B field outside the shield in that mode. Common mode inductance will change some. If the contact is poor, it'll go up on account of the mutual inductance of the turns, as usual. If the contact is good, it'll basically change from a long skinny wire to a short and very fat wire, so the inductance will go down.

Cheers

Phil Hobbs

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Dr Philip C D Hobbs 
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Phil Hobbs

...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Jim Thompson

I'm guessing (free, no charge) that it will behave just like a straight coax. Winding it on the drum should have no effect beyond any mechanical distortion, and a little resistance change as the shield turns scrape.

If the signal is balanced (no sneaky ground currents) the coax behaves like a N-ohm transmission line, where N can be calculated from the standard coax equation (or use Appcad.) There should be no additional inductance.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

This may not be practical for Syd, but I was thinking it would be nice to have data. Hit the big coax with a pulse or step, with the end open or shorted and see what the reflection looks like in both cases. (wound on spool and unwound.)

George H.

Reply to
George Herold

That's TDR, which can be done with a pulse generator and a scope for longish runs. That could be mocked up pretty easily, with some stripped coax.

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John Larkin         Highland Technology, Inc 
picosecond timing   precision measurement  
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John Larkin

A timely pointer for me, thanks Jim. The TV transmitter modules that recently came into my possession use a very similar structure, though for 200MHz, and I was trying to figure out how the impedance transformers work.

Clifford Heath.

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Clifford Heath

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