AoE x-Chapters, 4x.26, MOSFET current source, nodal analysis

The number of LCs goes as the square of Td/Tr. Gets ugly quick. And the step response rings.

LT Spice has two transmission line models that seem to work well. I use them all the time.

Not really.... An ODE (Ordinary Differential Equation) is pretty much any differential equation of one variable, in contrast to a Partial Differential Equation. Once the length of a line is fixed its usual partial differential equations become ODE.

What you probably mean is that an ideal transmission line is a pure delay. This means an exp(-tau.S) transfer function, which is not a rational function of S.

Spice has to do extra stuff, as in convolution, to handle TLines. It slows it down a tad...

Not really....

"Non local" pretty much universally means FTL (faster than the speed of light). A transmission line is most certainly local.

What you probably mean is that a transmission line requires spice to do some special stuff, as above.

-- Kevin Aylward

If Spice converged much faster, the transmission line could be a few hundred series inductors and a few hundred parallel capacitors... the practical hardware (a spool of good coax cable) is just 'way cheaper to operate than an accurate model is.

Nope. ODEs don't have memory, T-lines do. (At least from a circuits POV.)

Kirchoff's laws do not apply to transmission lines. Current disappears into one end and emerges from the other end sometime later. The currents into the circuit nodes don't sum to zero, and neither do the voltages around loops.

As I said, the approximation can often be patched up by hand like that.

Nope. It means that the governing equations require information from more than one space-time point. All differential equations are local. Nonlocal systems need integral equations.

A transmission line is most certainly local.

Nope, not from a circuits point of view. To find out what's going to come out at time t, you have to know the what went in at time t-tau.

Nope. I wasn't talking about SPICE at all.

Cheers

Phil Hobbs

The LT Spice tline part acts as if it has an ideal DC-true 1:1 transformer inside. The ends can be at radically different common-mode voltages and it works fine.

That avoids dealing with the em effects of the "shield". Modeling a twisted pair takes three tlines.

I've wondered how the tline is implemented inside, and how it relates to the current time step size.

Well, you're kind of using a private language there, as you say.

Cheers

Phil Hobbs

LTSpice lets you add parasitic aspects to basic parts, like resistors, but these are hidden on the schematic, so you have no idea whether the modeller made additions, or whether he made the right ones. I prefer seeing a SPICE schematic, with the parasitic parts explicitly showing, so you know what was taken care of, and how. To my mind, these aspects are up to the engineering doing the SPICE schematic drafting, and not up to the SPICE program. The LTSpice approach is bad, not good. So I disagree, it's not the sim that's wrong.

Where it is important, I add the parasitics externally.

Great. Now I have to go buy 20GB of SRAM!

So ports.....don't exist?

Tim

Which is what you expect as transmission lines correctly account for the fact the FTL is impossible, that is, it shows that signals take a finite time to propagate from A to B, thus transmissions lines behave *locally*. Period.

In contrast to Kirchoff's Laws. Those laws assume that all voltages and currents are instantaneously connected. They ignore propagation delay effects, thus Kirchoff's Laws are non-local. Period.

Thus... you have the definitions backwards.

We, apparently, have different definitions of locality.

Thus, to date the only accepted apparent FTL, i.e. non-local behaviour that I am aware of, is the statistical correlations between entangled particles, which cannot be used to transfer information at FTL

It is inherent in transmission lines that there is delay for transfer of information, which is *the* fundamental property of the definition of locality in physics, so it is indeed interesting that there are those that claim the opposite.

-- Kevin Aylward

A port is not a node. KCL talks about _nodes_.

Cheers

Phil Hobbs

In your dreams. Nonlocality doesn't imply FTL, whatever your Sunday supplement version of quantum field theory might tell you. The Boltzmann transport equation is also nonlocal, and that applies to your coffee cup. YCLIU.

You're inventing a private language to avoid being wrong in public. G'wan, Kevin, you're just hitting your stride, man!

No, that's a particular definition in relativistic quantum field theory. We're talking classical E&M here.

Cheers

Phil Hobbs

LTspice does have some parasitics like ESR for instance that helps keep the matrix from being ill conditioned as I understand it.

Also, there are places to add parasitics sometimes that do not take extra compute time as it would if you added them externally. That is why you should probably use them when available.

Hmm, I gotta agree with Kevin, Kirchoff's laws are non-local. Just like Newtons law of gravity is non-local. At least that's how I under stand how 'local' is used in physics. Non local implies action at a distance.. ( again my understanding.)

But then again I thought I agreed with everything you said... ? (Did you use the 'local' word?)

George H.

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Seems like there's a sort of double vision there--you're not applying the Kirchhoff approximation consistently.

Kirchhoff's voltage law: the sum of all voltages around any loop is zero. (Equivalently, curl E = 0)

Kirchhoff's current law: the sum of all currents entering a node is zero.

Being consistent about the low-frequency approximation, nodes and loops have negligible size. Otherwise you couldn't write a system of ODEs to describe the circuit. So Kirchhoff's laws are local in the physical sense.

I invite anyone here to write a system of ODEs for a circuit containing a transmission line.

Cheers

Phil Hobbs

Perhaps yes, and it has the useful property that for my "schematics", as far as I can tell, Kirchoff's laws do hold. Obviously I would like to find out any situations where that isn't true, as I don't want wrong predictions of the behaviour of a real system, but at that point I would probably declare the drawing to be no longer a valid "schematic" of the real system, again by my weird definition, and then I might try to fix it. Perhaps I am alone in holding these attitudes, but they have served me well so far.

Totally irrelevant.

Locality is defined by whether or not there is action at a distance faster that the speed of light, not by properties of any equation. Its a physical observational definition, not mathematical. If information is propagated FTL, its non-local. Period.

Whatever custom definition you are using for "locality" is not one that ever occurs in any Physics context, especially with regard to Maxwell's Equations.

-- Kevin Aylward

Twaddle.

Your are seriously confused. The *definition* of non-locality is FTL Period. No iffs or buts.

Twaddle.

Again, your are seriously confused. The *definition* of non-locality is FTL Period. No iffs or buts.

It has zero to do with Quantum Mechanics. You are really stepping out of your depth here.

Locality and non-locality are terms intimately connected with the construction of Special Relativity. Its not debatable.

I suggest you actually read the link:

"In physics, the principle of locality states that an object is directly influenced only by its immediate surroundings. A theory which includes the principle of locality is said to be a "local theory". This is an alternative to the older concept of instantaneous "action at a distance". Locality evolved out of the field theories of classical physics. The concept is that for an action at one point to have an influence at another point, something in the space between those points such as a field must mediate the action. To exert an influence, something, such as a wave or particle, must travel through the space between the two points, carrying the influence."

Don't even try to contradict it, it would only makes you come across as an "Einstein was wrong crank"

Classical E&M theory is a local theory, it is thus simply impossible for transmission lines to behave non-locally. End off.

Thus Kirchhoff's Laws are nonlocal. Kirchhoff's Laws require voltages and current information to be propagated instantaneously.

I suggest some refresher courses in physics, because you are only embarrassing yourself.

-- Kevin Aylward

Well yeah, a port is a branch that connects nodes.

So the port currents don't go anywhere?

Tim