Transmission line emulation.

Simulate a lossless line with just Ls and Cs... no Rs.

A discrete LC line tends to ring on a fast edge. The number of LC sections grows as the square of Tr/Td, which gets ugly fast.

It needs R, it's far from lossless and carries significant power too. (In fact most people in this business don't bother with the Ls, but I want a better emulation.)

So the question is, would a resistive inductor tapped with multiple Cs be closer to a real line than multiple discrete RLC stages? I can't test a real line.

Cheers

What kind of line are you emulating? What bandwidth, delay and impedance tolerance?

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Design Website:

What's the physics that you'd like to emulate?

If you can't test a real line, the sim will be a guess.

Unfortunately that's all NDA stuff.

But, assuming it's just a very long, very large coax, and I know its total C and R and can guess the L, the question is, as a generality, would you expect that a resistive inductor tapped with multiple Cs would make a better model than multiple discrete RLC stages? I can't test a real line.

Cheers

Maybe the trans atlantic cable :^)

George H.

Must be one odd customer to hold their coax so close to their chest.

Warning signs?...

Better for what? You've given us no way to tell.

Tapped inductors are generally better, where "better" is measured in terms of delay per stage.

Filters in general, analyze nicely when symmetry is taken advantage of. For example, a pi filter should be divided into half-stages of CL and LC, and those further into RC || CL + LC || CR, where the R's are the source and load impedance.

The inductor doesn't need to be resistive, but if that's what happens to give a closer result, so be it. It would seem more inconvenient to use an inductor of specific loss, until you know how lossy it actually needs to be.

Note that lossy inductance doesn't give complementary lossy capacitance. This may or may not be correct. In average coax, there's far more ESR than EPC (equivalent parallel conductance), because typical dielectrics are much better capacitors than copper an inductor.

If it's, say, ferro(electric|magnetic) loaded line, the equivalent case may involve some combination.

(The ratio of parallel and series equivalent losses affects dispersion.)

Tim

I don't really know. But I know that in all-pass delay network synthesis, a negative inductor will show up. And were that occurs, the negative inducto r is resolved by a coupled inductor transformation.

But you're asking, perhaps, more about a low-pass type structure.

Do you recall with the Brune process (Brune cycle or one of the Darlington networks) where coupled inductors result? Maybe some intuition can be sourc ed there. The structure looks low-pass-ish. See Fig. 6 in:

No, it's just very special and very expensive for a specific purpose and still under development.

Better (lower) attenuation at higher frequencies.

If I make a lumped line model in LTspice and compare this with the LTRA (lossy transmission line) model provided using a simple AC analysis, after following each other nicely for a while, the lumped line response goes south at an alarming rate while the LTRA flattens out.

I don't know why this is, and that's why I'm asking here, it's not my specialism. Is the LTRA model faulty, or is a lumped model not that representative?

My two physical test lines behave quite differently despite having the same overall RLC, the 10 section lumped RLC is worse at HF than my long resistive inductor with Cs on taps. Is the latter behaving more like the LTRA model?

For me, delay and phase don't matter, in that they're critical so are compensated for :-) Attenuation is the issue.

I do know the total resistance of the line, and my long tapped inductor is designed to have that resistance and also the total line inductance as estimated.

There is some parallel conductance too, but at this stage I'm ignoring it.

Thanks

How are you going to emulate the bidirectional amplifiers?

They are all fiber now!

If it's long enough, just Rs and Cs.

So, reduce R towards zero?

Mind, in a simulation, that depends on the accuracy and integration method. Accuracy is the various *TOL parameters.

LTSpice claims to have a hacked integration method, that gets losses closer than original TRAP (which tends to underestimate). It also runs faster, but has a number of... quirks consequential to that speed. GEAR tends to overestimate losses, though to a much lesser degree, especially at higher levels. Of course, it goes much slower at higher levels.

The more precise of a measurement you are trying to make, the more these will matter. Loss is a subtractive element, so you need an extremely precise simulation to model it correctly, when it's a small part of the total.

The model needs to be that much more representative, as well.

"Goes south" when? At frequencies on the order of 1 / sqrt(L*C)?

I ask again, what delay to risetime ratio are you looking at? Because that's exactly as important as JL noted!

Doubt it: the transmission line models are propagating-wave hacks. They are not built from RLC sections! They should be more accurate, around the cutoff, than a lumped equivalent.

RTFM for more details? I don't know how exactly the LTRA is built. Finding the equations used, would be the best reference.

Sure, but is that the same as the AC resistance? Is that distributed the same as in the real article?

This seems like a lot of folly, like trying to use the wrong tool for the job -- it sounds like a proper field solver would be more effective, and that would get you the parameters of an incremental TL segment, including correct AC resistance.

Assuming they've been kind enough to give you any mechanical information at all, of course.

Tim

NEVER gonna make that work. An inductor with taps is a MUTUAL inductor, you need decoupled ones. Looks similar on a diagram, but not at all the same.

There are ferrite beads you can string on a wire, and swage a tube around it... to make a delay line with real delay (and hundreds of 'sections'). Or, two-hole beads to make a differential (twinax style) line. How much delay do you want, and with what signal bandwidth?

Heck, you can buy acoustic (reverb module) lines and couple into 'em with transformers. Not gonna be a good electrical-resistance model, if you do it that way, but you get lots of milliseconds for the dollar.

est a

Huh, I know nothing of what you do, but attenuation is about resistance. (I guess you can have an antenna with radiation resistance.)

The difference between the lumped L and tapped L is that the tapped L has more linkage/ coupling. Is that right?

So how much coupling is in the real cable?

George H.

o use

ESR

Is that the trans atlantic cable..? Heaviside figured out how to load it.

One good way to make a delay line is with t-coils.

Take a look at the delay line in the old Tektronix 545 scope. There are manuals online.

Yeah, but you have to orient them so the alternate ones don't couple, only the adjacent pairs. Solenoids are a bad way to put together lumped-constant delays, ferrite beads (toroidal magnet circuit, self-shielding) are easier to work with when you're stacking a dozen or three. A t-coil is an inductor with A tap, not 'taps'.