Simple transmission line question

I'll have to try it. Does LT spice have directional couplers? It'd be nice to see what's coming and going....

George H.

Thanks Jeroen, Let's just call it 'brain dead Monday' on my part.

George H.

t the Hi-Z termination at the monitor is because there can be no net curren t into the Hi-Z, so the superposition of the forward and backward wave curr ents must sum to zero there, meaning the backward wave current must equal t he forward wave current thereby developing the same voltage across the line as the forward wave (V=IxZ), making the superposition of the two twice t he voltage of the forward wave. So assuming the logic is 5V your circuit la unches a 5 x 25/475= 263mV voltage down the line, which transforms to 2 x 263= 526mV at the Hi-Z with a backward wave amplitude of 263mV. All but

-reflected back down the line again to the Hi-Z.

Thanks... Is the 5% re-reflection bit because I 'should' have the parallel combo of the two source resistors equal to 50 ohms... so 450 and (tap tap tap) 56 ohms?

George H.

Probably not; LT doesn't do much RF.

An ACT part, like an '04 or '240 type, with 3 or 4 sections in parallel and powered from 5 volts, is a pretty fierce driver. Add about 20 ohms to get a 5-volt, 50 ohm, 1 ns driver. Of course, that's overkill for triggering an oscilloscope!

A divider, like 150:75 ohms, is a more sensible way to make a 50 ohm scope trigger from a logic level. Reduce that 150 ohms by the expected internal impedance of your logic chip, to be compulsive.

the Hi-Z termination at the monitor is because there can be no net current into the Hi-Z, so the superposition of the forward and backward wave currents must sum to zero there, meaning the backward wave current must equal the forward wave current thereby developing the same voltage across the line as the forward wave (V=IxZ), making the superposition of the two twice the voltage of the forward wave. So assuming the logic is 5V your circuit launches a 5 x 25/475= 263mV voltage down the line, which transforms to 2 x 263= 526mV at the Hi-Z with a backward wave amplitude of 263mV. All but 5% of the 263mV is absorbed by your

RPN:

R1 1/x R2 1/x - 1/x

Directional couplers in spice are trivial! Basically, it's nothing more than a Wheatstone bridge. Replace one of the sides by the tline and apply twice the signal to the top of the bridge. The reflection appears across the bridge. Below an LTspice example to play with.

Jeroen Belleman ================================================== Version 4 SHEET 1 880 680 WIRE 32 112 -320 112 WIRE 224 112 32 112 WIRE 32 144 32 112 WIRE 224 144 224 112 WIRE -320 224 -320 112 WIRE 32 240 32 224 WIRE 128 240 32 240 WIRE 160 240 128 240 WIRE 224 240 224 224 WIRE 336 240 224 240 WIRE 576 240 432 240 WIRE 32 272 32 240 WIRE 336 272 272 272 WIRE 496 272 432 272 WIRE 496 288 496 272 WIRE 576 288 576 240 WIRE 272 304 272 272 WIRE -320 336 -320 304 WIRE 336 352 272 352 WIRE 400 352 336 352 WIRE 32 368 32 352 WIRE 272 368 272 352 WIRE 400 368 400 352 WIRE 224 384 224 240 WIRE 576 384 576 368 WIRE 160 432 160 240 WIRE 224 432 160 432 WIRE 272 464 272 448 WIRE 400 464 400 448 FLAG 272 304 0 FLAG 496 288 0 FLAG 32 368 0 FLAG 576 384 0 FLAG -320 336 0 FLAG 272 464 0 FLAG 400 464 0 FLAG 336 352 reflected FLAG 128 240 incident SYMBOL res 16 128 R0 SYMATTR InstName R1 SYMATTR Value 50 SYMBOL res 16 256 R0 SYMATTR InstName R2 SYMATTR Value 50 SYMBOL res 208 128 R0 SYMATTR InstName R3 SYMATTR Value 50 SYMBOL tline 384 256 R0 SYMATTR InstName T1 SYMBOL res 560 272 R0 SYMATTR InstName R4 SYMATTR Value 1meg SYMBOL voltage -320 208 R0 WINDOW 123 0 0 Left 2 WINDOW 39 0 0 Left 2 SYMATTR InstName V1 SYMATTR Value PULSE(0 2 10n 1n 1n 10n) SYMBOL e 272 352 R0 SYMATTR InstName E1 SYMATTR Value 1 SYMBOL res 384 352 R0 SYMATTR InstName R5 SYMATTR Value 1 TEXT 80 40 Left 2 !.tran 200n

El 22-04-13 21:25, George Herold escribió:

Measure both current and voltage through the line at the same point (that means between two connected transmission line sections.

Forward voltage = 0.5*(50*I+U)

When you add a minus sign (or flip the current measurement) you get the reverse voltage.

The formula you can put in arbitrary source to get the output as a voltage or current.

On Apr 22, 9:25 pm, George Herold wrote:

something like this?

Version 4 SHEET 1 880 680 WIRE -192 64 -304 64 WIRE 176 64 -112 64 WIRE 352 64 256 64 WIRE 480 64 352 64 WIRE 720 64 576 64 WIRE 720 80 720 64 WIRE 480 128 480 96 WIRE 576 144 576 96 WIRE 176 160 96 160 WIRE 320 160 256 160 WIRE 720 176 720 160 WIRE 96 224 96 160 WIRE 96 288 96 224 WIRE 176 288 96 288 WIRE 352 288 352 64 WIRE 352 288 256 288 WIRE 0 384 -32 384 WIRE 176 384 0 384 WIRE 320 384 320 160 WIRE 320 384 256 384 WIRE 416 384 320 384 WIRE 464 384 416 384 FLAG 0 464 0 FLAG 416 464 0 FLAG 96 224 0 FLAG 720 176 0 FLAG -304 144 0 FLAG 480 128 0 FLAG 576 144 0 FLAG -32 384 forward FLAG 464 384 reverse SYMBOL ind2 160 176 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L1 SYMATTR Value 100µ SYMATTR Type ind SYMBOL ind2 272 48 R90 WINDOW 0 5 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName L2 SYMATTR Value 1µ SYMATTR Type ind SYMBOL ind2 272 272 R90 WINDOW 0 5 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName L3 SYMATTR Value 100µ SYMATTR Type ind SYMBOL ind2 160 400 R270 WINDOW 0 32 56 VTop 2 WINDOW 3 5 56 VBottom 2 SYMATTR InstName L4 SYMATTR Value 1µ SYMATTR Type ind SYMBOL res -16 368 R0 SYMATTR InstName R1 SYMATTR Value 50 SYMBOL res 400 368 R0 SYMATTR InstName R2 SYMATTR Value 50 SYMBOL res 704 64 R0 SYMATTR InstName R3 SYMATTR Value 1000 SYMBOL voltage -304 48 R0 WINDOW 0 -56 -5 Left 2 WINDOW 123 0 0 Left 2 WINDOW 39 24 124 Left 2 SYMATTR InstName V1 SYMATTR Value SINE(0 1 10e6 100n 0 0 1) SYMBOL tline 528 80 R0 WINDOW 3 -5 -93 Top 2 SYMATTR InstName T1 SYMATTR Value Td=500n Z0=50 SYMBOL res -96 48 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R4 SYMATTR Value 50 TEXT 64 112 Left 2 !k1 L1 L2 1 TEXT 56 336 Left 2 !K2 L3 L4 1 TEXT -242 488 Left 2 !.tran 1.5u

-Lasse

Exactly, 56R is a better match, but even your resistor precision gets a 5% tolerance too. Just pretend you don't see the 10mV overshoot, it's just monitor front end parasitics ....

Depends on your definition / configuration. Config #1: generator is 50 ohms and you consider that as "source termination" --> signal bounces back, one sees it at generator. Config #2: generatro is 50ohms, then a 50 ohm resistor ("source termination") in series to drive coax -->signal bounces back and all looks kosher at coax source end.

Check; case #1 in my posted list.

to the op.

as long as you are sending the signal into the cable at one end taking the signal out at one end (no taps etc) then you need to terminate the cable at only one end, either end will do...

when terminating the cable on the source end as you described, there will be reflections on the cable all along the way, but it is just as you said, the Rx end will have a clean waveform. There will be no RE reflection because the source is terminated.

Mark

Appendix A on "how to drive a scope" in Art of Electronics, 2nd edition, by Horowitz and Hill looks like a reasonable student guide to me. I must warn you that I have never tried it on, or seen it tried on, real live physics students. :) Because of when it was written, it's for analog scopes.

Standard disclaimers apply; I don't get money or other consideration from any companies mentioned.

Matt Roberds

"George Herold"

So I have a digital pulse and I want to send a monitor signal out to a 'scope. But the digital logic doesn't have enough poop to drive 50 ohms,

That job is just what they are designed to do.

The only alternative is an active probe.

Unterminated co-axial cable has a null ( ie zero impedance ) at the 1/4 wave length frequency - about 50MHz for a 1 meter length.

... Phil

Here is an interactive tutorial about how reflection works:

Based on my experience with new engineers, they need to be taught stuff like this. Hiding the "gotchas" isn't doing them any favors. Last thing I'd want is a Mindless Engineer.

Switcing to 75 Ohm or higher impedance coax would seem like a good first step then.

...which is why he sensibly wants to source terminate.

The current generation digital scopes are TERRIBLE from a human-factors design perspective, In my generation, students who had experience with television sync twiddling were handling analog trigger controls on the front panel, with one-and-only-one display mode running just left of the knobs. Except for left-handers, this was an easy task.

Unlabeled 'multipurpose' knobs, mixed mode (letters-on-screen/trace-on-screen, cursors-on-screen/pointers-on-screen/colorcodes) readouts, menu hierarchy, and 'automatic' features make a modern digital scope a morass for a struggling newbie. Us oldsters get trapped, too.