Impedance, Length, Reflection in cables (coax or others) - Page 1
Forum section: electronics / general

author: eV1Te
author_rank: Regular Contributor
timestamp: April 20, 2012, 02:51:33 pm
content: I would like to see a video that explains why you need and how terminating resistors work for signal cables (what determines the impedance of a cable)How you measure impedance of coax cables, can you also measure it for a PCB? What is the impedance of a PCB that handles high frequency signals (should you have terminating resistors with a matching value of the PCB in some cases)?Measure length of cables by reflectivity? How do you detect defects in cables by measuring reflecting signal? etc. etc. (speed of the electricity?)

author: free_electron
author_rank: Super Contributor
timestamp: April 20, 2012, 03:48:57 pm
content: An interesting machine for all these kinds of questions is a TDR : Time Domain Refelectometer.you can make this yourself with a sufficiently fast oscilloscop eand a 'step' generator ( a little circuit that makes a 1 volt step with a very high rise time. they are typically built from a 60 volts source and use avalanche breakdown effect of a transistor. Jim williams and bob Pese have circuits. you can find them using google. )yby injecting the step pulse on one side of the cable and looking for the echo you can measure the 'flight time' : time for the signal to travle to the other end , find nothing there , and come back ( that is essentially reflection. the energy wave has nowhere to go so it bounces back . throw a stone in water : you get ripples , when they hit the land , they bounce back. on their way back they intermodulate with ripples still going out.. )since you know the speed of the wave ( in copper it is less than lightspeed ) and the time it took to go out and come back : divide time by two , apply mathematics to bring in the traveling speed and you know the distance.so that is how you find out where the 'break in the cable is.you can do the same on circuit boards. but , since distances there are very short you will need a scope that can sweep fast enough ( think : GHz range.... )as for impedance of the PCB : you need to know 3 things : The permeability of the used material (so you need to know what material is used : FR4 , rogers , teflon and exactly what type ( there are over 400 kinds of FR4 ... some has coarse strands ,some has fine strands , )the thickness of the material, and the width of the trace.here is a tool that lets you do the calculationhttp://www.eeweb.com/toolbox/microstrip-impedanceto measure it you need a network analyser. in the PCB industry this is typically a machine made by Zmetrixwww.zmetrix.com

author: Nermash
author_rank: Frequent Contributor
timestamp: April 20, 2012, 05:45:27 pm
content: There is this very nice video made by a member of this forum:

author: eV1Te
author_rank: Regular Contributor
timestamp: April 20, 2012, 09:52:48 pm
content: In the video "w2aew" mentions that the propagation speed depends on the dielectric material in the insulator between shield and core. But "free_electron" mentioned that it is the speed in copper...Which one is it (if you have a silver cable or some other metal, does it change)? I mean the electric signal or "wave" propagates in the copper core, but the charging of the cable capacitance along the length depends on the insulator?!

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 21, 2012, 01:49:07 am
content: Quote from: free_electron on April 20, 2012, 03:48:57 pmi'm thinking of building a PC software for this so i dont have to go online to do it. the UI is very simple, just 4 input and auto calculate output. but i dont know whats the formula used. if i know the formula, i can build the software. any reference?

author: amspire
author_rank: Super Contributor
timestamp: April 21, 2012, 02:41:10 am
content: Quote from: Mechatrommer on April 21, 2012, 01:49:07 amThere are a number of free PC based microstrip calculator programs that already do this. If you install KiCad, you get one called PCB Calculator.

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 21, 2012, 03:16:35 am
content: Quoteand i dont want to install and load KiCad either. i may/can "extend" the func of my app if i want to to suite per need,... knowledge is power. and whats the point of this kind of advice? is it an implicit meaning of... that formula is soo complex that it'll need 3rd order partial differentiation number crunching numerical finite element analysis to calculate? pls clarify, that may stop me from asking and live with online calculatori love you mate.

author: amspire
author_rank: Super Contributor
timestamp: April 21, 2012, 04:39:08 am
content: Try thishttp://web.awrcorp.com/Usa/Products/Optional-Products/TX-Line/There are formulas online at sites like this:http://www.rfcafe.com/references/electrical/microstrip-eq.htmhttp://chemandy.com/calculators/microstrip-transmission-line-calculator-ipc2141.htmAnd you could get the code of PCB Calculator and just lift the code out. Each type of transimission line is in a separate C++ file and so you could use them directly or just copy the actual calculation section.Richard

author: robrenz
author_rank: Super Contributor
timestamp: April 21, 2012, 04:51:28 am
content: Or this,www.lg-advice.ro/pdf/pdf_design/IPC-2141A-cuprins.pdf

author: vk6zgo
author_rank: Super Contributor
timestamp: April 21, 2012, 06:39:43 am
content: Quote from: eV1Te on April 20, 2012, 09:52:48 pmI think "free_electron" was using "in copper",more as a "slang term",rather than to refer to any special qualities of copper.Copper is a good compromise between conductivity,mechanical qualities,& not least,price.Cables are made from other metals,but very few can match copper forallof the above qualities.As w2aew says,propagation velocity depends on the dielectric constant of the insulating material.It does not depend on the type of conductor material.As an example,in a common small coaxial cable,RG58 ,the signal travels at approx 0.66 of the free space velocity.In air spaced cables,the propagation velocity approaches more closely that in free space.

author: jahonen
author_rank: Super Contributor
timestamp: April 21, 2012, 06:53:02 am
content: Quote from: Mechatrommer on April 21, 2012, 03:16:35 amRemember that all those "simple" formulas are just approximations, they are not closed form solutions based on Maxwell's equations. Closed form solutions exist only for quite simple cases.If you want to do better than that, you'll need to build some kind of field solver to accurately calculate the impedance. I have been recently playing around with ATLChttp://atlc.sourceforge.net/,which is a very nice open source field solver based impedance calculator. It can calculate impedance of just about any uniform transmission line structure, and even differential and common mode impedance of differential structures. Drawback is of course that it takes quite a while to calculate the field. ATLC seems to agree quite well with commercial strength software like Hyperlynx and Sonnet lite.Open structures like a microstrip or differential microstrip require a quite large empty area above the line to calculate correctly due to field extending quite far from the line.Regards,Janne

author: free_electron
author_rank: Super Contributor
timestamp: April 21, 2012, 12:10:18 pm
content: Quoteexactly. I meant 'in copper' as in ' in the copper trace on a pcb'.Depending on the pcb buildup, material used the wave propagation alters.People always think 'electricity runs at the speed of light' .. Eh not quite true...Besides, it is the wave that runs, the electrons (particle) are much slower ! They can even run backwards in respect to the wave direction. Think of it like this : take cup of coffee, pour a bit of milk in it. Now move a spoon from left sode of the cup to the right side and observe. The wave travels left to right... But you will see slivers of milk curling back in the opposite direction.same happens with electrons.And we haven't even tackled electron dispersion yet (more commonly called the 'skin effect')

author: eV1Te
author_rank: Regular Contributor
timestamp: April 22, 2012, 02:19:57 pm
content: I took out my old books and did a rough calculation on the velocity of electrons:Mean velocity of single electron at room temperature (but jumps back an forth between "atoms")=~0.5 % of speed of light in vacuumDrift velocity at normal current densities (useful speed after canceling out all the jumping) =~1 meter per hourI am not an electrical engineer, but I do have a masters in material science, (so I have focused mostly on practical measurements of conductivity in superconducting to insulating materials or their magnetic properties at different frequencies etc.). But I did not know what the practical speed was for a signal to go through a cables (my first guess would not have been that it was determined by the insulating material), so i guess many others out there also lacks the knowledge about this interesting subject!Isn't Dave getting the 1 GHz upgrade to his Agilent scope? Maybe possible to do some cool and fast practical measurements on cables or PCBs?

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 22, 2012, 10:29:41 pm
content: Quote from: eV1Te on April 22, 2012, 02:19:57 pmi believe electron will not move from one end to the other? it is its force wiggling between the one to the next creating this "electric field". if thinking of your ideal 0.5% light speed electron speed, its difficult to achieve 0.66 SOL signal movement. but again i'm not scientist or engineer.

author: eV1Te
author_rank: Regular Contributor
timestamp: April 23, 2012, 08:17:34 am
content: Quote from: Mechatrommer on April 22, 2012, 10:29:41 pmThink of it as a train, even if the locomotive starts going forward slowly, the time it takes until the last cart/unit "feels" this forward movement is very short, since they are all pushing on each other.Hence the speed of the "signal" is much higher than the speed of cart/unit in the train

author: ejeffrey
author_rank: Super Contributor
timestamp: April 23, 2012, 09:15:55 am
content: Quote from: Mechatrommer on April 22, 2012, 10:29:41 pmElectrons move at 0.5% c even in a conductor that is carrying zero current. That is just the mean thermal velocity aka brownian motion. For the most part the electrons move in random directions and the average velocity is zero. When you apply a voltage and therefore a current, the electrons acquire a very small average velocity. They are still moving mostly randomly in every direction at high speed, but on average they go slightly more often in one direction than the other.0.66 c signal propagation is roughly what you get in any insulated transmission line. You don't have to work hard to get it. It is actually difficult to go much faster or slower than that simply because most of the materials we want to use as dielectrics are pretty similar. Free space (non guided) waves travel at c, and there are vacuum/air dielectric transmission lines that have near light speed signal propagation. None of that depends on the drift velocity or the thermal velocity of the electrons.

author: vk6zgo
author_rank: Super Contributor
timestamp: April 23, 2012, 11:22:04 am
content: There are some coaxial lines that are 0.78 & 0.83c!I got caught out by assuming 0.66 & cut some lines too short!

author: Mechatrommer
author_rank: Super Contributor
timestamp: April 23, 2012, 11:59:58 am
content: Quote from: vk6zgo on April 23, 2012, 11:22:04 amyeah i learnt that while mixing with people here, so i dont have to do it the hard wayw2aew vid is very usefull 3in1 measuring coax: impedance, length and the %c (if the length is known/measured). sadly i dont follow utube so i cant give him thumbs up.

author: vxp036000
author_rank: Regular Contributor
timestamp: April 30, 2012, 11:16:01 pm
content: I have not seen anyone give a straight-forward answer to when termination is required, so here you go. At frequencies where the circuit dimensions or signalling lines approach lengths greater than 0.1 wavelengths or so, you need to terminate the cable with a matched resistor. A more precise answer, is to choose the maximum parasitic capacitance or inductance acceptable in your design and back-calculate to get a maximum trace / signalling line length. No simulation required, it's a simple back-of-the-hand calculation. Without a termination, a significant amount of the signal will be reflected back to the source, which can lead to all sorts of unexpected behavior (in an RF amplifier, for example, the power FET could easily be damaged).It's difficult to calculate impedance of board traces by hand; typically something like AWR or Sonnet is used to size the trace to get the intended impedance.Quote from: eV1Te on April 20, 2012, 02:51:33 pm