Re: Transmission Line Theory
Alright, I'll take a stab ... At 02:30 PM 5/12/99 -0700, Allen Tudor wrote: > >Greetings group, > >I am trying to draw a parallel between transmission line theory and radiated emissions. > >From what I understand, a transmission line can be terminated at the source or the load with an impedance that equals the characteristic impedance of the transmission line itself. The important thing to remember is that once the signal energy has reached the far end of the trace, you want to dump that energy into a resistive load. Loading a line at the source end ... I'm not familiar with that. Even so, once the signal gets to the unterminated far end of the trace, there's an impedance mismatch and you will get reflection. Ideally, you will get 100% reflection. Tau(reflected) = E(reflected)/E(incidence) = (RLoad - RSource)/(RLoad + RSource) RLoad = infinity (unterminated) means 100% reflection. Tau(transmitted) = E(transmitted)/E(incidence) = (RLoad^2)/(RLoad + RSource) >With this in mind, consider this scenario. A printed circuit card drives a clock signal down a trace on a backplane. The length of the backplane trace is long enough to be considered a transmission line. The driver on the printed circuit card is located within ½ inch of the edge connector (mating with the backplane) and is terminated with an impedance equal to the characteristic impedance of the backplane trace. However, the backplane trace is open ended (there is nothing connected to the end of the trace). Transmission line theory says the signal integrity will be maintained in this case. Forgive me on this, but I don't see how. I'm sort of a purist. You'll get 100% reflection under ideal conditions. >Now for the questions: > >(1) How much, if any, of the energy will be radiated into free space when it gets to the end of the open transmission line? To me, this looks like a monopole antenna. I don't have a very good understanding of antenna theory, so this could very well be an invalid assumption. Of the cuff? Unknown. There's too many variables. Single wire antenna (monopole) embedded in FR-4 ... >(2) If radiation does take place as stated above in question (1), which is better for reducing the radiation, termination at the source or termination at the load of the transmission line, or does it matter? Termination at load end. >(3) If the characteristic impedance of the trace on the printed circuit card differs from the characteristic impedance of the trace on the backplane, how is this handled? Is a termination needed at each end in this case? I've seen small resistors put in series with the trace. Keep in mind that this "characteristic impedance" is not really a purely resistive thing. - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
RE: Transmission Line Theory
I don't think the source end of a transmission line has to be terminated
unless there is going to be a reflection. If there is a reflection, though,
impedance matching at the source should insure that there are only two
bounces along the line. In order to match impedance at the source end
either a series or parallel resister might be required, which could effect
circuit performance.
In any case, I tend to agree with Qu, that common mode radiation is probably
the most significant problem-especially if the circuit is connected to an
external cable.
Max Kelson
Peripherals Engineer
Evans & Sutherland
600 Komas Drive, Salt Lake City, UT 84158
http://www.es.com/ <http://www.es.com/>
Telephone: 801-588-7196 / Fax: 801-588-4531
E-Mail: mkel...@es.com <mailto:mkel...@es.com>
-Original Message-
From: Qu Pingyu [mailto:pin...@ime.org.sg]
Sent: Wednesday, May 12, 1999 10:55 PM
To: 'emc-p...@ieee.org'
Subject:FW: Transmission Line Theory
Hello, Group,
My first post was bounced back by the server. Don't know
what's wrong. Here
I send it again. I appologize if you already recieved the
first post.
Regards
Qu Pingyu
> -Original Message-
> From: Qu Pingyu
> Sent: Thursday, 13 May 1999 9:31
> To: emc-p...@ieee.org
> Subject: RE: Transmission Line Theory
>
> Hello:
>
> Regarding your question, you can refer to the paper by
C.R. Paul on IEEE
> Trans. EMC, Vol. 31, No.2, May 1989.
>
> As I understand, the radiation emission in this case is
due to the
> differential mode (DM) current and common mode (CM)
current. According to
> Paul, the contribution from common mode current is far
severe than that
> from the DM current. The reason is CM current on signal
trace and return
> path tends to enhance each other, while DM current tend to
cancel each
> other. In his paper, he gave the formular to calculate the
radiated
> electric field due to both CM and DM current.
>
> As to the termination at the end of the traces, I don't
have a thorough
> study myself. But I guess that at least one end of the
traces should be
> terminated with impedance which matches the characteristic
impedance of
> the trace. If both end are not matched, the transmission
line will act as
> an resonator and the radiation will be very strong for
those frequencies
> that satisfy the following fomular:
>
> L = n x lamda/4,
>
> Where n is an integer and lamda is the wavelength of
radiated field.
>
> Hope this helps.
>
> Regards
>
> Qu Pingyu
>
> -Original Message-
> From: Allen Tudor [SMTP:allen_tu...@pairgain.com]
> Sent: Thursday, 13 May 1999 5:31
> To: emc-p...@ieee.org
> Subject: Transmission Line Theory
>
>
> Greetings group,
>
> I am trying to draw a parallel between transmission line
theory and
> radiated emissions.
>
> From what I understand, a transmission line can be
terminated at the
> source or the load with an impedance that equals the
characteristic
> impedance of the transmission line itself.
>
> With this in mind, consider this scenario. A printed
circuit card drives
> a clock signal down a trace on a backplane. The length of
the backplane
> trace is long enough to be considered a transmission line.
The driver on
> the printed circuit card is located within ½ inch of the
edge connector
> (mating with the backplane) and is terminated with an
impedance equal to
> the characteristic impedance of the backplane trace.
However, the
> backplane trace is open ended (there is nothing connected
to the end of
> the trace). Transmission line theory says the signal
integrity will be
> maintained in this case.
RE: Transmission Line Theory
Your first assumption is incorrect: the transmission line only operates as a transmission line if it has a source at one end and a load at the other end. If the source, line and load are all the same impedance, maximum energy is transferred to the load. Assuming all impedances are matched, the load is near the source and some unknown length of line is also connected to the load, that line could act in a number of ways: depending on its length, the frequency of the applied signal and whether or not the line is open or shorted, it would act as a series or parallel LC circuit connected to the load. If this extra line is a single line and not paired with a return, it will likely operate as an antenna. If that line were the same impedance as the load, half the energy from the source would go to the "antenna" and half to the load... Mike Hopkins mhopk...@keytek.com > -Original Message- > From: Allen Tudor [SMTP:allen_tu...@pairgain.com] > Sent: Wednesday, May 12, 1999 5:31 PM > To: emc-p...@ieee.org > Subject: Transmission Line Theory > > > Greetings group, > > I am trying to draw a parallel between transmission line theory and > radiated emissions. > > From what I understand, a transmission line can be terminated at the > source or the load with an impedance that equals the characteristic > impedance of the transmission line itself. > > With this in mind, consider this scenario. A printed circuit card drives > a clock signal down a trace on a backplane. The length of the backplane > trace is long enough to be considered a transmission line. The driver on > the printed circuit card is located within ½ inch of the edge connector > (mating with the backplane) and is terminated with an impedance equal to > the characteristic impedance of the backplane trace. However, the > backplane trace is open ended (there is nothing connected to the end of > the trace). Transmission line theory says the signal integrity will be > maintained in this case. > > Now for the questions: > > (1) How much, if any, of the energy will be radiated into free space when > it gets to the end of the open transmission line? To me, this looks like > a monopole antenna. I don't have a very good understanding of antenna > theory, so this could very well be an invalid assumption. > > (2) If radiation does take place as stated above in question (1), which is > better for reducing the radiation, termination at the source or > termination at the load of the transmission line, or does it matter? > > (3) If the characteristic impedance of the trace on the printed circuit > card differs from the characteristic impedance of the trace on the > backplane, how is this handled? Is a termination needed at each end in > this case? > > I look forward to your responses. > > Thanks. > > Allen Tudor, Compliance Engineer > PairGain Technologies tel: (919)875-3382 > 2431-153 Spring Forest Rd. fax: (919)876-1817 > Raleigh, NC 27615 email: > allen_tu...@pairgain.com > > > > - > This message is coming from the emc-pstc discussion list. > To cancel your subscription, send mail to majord...@ieee.org > with the single line: "unsubscribe emc-pstc" (without the > quotes). For help, send mail to ed.pr...@cubic.com, > jim_bac...@monarch.com, ri...@sdd.hp.com, or > roger.volgst...@compaq.com (the list administrators). > - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
RE: Transmission Line Theory
Hi Allen, I suppose I can take a stab at this one. (1) How much, if any, of the energy will be radiated into free space when it gets to the end of the open transmission line? To me, this looks like a monopole antenna. I don't have a very good understanding of antenna theory, so this could very well be an invalid assumption. >>I am not really sure, maybe someone else can shed some light on this. (2) If radiation does take place as stated above in question (1), which is better for reducing the radiation, termination at the source or termination at the load of the transmission line, or does it matter? >> If you do get a lot of emission form this it will probably be due to standing waves on the line and I would say that a termination at the end of the line would be most useful to stop the occurrence of reflections (which will cause your standing wave). Ideally what you want is to have your source and load matched to the characteristic impedance of your line. (3) If the characteristic impedance of the trace on the printed circuit card differs from the characteristic impedance of the trace on the backplane, how is this handled? Is a termination needed at each end in this case? >> In this case I would not worry too much about the length of the trace from the driver to the edge connector (unless you are working with a several GHz signal). At this point if you are going to match something I think you would have to consider matching the PCB to the characteristic impedance of the backplane. Good luck! Jeff Bailey EMC Technologist SST Division of Woodhead Canada Phone: (519) 725 5136 ext. 363 Fax: (519) 725 1515 Email: jbai...@sstech.on.ca Web: www.sstech.on.ca All comments here are my own and do not necessarily express the views of SST. -Original Message- From: Allen Tudor [mailto:allen_tu...@pairgain.com] Sent: Wednesday, May 12, 1999 5:31 PM To: emc-p...@ieee.org Subject: Transmission Line Theory Greetings group, I am trying to draw a parallel between transmission line theory and radiated emissions. >From what I understand, a transmission line can be terminated at the source or the load with an impedance that equals the characteristic impedance of the transmission line itself. With this in mind, consider this scenario. A printed circuit card drives a clock signal down a trace on a backplane. The length of the backplane trace is long enough to be considered a transmission line. The driver on the printed circuit card is located within ½ inch of the edge connector (mating with the backplane) and is terminated with an impedance equal to the characteristic impedance of the backplane trace. However, the backplane trace is open ended (there is nothing connected to the end of the trace). Transmission line theory says the signal integrity will be maintained in this case. Now for the questions: (1) How much, if any, of the energy will be radiated into free space when it gets to the end of the open transmission line? To me, this looks like a monopole antenna. I don't have a very good understanding of antenna theory, so this could very well be an invalid assumption. (2) If radiation does take place as stated above in question (1), which is better for reducing the radiation, termination at the source or termination at the load of the transmission line, or does it matter? (3) If the characteristic impedance of the trace on the printed circuit card differs from the characteristic impedance of the trace on the backplane, how is this handled? Is a termination needed at each end in this case? I look forward to your responses. Thanks. Allen Tudor, Compliance Engineer PairGain Technologies tel: (919)875-3382 2431-153 Spring Forest Rd. fax: (919)876-1817 Raleigh, NC 27615 email: allen_tu...@pairgain.com - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators). - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
RE: Transmission Line Theory
Hello: Regarding your question, you can refer to the paper by C.R. Paul on IEEE Trans. EMC, Vol. 31, No.2, May 1989. As I understand, the radiation emission in this case is due to the differential mode (DM) current and common mode (CM) current. According to Paul, the contribution from common mode current is far severe than that from the DM current. The reason is CM current on signal trace and return path tends to enhance each other, while DM current tend to cancel each other. In his paper, he gave the formular to calculate the radiated electric field due to both CM and DM current. As to the termination at the end of the traces, I don't have a thorough study myself. But I guess that at least one end of the traces should be terminated with impedance which matches the characteristic impedance of the trace. If both end are not matched, the transmission line will act as an resonator and the radiation will be very strong for those frequencies that satisfy the following fomular: L=n x lamda/4, Where n is an integer and lamda is the wavelength of radiated field. Hope this helps. Regards Qu Pingyu > -Original Message- > From: Allen Tudor [SMTP:allen_tu...@pairgain.com] > Sent: Thursday, 13 May 1999 5:31 > To: emc-p...@ieee.org > Subject: Transmission Line Theory > > > Greetings group, > > I am trying to draw a parallel between transmission line theory and > radiated emissions. > > From what I understand, a transmission line can be terminated at the > source or the load with an impedance that equals the characteristic > impedance of the transmission line itself. > > With this in mind, consider this scenario. A printed circuit card drives > a clock signal down a trace on a backplane. The length of the backplane > trace is long enough to be considered a transmission line. The driver on > the printed circuit card is located within ½ inch of the edge connector > (mating with the backplane) and is terminated with an impedance equal to > the characteristic impedance of the backplane trace. However, the > backplane trace is open ended (there is nothing connected to the end of > the trace). Transmission line theory says the signal integrity will be > maintained in this case. > > Now for the questions: > > (1) How much, if any, of the energy will be radiated into free space when > it gets to the end of the open transmission line? To me, this looks like > a monopole antenna. I don't have a very good understanding of antenna > theory, so this could very well be an invalid assumption. > > (2) If radiation does take place as stated above in question (1), which is > better for reducing the radiation, termination at the source or > termination at the load of the transmission line, or does it matter? > > (3) If the characteristic impedance of the trace on the printed circuit > card differs from the characteristic impedance of the trace on the > backplane, how is this handled? Is a termination needed at each end in > this case? > > I look forward to your responses. > > Thanks. > > Allen Tudor, Compliance Engineer > PairGain Technologies tel: (919)875-3382 > 2431-153 Spring Forest Rd. fax: (919)876-1817 > Raleigh, NC 27615 email: > allen_tu...@pairgain.com - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
Re: Transmission Line Theory
Hi Allen: I'll take a shot at this one! (At one time I dealt with high- fidelity high-frequency signals where the effects of connectors could be observed in the waveforms.) The ideal transmission line is terminated at both ends with its characteristic impedance. In some circumstances, you can "cheat" by not providing a termination (i.e., open or short) at either the source end or the load end, depending on what you want to accomplish. If you terminate at the load end, then no standing waves and no reflections back to the source. So, the source can be any impedance without degrading the signal, and you get twice the voltage. If you terminate at the source end, then you get a standing wave, and reflections. The reflections are terminated in the source impedance and don't distort the pulse at the source. I presume this is your situation -- terminating at the source. I'm afraid I have no idea of what is radiated from the transmission line, but I would guess that it would be a lot because of the standing wave. If you change impedance, you get standing waves or reflections, but not as much amplitude as a short or open. There are impedance-matching attenuators which act as a termination for one transmission line and as a source impedance for the other transmission line. Lossy. There is leakage from transmission lines. I would expect rather high leakage from a PWB transmission line (compared to coax). As one colleague once stated, the whole world is a transmission line. In fact, he calculated the characteristic impedance between the earth and the moon! 377 ohms??? Best regards, Rich - Richard Nute Product Safety Engineer Hewlett-Packard Company Product Regulations Group AiO Division Tel : +1 619 655 3329 Effective 6/12/99: +1 858 655 3329 16399 West Bernardo Drive FAX : +1 619 655 4979 Effective 6/12/99: +1 858 655 4979 San Diego, California 92127 e-mail: ri...@sdd.hp.com - - This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
