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).
