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: [email protected] <mailto:[email protected]> -----Original Message----- From: Qu Pingyu [mailto:[email protected]] Sent: Wednesday, May 12, 1999 10:55 PM To: '[email protected]' 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: [email protected] > 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:[email protected]] > Sent: Thursday, 13 May 1999 5:31 > To: [email protected] > 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: > [email protected] --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators). --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to [email protected] with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to [email protected], [email protected], [email protected], or [email protected] (the list administrators).
