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).
