Issue link: https://iconnect007.uberflip.com/i/1467185
26 DESIGN007 MAGAZINE I MAY 2022 Figure 5 (le) illustrates the ringing (red) in an unmatched transmission line. is ringing, which also causes electromagnetic radiation (right), is dramatically reduced by terminat- ing the transmission line with an 18.7 Ω series resistor. Crosstalk Another culprit is crosstalk, particularly on long parallel trace segments. Crosstalk arises as a result of the unintentional coupling of electromagnetic fields and causes both for- ward and reverse reflections. e easiest way to reduce crosstalk from a nearby aggressor signal, of course, is by increasing the spac- ing between the signals in question. Crosstalk falls off very rapidly with distance. Cross- talk plummets roughly quadratically with increased separation. Doubling the spacing cuts the crosstalk to roughly a quarter of its original level. A good rule of thumb for this is Gap = 3 x trace width. However, in today's complex designs it is not always possible to use up valuable real estate to satisfy the above. Reducing the signal trace to reference plane dielectric thickness can also reduce crosstalk while not requiring additional space. Also, different technologies should not be mixed as higher voltages create higher crosstalk. And long parallel trace segments should be avoided. Crosstalk also depends on the load which may vary considerably when driving banks of memory modules, for example. Keep in mind that the total crosstalk on each victim trace is the total crosstalk from each of several nearby aggressors, all of which sum to produce the maximum value. Plane Cavity Resonance Plane pairs, in multilayer PCBs, are essen- tially unterminated transmission lines—just not the usual traces or cables we may be accus- tomed to. ey also provide a very low imped- ance path, which means that they can present logic devices with a stable reference voltage at high frequencies. But, as with signal traces, if the transmission line is mismatched or unter- minated, there will be standing waves—ring- ing. e bigger the mismatch, the bigger the standing waves and the more the impedance will be location-dependent. When the cavity has open end boundary conditions, resonances arise when a multiple of half wavelengths can fit between the ends of the cavity. Figure 6 shows the cavity reso- nance of a plane pair with a resonant frequency of 1 GHz. If the signal clock frequency (or har- Figure 5: Ringing and radiation are reduced dramatically by adding a series terminator.