3.7 Notes on Radiation

Radiation and cross coupling occurs where there are changing field patterns. When the rise time is 1 ns, a wave can travel 20 cm during this time. For many circuit boards, logic lines are in transition over their entire length. In the section on cross talk, very short rise times are used to make it easier to describe the phenomena.

For outer traces (microstrip), most of the field is in the dielectric and a small part of the field is in air. The portion in air travels faster. This has the effect of smearing the leading edge and changing the nature of the cross talk.

When transmission lines (traces) are terminated, the energy carried on that line is dissipated in the termination. Cross coupling and radiation stops when wave action stops. For series (source) termination, the outgoing wave is 50% of the supply voltage. The radiation level is half the case where the line is parallel terminated. This radiation lasts for the round trip time of the wave.

For short transmission lines, terminating resistors may not be necessary. The energy stored on this line is then dissipated in the line and switch resistances. Again, the need for a terminating resistor is controlled by rise time and line length. Figure 3.5 shows the wave forms that take place on an unterminated transmission line with different rise times. This figure can be scaled. If the rise time is doubled the line line length is doubled.

Figure 3.5 Rise time and the reflections on an unterminated transmission line.

The Rise-Time Rule. If the length of the transmission line is less than one-third the distance a wave travels in one rise time, then terminating the line in its characteristic impedance is not necessary. For a rise time of 100 ps, the rise-time distance is 1.5 cm.

One-third this distance is 0.5 cm. This means that terminations are needed for gigahertz clock rate logic. Series terminations are preferred. This same rule applies in the case of stub lengths.