ABOVE BOARD
Winter 1998

Trace Routing

There are numerous techniques for controlling higher order harmonics caused by fast rise times. Here UltraCad will focus on some of the more fundamental ones that every designer should know about.

Antennas:

Dead end stubs: Never allow a stub trace to exist without a terminating point. Such a  stub trace is an antenna and its uncontrolled impedance can cause signal reflections whose results will be absolutely unpredictable (but those results will never be positive).

Adding trace delay times: It is not uncommon to adjust the timing delay on a trace by adding trace length. Care must be taken in how the excess trace length is routed. An innocent looking trace that extends beyond a pad and then doubles back to it can look just like an antenna and radiate badly from that point. A trace that snakes back and forth can look just like a dish antenna.

Right angle turns and "T"s: A trace that extends in a straight line is relatively clean. One that extends straight and then turns 180 degrees back on itself looks just like an antenna (like those on a tall building). A line that makes a right angle turn begins to look like, and have the characteristics of, an antenna. It is admittedly not a real good antenna. The point is that we don't want even poor antennas on the board! If you probe a board with an  EMI  detector, the strongest radiating points will almost invariably be at 90 degree corners and "T"s. A board should never have signal  lines that turn more than 45 degrees and all trace corners should be  mitered .

Remember: Antennas work both ways. If a stub or corner emits well, it also receives well. So these are the points where noise can be injected into the board also.
 

Signal Return  Paths

Every signal has a return path. So an interesting question for each trace is "Where is its return path?" The higher the frequency (including the higher order harmonics) the closer the return path will be to the signal trace. So it is wise to make provision for it!

Power/Ground Planes: The best provision is a power distribution plane directly under the signal. Studies have shown that if there is a ground plane under the trace, for very high frequencies, the return signal is directly under the trace. Note the implication - anything that breaks the continuity of the ground plane under the trace will cause the return signal to deviate around the interruption. It will return under the trace as soon as possible. The path the return signal takes could look just like an antenna! Thus an otherwise seemingly careful design, one that seems to take everything into consideration, might inadvertently inject an antenna affect just where you would least expect it... on the ground plane itself.

Guard Banding: The next best provision is sometimes called a  guard band. A guard band is a parallel trace immediately beside the signal that is tied to the ground plane. Care should be taken to make it  as nearly as possible the same length. Different designers like to tie the plane to ground differently. Some tie it only at each end, some stitch it to the ground along the trace. A few will tie it to ground at only one end ... a practice we do not recommend since it defeats the purpose of providing a return signal path.

Ground Loops: Beware of the possibility of multiple signal return paths that can set up the possibilty of ground loops. Ground loops can be especially destructive in absorbing power and radiating stray signals into other traces and FCC compliance testing equipment!

Note that if you make no provision for the return path, the signal will return by some way anyway. If it is uncontrolled, you have no idea where it is going, how it is radiating, and what other signals it is interfering or combining with.

This series is contributed by UltraCad design Tel: (206) 450-9708