5.7 Measuring the Performance of a Net
When a trace configuration is open to question it may be necessary to construct the net and measure its actual performance. As the rise times get smaller, the difficulty in making useful measurements increases. The rise time of the measuring hardware becomes an issue and also the techniques used by the operator. An oscilloscope probe is really a field measuring device. Where the probe ground is connected makes a big difference. The loop areas formed by the probe at the point of measure must be kept very small. It is necessary to verify that no signal is sensed when the probe is connected to the probe ground at the point where it connects to the board. If there is signal then the loop formed at the tip may be excessive. It is also possible that current in the probe shield may be the source of error. If there is direct probe coupling then a better probe may be necessary.
It is a good idea to test a net without its terminating load (terminating gate). Clamping action obscures overshoot on any arriving signal. When the gate is removed, a small capacitor may have to be used as a dummy termination. In high speed circuits, the loop area formed by this termination is important. If the connection is too long, it amounts to a stub on the transmission line. Any terminating capacitor should be a surface mounted component to keep the loop area as small as possible.
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