In high-speed digital systems, simple passive circuit elements might significantly affect signal quality. These elements include, for example, wires, cables, and chip PCB interconnections. High-speed digital edges contain frequency components that are several times the effective toggle rate of that signal. For example, a digital edge with a rise time of 1.5 ns contains significant energy in frequencies up to 333 MHz, regardless of toggle rate.

When designing systems using digital waveform generators and analyzers, you must have a basic understanding of both transmission lines and termination so that you can maximize signal quality and minimize the effects of signal reflections.

The signals in the following figures show identical digital waveforms generated by an PXI-6552. The first figure shows a properly terminated waveform. In this case, the test system is designed by considering the effects of transmission lines and termination. The second figure shows a waveform from an unterminated system. In this case, the test system is designed without considering the effects of transmission lines and termination.

When designing your test system, you must control the following key areas:

  • Zs—The impedance at the source of the transmission line
  • Z0—The characteristic AC impedance of the transmission line
  • Zt—The impedance at the destination of the transmission line

If you do not control these elements, the signal becomes distorted. This results in the following issues:

  • Signals that exceed specified high-level and low-level thresholds (overshoot and undershoot)
  • Signals that have false edges (ringing)
  • Signals that have reduced operating margins (degraded eye diagram caused by inter-symbol interference)
  • Potential physical damage or overheating of driver/receiver components in extreme cases

Refer to Terminating Your Module for information about series and parallel resistor termination that is commonly applicable to your digital waveform generator/analyzer.