The Need for Properly Understanding Signal Types and Terminations

Printed circuit board traces behave like transmission lines that can filter a clock signal, attenuating and distorting the clock signal as it moves along the length of the trace. Higher frequency clock signals are more susceptible to attenuation, distortion, and noise. However, the preferred method to improve jitter clock edges is with higher slew rates, which creates challenges to implementing a clock solution.

To correctly implement a high quality clock source the following should be considered:

• Isolate clock sources from each other
• Use proper grounding and power supply filtering
• Use short PCB traces for clock signals
• Place the device to be clocked as close to the clock as possible
• Ensure that the correct clock output type has been selected for the application
• Ensure that output drivers are terminated correctly and impedance matching techniques are employed

The last two points are the subject of this application note. Reflections and attenuation occur when the traces are not properly terminated. Reflections will increase jitter, while attenuation can further degrade the clock waveform and overall performance. Maintaining signal integrity is paramount in realizing the performance of a low phase noise oscillator.