EMC Question of the Week: July 2, 2018
Which two parameters should be compared in order to determine whether it is necessary to provide a matched termination for a digital signal?
- wavelength and path length
- wavelength and conductor separation
- propagation delay and bit width
- propagation delay and transition time
Answer
The correct answer is "d." If the propagation time is long relative to the transition time, the transition occurs before the source can see what the load impedance is. The source initially provides a voltage and current consistent with the characteristic impedance of the transmission line rather than the load impedance. This can result in ringing as reflections between the source and load are needed to settle on steady state values for the voltage and current. Typically matching is needed when the transition time is less than 2 - 5 times the propagation delay. Most circuits operating at speeds below 100 Mbps on a single printed circuit board do not require matching if the transition times and trace lengths are controlled.
For analog or RF circuits with a well-defined frequency (and hence a well-defined wavelength), the first choice would be correct. Digital signals do not have a specific wavelength. Nevertheless, some design guidelines in the open literature relate a "minimum wavelength of interest" to the transition time and velocity of propagation; thereby effectively performing the comparison in choice "d".
Have a comment or question regarding this solution? We'd like to hear from you. Email us at