EMC Question of the Week: November 10, 2025
A digital clock source (5 Mbps, 2 Ω) provides a signal to three devices on the same printed circuit board. Each of the three device inputs is a 5-pF capacitance and is connected to the source with a 10-cm microstrip trace (∼10 pF of trace capacitance). Which of the components listed below should be added to this circuit?
- 100-Ω series resistor
- 3 matched terminations
- both of the above
- nothing
Answer
The best answer is “a.” Without any added components, the transition time (2.2RC) will be approximately 200 picoseconds. This is much less than the round-trip propagation delay (∼1.4 ns) and will enable ringing in the received clock signals. With a 100-Ω resistor in series with the clock driver, the transition time is increased to ∼10 ns. This is slow enough to prevent strong reflections and ringing, but fast enough compared to the 200-ns period of the clock signal to ensure good signal integrity.
Three matched terminations would also eliminate reflections, but that approach would significantly increase the power dissipation. Using controlled-impedance traces would also increase the cost of the board and take up more space.
Finally, note that "both of the above" would significantly reduce the amplitude of the clock signal.
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