EMC Question of the Week: January 27, 2025
A 5-Ω digital signal source drives a 10-nF load through a lossless, 1-m cable with a propagation delay of 5.6 ns. The characteristic impedance of the cable is 50 Ω. What is the transition time of the received signal?
- 5 ns
- 11 ns
- 110 ns
- 1.2 μs
Answer
The best answer is “c.” The transition time is calculated as 2.2RC or 2.2 (5 Ω)(10 nF) = 110 ns. The transmission line is electrically short because the transition time of the signal (110 ns) is much greater than the propagation delay (5.6 ns). Even if the source voltage transitions instantaneously, the effect of the reflections on the voltage at the load end is negligible.
Note also, that the total capacitance of the transmission line is 1.1 pF/cm x 100 cm = 110 pF. Compared to the load capacitance of 10 nF, this is also negligible. In this situation, the cable has very little effect on the signal.
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