EMC Question of the Week: February 2, 2026
A digital clock driver transmits a signal on a 50-Ω coaxial cable to a 50-Ω load. What value of source resistance maximizes the power delivered to the load?
- 0 Ω
- 50 Ω
- 100 Ω
- depends on cable length
Answer
The best answer is “a.” A zero-ohm source provides the full open-circuit voltage to the load. Any other value provides less voltage (and therefore less power) to the load.
On the other hand, when we're trying to maximize the power delivered to a load from a given source, we want to match the load impedance to the source impedance. So, for example, if the source impedance had been 20 Ω, we would want a 20-Ω transmission line and a 20-Ω load in order to maximize the power delivered.
Note that there is no reflection of power (no standing wave) when the transmission line is matched at the load end. It is not necessary to match the source to the transmission line when the load is matched and there is only one source.
Have a comment or question regarding this solution? We'd like to hear from you. Email us at