What component is represented by the blank box marked "X" in the Line-Driver/Coax/Line-receiver circuit?

The correct choice for the blank box "X" in the Line-Driver/Coax/Line-receiver circuit is a 51-ohm resistor. In the context of transmission lines and matching impedances, particularly in RF applications, it is common to match the characteristic impedance of the coaxial cable used in the circuit, which is typically 50 ohms or 75 ohms.

The 51-ohm resistor serves as a termination resistor that minimizes signal reflection in the line by providing a close impedance match to the transmission line. This matching is crucial for ensuring that the maximum amount of signal power is transferred from the line driver to the line receiver without significant loss or distortion caused by reflections.

When the correct value resistor, like the 51-ohm, is used, it helps in maintaining signal integrity and optimizing performance in high-frequency applications. Therefore, selecting a termination resistor that effectively matches the impedance of both the line and the equipment helps to ensure the overall reliability and efficiency of the entire system.