In a RADAR unit, if the crystal mixer is defective, what should be replaced?

The correct answer focuses on the replacement of components associated with the crystal mixer in a RADAR unit. The crystal mixer is a critical part of the RADAR's signal processing, as it combines or "mixes" the received signals with the transmitted signals to produce intermediate frequencies that can be processed further.

When the crystal mixer is defective, it often implicates not just the crystal itself but also the associated transceiver units, notably the TR (Transmitter-Receiver) tube. The TR tube is responsible for handling the switching between transmitting and receiving modes in the RADAR system. If the mixer is malfunctioning, it may indicate that the signals are not being managed correctly, which could damage or lead to improper functioning of the TR tube as well.

Therefore, replacing both the crystal and the TR tube ensures that the entire signal path associated with the mixer is functioning correctly after the defect. Replacing only the crystal might not resolve the underlying issue if the TR tube has also been affected.

Other answers involve components that aren't as directly related to the crystal mixer in terms of functionality. For instance, the ATR tube, used in some applications for amplification of signals, might not be needed if the mixer is the main concern. Similarly, the klystron,