In a feedthrough encircling coil eddy current system, the test frequency is controlled by the:

In a feedthrough encircling coil eddy current system, the test frequency is primarily controlled by the oscillator. The oscillator generates an alternating current at a specific frequency, which is necessary for creating the changing magnetic fields that induce eddy currents in the conductive materials being tested. These induced currents are then detected and evaluated, making the control of frequency vital for the effectiveness of the testing method.

Understanding the role of the oscillator is crucial because the frequency of the generated signal directly impacts the penetration depth and sensitivity of the eddy current testing. Different materials and testing conditions may require specific frequencies to optimize the detection of flaws or characteristics within the material.

In contrast, the other options serve different functions. The oscilloscope is used to visualize the signal but does not control its frequency. The sensitivity setting adjusts the response of the system to detected signals rather than defining the frequency. The modulation analysis setting relates to the techniques for processing the received signals but does not determine the operating frequency. Thus, the oscillator is the key component that directly defines the test frequency in an eddy current testing system.