Principle of Operation - AM System
The probe is a triaxial device with the centre wire being connected to the probe tip. The guard surrounds the centre wire and probe tip, and the outer screen surrounds the guard. The locally mounted oscillator drives the guard at 16kHz at constant amplitude. This induces an identical sinusoidal oscillation on the centre wire. As soon as a target comes into range of the probe tip the additional capacitive couple between the probe tip and target modifies the electric field. This causes a reduction of the amplitude of oscillation of the induced voltage on the centre wire.
Output from the oscillator is a reference oscillation which, when compared with the signal oscillation from the centre wire, gives a difference proportional to the capacitance at the probe tip.
The oscillator also contains a differential amplifier into which both reference and signal oscillations are fed. Output of the differential amplifier is a 16kHz sine wave, the amplitude of which is the difference between the amplitude of the reference and signal oscillation. This signal is fed down a 50 Ohm coaxial cable to a receiver with the oscillator containing line drive amplifiers to enable 490 feet (150 metres) of cable to be driven.
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