Grid dip oscillator

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"Dipmeter" could also refer to an influential early commercial expert system called Dipmeter Advisor.

Grid dip oscillator (GDO), also called grid dip meter, dip meter, dipmeter, or just dipper, is a measuring instrument to measure resonant frequency of radio frequency circuits. It measures the amount of absorption of high frequency electromagnetic field by nearby objects. It is an oscillator whose output energy changes in the vicinity of a circuit which is in resonance with the frequency the oscillator generates; somewhat similar to an acoustic tone becoming louder when generated in the vicinity of a resonant cavity or a string tuned to the same frequency.

As an alternative, an absorption wavemeter can be used.

The first GDOs were based on vacuum tubes, therefore the grid, as the measured value was the tube's grid current; their use dates back to 1920's. The more modern GDOs are solid-state and more versatile, and are sometimes called gate dip oscillators or emitter dip oscillators.

GDOs were widely used by amateur radio operators for many years, for measuring the properties of resonant circuits, filters, and antennas.

A GDO is a tunable LC circuit with a coil that serves as a loose coupling to the measured LC resonant circuit. Resonance is indicated by the dip of the GDO output. The output is usually indicated by a microammeter.

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