LC_CHECKER

Published:2009/7/7 23:03:00 Author:May | From:SeekIC

The circuit is based on the grid-dip or absorption effect, which occurs when a parallel resonant circuit is coupled to an oscillator of the same frequency. Q1 operates in a conventional Colpitts oscillator circuit at a ftxed frequency of approximately 4 MHz. A meter connected in series with the transistor's base-bias resistor serves as the dip or absorption indicator.The variable measuring circuit consists of C1, C2, and L2 and is connected to panel terminals as shown. L2 is loosely coupled to L1 in the oscillator circuit. This measuring circuit is tuned to the oscillator frequency with variable capacitor C2 set at full capacitance. When power is applied to the oscillator, the meter shows a dip caused by power absorption from the measuring circuit.Connecting an unknown capacitor across the test terminals lowers the resonant frequency of the measuring circuit. To restore resonance, tune capacitor C2 lower in capacitance. The meter will dip again when you reach this point. Determine the capacitance across the text terminals by calibrating the dial settings of C2.Capacitor C4, a small variable trimmer in the oscillator circuit, compensates for drift or other variations and is normally set at half capacitance. The capacitor is a panel control, labeled zero, and it is used to set the oscillator exactly at the dip point when C2 is set at maximum capacitance. This corresponds to zero on the calibration scale.



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