Posted on May 22, 2012

The circuit is based on the grid-dip or absorption effect, which occurs when a parallel resonant circuit is coupled to an oscillator of tbe same frequency. Ql operates in a conventional Colpitts oscillator circuit at a fixed frequency of approximately 4 MHz. A meter connected in series with tbe transistor`s base-bias resistor serves as the dip or absorption indicator. The variable measuring circuit consists of Cl, C2, and 12 and is connected to panel terminals as shown. 12 is loosely coupled to 11 in the oscillator circuit.

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This measuring circuit is tuned to the oscillator frequency with variable capacitor C2 set at full capacitance. When power is applied to the oscillator, tbe 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 tbe measuring circuit. To restore resonance, tune capacitor C2 lower in capacitance. The meter will dip again when you reach this point. Determine tbe capacitance across tbe text terminals by calibrating tbe dial settings of C2. Capacitor C4, a small variable trirruner in tbe 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 tbe oscillator exactly at tbe dip point when C2 is set at maximum capacitance. This corresponds to zero on the calibration scale.

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