What is a Tuned Capacitance Box | How to Construct a Tuned Capacitance Box

A tuned capacitance box is an electronic tool, which helps to find out small capacitance values in a circuit. This is an important tool in radio frequency electronics where capacitance values are very small, which are in the picofarad range. This instrument is different from a capacitance decade box in that it is variable and it measures smaller capacitance values. This capacitance box helps in finding out the ideal capacitance value in place of an actual capacitor in a circuit. The physical isolation of the variable capacitor in the box helps to prevent introduction of hand capacitance to the circuit. The connecting leads to the circuit are shortened so as not to introduce additional inductance or load to the circuit under test.

Capacitance Box Tuning an LC Tank Circuit

Construction and Working of Tuned Capacitance Box
The device consists of a variable capacitor which is a 2J gang capacitor; the two sections of it are connected in parallel. The variable capacitor measures between 5 PF to 500 PF. It is housed inside a box so that the shaft can be manually rotated from outside the box. Two screw terminals are taken out from the leads of the variable capacitor through which the circuit to be tested is connected. One end of the connecting wire is attached to the connector by means of screws and the other end of the wire is connected to the circuit under test. The variable capacitor is rotated to find out the ideal value of capacitance in that circuit. This specific capacitance that was found out, is measured using an LC meter and an appropriate fixed value capacitor of high Q is replaced in the circuit.

Completed Capacitance Box

 

Capacitance Box Tuning to the Resonant Capacitance Value

 

Measuring the Capacitance Value With an LCR Meter

Uses of a Capacitance Box

1. The capacitance box helps to measure small capacitance values, particularly in an RF circuit.
2. It helps to find out the capacitance value of a tuned circuit or tank circuit in an oscillator, RF bandpass filter, IF tuned amplifier, antenna input tuned circuits etc.of a radio receiver.
3. It helps to find out the capacitance value of a tuned circuit in an oscillator, bandpass filter, tuned amplifier stages, RF output filter, antenna tuner etc. of a transmitter.

 Watch the Video: https://youtu.be/yRbFg5q31Qw