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When the junction diode
is reverse biased, the
insulating barrier
widens.
The higher the reverse
voltage the wider the
barrier becomes.
The barrier forms the
dielectric, of variable
width, of a capacitor.
The N and P type cathode
and anode are the two
plates of the capacitor.
In the diagram, the
diode and coil form a
resonant circuit.
The capacitance of the
diode, and thereby the
resonant frequency, is
varied by means of the
potentiometer
controlling the reverse
voltage across the
varicap.
The capacitor prevents
the coil shorting out
the voltage across the
potentiometer.
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