Epicap Tuning Diodes in RF Electronics

The Epicap Tuning Diode (also called the voltage-variable capacitance diode, varicap diode, tuning diode, and varactor diode) is widely used in television receivers, radio receivers, and other communications equipment because it can be used for electronic tuning.

The Epicap Diode has a variable capacitance as the capacitance value can be controlled by adjusting its bias voltage. 

It is in fact a type of PN diode usually used as a  'tuning capacitor' in radio circuits.

The Epicap Diode is connected in reverse bias because when a reverse bias voltage is applied, the depletion zone in the P-N junction varies depending on the magnitude of the voltage (see an application example below - click image to enlarge). 

These diodes are available in a wide range of capacitance values from nearly 22pF to 33pF.

And, low capacitance diodes for VHF and UHF circuits are still reasonably easy to find.

The SVC201SPA from Sanyo is a quite popular quondam Epicap/Varactor Diode.

This Diffused Junction Type Silicon Diode is intended for use in FM receiver electronic tuning applications.

Next one is the MV209 Silicon Epicap Diode which is designed for general frequency control and tuning applications to provide solid−state reliability in replacement of mechanical tuning methods.

See its Diode Capacitance Curve below.

In a nutshell, an Epicap Tuning Diode operates using a reverse bias voltage applied to the P-N junction of the diode. 

As this reverse bias voltage increases, the depletion layer at the P-N diode junction widens.

Thus the resultant is a device which can be used as an 'electronic' variable capacitor or tuning capacitor.

Finally, to the practical project of a Miniature FM Transmitter ⇲

It consists of a simple oscillator with silicon planar RF PNP transistor, and a Varicap Diode is used instead of the conventional tuning capacitor.

You will see the full project details here → https://danyk.cz/sten3_en.html . Have fun!