Variable capacitor

By choosing the shape of the rotary plates, various functions of capacitance vs. angle can be created, e.g. to obtain a linear frequency scale.

Various forms of reduction gear mechanisms are often used to achieve finer tuning control, i.e. to spread the variation of capacity over a larger angle, often several turns.

The movable part (plunger) is mounted on a flexible metal membrane that seals and maintains the vacuum.

The vacuum not only increases the working voltage and current handling capacity of the capacitor, it also greatly reduces the chance of arcing across the plates.

This has practical advantages for makeshift or home construction, and may be found in resonant-loop antennas or crystal radios.

The closely related split stator variable capacitor does not have the limitation of 90° angle since it uses two separate packs of rotor electrodes arranged axially behind one another.

Any diode exhibits this effect (including p/n junctions in transistors), but devices specifically sold as variable capacitance diodes (also called varactors or varicaps) are designed with a large junction area and a doping profile specifically designed to maximize capacitance.

They have a dedicated analogue control input and therefore introduce less non-linearities than varactor diodes, especially for higher signal voltages.

MEMS, BST and SOI/SOS devices are available from a number of suppliers and vary in capacitance range, quality factor and resolution for different RF tuning applications.

The capacitance values are designed for antenna impedance matching in multi-band LTE GSM/WCDMA cellular handsets and mobile TV receivers that operate over wide frequency ranges, such as the European DVB-H and Japanese ISDB-T mobile TV systems.