User:Elcap/Playground

Variable capacitor
A variable capacitor is a capacitor whose capacitance may be intentionally adjustable mechanically or electronically within defined limits. They are predominantly used in filters and oscillators for the tuning of transmitters or receivers, as well as for impedance matching. Due to the possibility of adjustability variable capacitors can replace a number of fixed capacitors which are needed in circuits for different frequencies. There are mechanically and electrically variable capacitors. The mechanical-variable capacitors belong to the passive components and are distinguished in Most types of mechanical-variable capacitors have only a historical significance. Nowadays (2017) still required designs of mechanical types are SMD trimmers in circuits with very low capacitances, multiturn trimmers for very high frequencies and variable vacuum capacitors for devices with higher power demand, for example in  MRI scanners.
 * tuning capacitors, which are designed for the manual adjustment in radio receivers for frequent and repetitive operations and
 * trimmer capacitors designed for one-time or rare operations for fine tuning.

The quite large and expensive mechanical variable capacitors meanwhile mostly have been replaced by electrically variable capacitors, also called varactors, with adjustable capacitance values which are generated by electrical control. These capacitors belong to the active components and exploit the properties of semiconductor technology to achieve variable capacitance.

The following types belong to electrically variable capacitors :
 * Varicaps
 * Capacitors with tunable dielectrics like BST varactors   ,


 * Digitally tunable capacitors (DTC) and


 * Electrically tunable RF MEMS

The special features of the semiconductor technology influences the parameters of these electrically variable capacitors strongly. For example the significantly smaller dimensions results in lower capacitance values, however that makes these capacitors more usable for higher frequencies up to a few hundred GHz. They are used in all modern stationary and mobile receivers in filters for frequency selection for all commercially and industrially used channels.

Mechanical-variable capacitors
The mechanical-variable capacitors are distinguished in Mechanical-variable capacitors are constructed as "plate capacitors" which capacitance formula is:
 * tuning capacitors, which are designed for frequent and repetitive operations in LC circuits (impedance matching) or tuning networks, for the manual adjustment in radio receivers or the automatic motor-controlled frequency adjustment in transmitters, and
 * trimmer capacitors or short "trimmer", which are intended for one fine tuning adjustment of the intermediate frequency (IF), and radio frequency (RF) circuits at the initially calibrating of radio and television receivers after manufacturing or in frequent and seldom adjustment of oscillating circuits. Trimmers are connected in parallel to the main tuning capacitor in a radio frequency network, although an alternative form, known as a padder, can be connected in series.
 * $$C = \varepsilon \cdot \frac{A}{d}$$

The capacitance C increases with the area $$A$$ of the plates and with the permittivity $$\varepsilon$$ of the dielectric material and decreases with the plate separation distance $$d$$. The capacitance is therefore greatest in devices made from materials with a high permittivity, large plate area, and small distance between plates. The capacitance value of a capacitor can be mechanically influenced in two different ways, by
 * changing the capacitive effective electrode area,
 * changing the electrode distance to each other, and
 * choosing of a suitable dielectric material.

Tuning capacitors


In the early radios the multiple tuned circuits required multiple knobs to be adjusted to tune in a new station. One of the most important ease-of-use innovations was "single knob tuning", achieved by linking the tuning capacitors together mechanically.