HOW DO TUNING CAPACITORS WORK?

The term “Capacitive Tuning” generally relates to the use of variable capacitors in tuning circuits or capacitive touch systems. These capacitors are of low loss types, and different ratings, depending on application.

1. In tuning circuits (such as radio receivers), capacitors are used to select or “tune” a particular frequency from a range of frequencies. By adjusting the capacitance, the resonant frequency of the circuit changes, allowing the circuit to resonate at a desired frequency. This enables selection of a specific signal, like tuning a radio to a particular station. Typically, this involves an inductor-capacitor (LC) resonant circuit where the capacitor is often variable to enable tuning flexibility.
2. In capacitive touch applications (such as touchpads and buttons in electronics), tuning involves adjusting capacitive values to optimize sensor performance based on environmental conditions and hardware layout. This tuning process is done using software and hardware tools to measure capacitive changes in touched and untouched states, ensuring accurate and responsive touch sensing.

Radio Tuning with Variable Capacitor

Capacitance of a variable capacitor can be intentionally and repeatedly changed mechanically or electronically. It is commonly used in LC (inductor-capacitor) circuits to set resonance frequency, such as tuning a radio. Hence it is sometimes called a tuning capacitor or tuning condenser.

In radio tuning, a variable capacitor typically consists of a group of semicircular plates arranged on a rotary axis (rotor) positioned between a set of stationary plates (stator). Rotating the rotor changes the overlapping area between plates, thereby varying the capacitance. This variation in capacitance adjusts the resonant frequency of circuit, thereby tuning a radio to different stations.

• The rotor is rotated to change the capacitance, with maximum capacitance when plates are meshed (interlaced) and minimum when unmeshed.
• Reduction gears often allow fine tuning over a larger rotational angle.
• Dielectric materials can be air or plastic foils.
• Vacuum variable capacitors are used for high power and high voltage applications, sealed under vacuum to reduce arcing.
• Smaller, fixed tuning capacitors called trimmers can be adjusted once for precise calibration.

As a tuning device in radios, variable capacitors adjust the frequency of the LC circuit to select the desired radio frequency for reception. This mechanism was invented by Dezső Korda, Hungary in 1893 and remains a fundamental component in classic analog radio tuning despite digital synthesizer advances today.

A variable capacitor used in radio tuning circuits works by changing the capacitance in an LC (inductor-capacitor) circuit to set the resonant frequency, enabling selection of a desired radio frequency station.

Above is a simple crystal radio circuit, which works without a battery or power supply, It draws power from the RF signals it gets at its antenna. The earphone is very low power, and one may add amplifier to listen on a speaker. This basic tuning principle is used in radio receivers, TV sets, and other devices requiring frequency selection.

Variable capacitors in radios often have a dial or knob that manually rotates the plates for tuning, changing capacitance typically in the picofarad (pF) range. Gang capacitors with multiple linked sections are used to tune multiple stages simultaneously.

FM Band Radio Tuning Capacitor Ranges

For tuning the FM radio band from 88 MHz to 108 MHz with a typical inductor of 100 nH, the required capacitance variation range is generally from 21.7 pF for 108 MHz to about 21.7 pF for 88 MHz This range allows the LC circuit to resonate across the entire FM band (88–108 MHz) Adjusting the capacitor within this range changes the resonant frequency for tuning different stations in the FM spectrum.

Varicap Diode for Tuning

Varicap diode, (or Varactor diode / Tuning diode) is a special reverse biased diode whose capacitance changes with applied voltage. The depletion region of a diode changes with applied voltage. The depletion region gap of a diode changes with applied voltage. This thickness acts as a dielectric between two sides of reverse biased diode. Thus, the capacitance varies with voltage. This is very useful in electronic tuning of a radio of TV.

Varicap diode, when connected in an LC tank circuit, can be used for tuning by varying the voltage across varicap diode.

Benefits of Varicap Diodes

• Compact size and high reliability vs. mechanical variable capacitors.
• Electronic tuning capability enabling remote or automatic control.
• Smooth and continuous tuning over a wide capacitance range.
• Commonly used in modern FM radios, TV tuners, and communication devices.