What are RF Capacitors?
RF capacitors are capacitors that are optimized for high-frequency performance and have characteristics such as very low equivalent series resistance (ESR) and a high-quality factor (Q). They ensure minimal energy loss, stable capacitance, and efficient operation in high-frequency environments. Regular capacitors may perform poorly at high frequencies due to parasitic inductance and resistance. RF capacitors are suitable for use in circuits of RF devices like filters, oscillators, and impedance-matching networks.
Working of an RF Capacitor
Like regular capacitors, RF capacitor consists of two conductive plates called electrodes separated by a non-conductive material called a dielectric. When voltage is applied across the plates, an electric field builds up between them. This electric field stores electrical energy and this process is called charging. When the voltage is removed, the electric field collapses, and the stored energy is released back into the circuit. This is called discharging.
The dielectric acts as the insulating material between the two electrodes. It increases the capacitor's ability to store electrical energy by allowing more charge to be stored on the plates for a given voltage. Hence, the material chosen for the dielectric can affect the overall performance of the capacitor in many ways. The dielectric constant of the material determines how much charge the capacitor can hold. A higher dielectric constant means higher capacitance. In RF capacitors, the dielectric material is chosen to have low dielectric losses at high frequencies. This ensures the stored energy is released efficiently back into the circuit.
The capacitance (C) of a capacitor is a measure of its ability to store charge and is measured in farad (F). It is given by the formula:
Where:
Ε is the permittivity of the dielectric material,
A is the area of the plates,
d is the distance between the plates.
Capacitive reactance is the property of a capacitor to oppose alternating current. It is inversely proportional to both the capacitance (C) of the capacitor and the frequency (f) of the AC signal. Capacitive Reactance is measured in Ohms, just like resistance.
The capacitive reactance (XC) of a capacitor in an AC circuit is given by:
Where:
f is the frequency of the AC signal
C is the capacitance of the capacitor
Key Features of RF Capacitors
Very Low Equivalent Series Resistance (ESR) at High Frequencies: ESR is the resistance of the capacitor and includes resistance due to losses in the dielectric and electrodes. ESR is directly related to power dissipation in the form of heat. Higher ESR can cause excessive heating in the capacitor, hence RF capacitors must have the lowest possible ESR at high frequencies. To achieve low ESR, RF capacitors typically use Class I C0G/NPO dielectric materials due to their minimal loss properties.
High Quality Factor (Q) at High Frequencies: The quality factor (Q) represents the efficiency of a capacitor and indicates the ratio of energy stored versus energy dissipated as heat due to ESR. Q is inversely proportional to ESR, hence lower ESR means higher Q. RF capacitors, also referred to as high-Q capacitors, exhibit very high Q factors due to their low capacitance and exceptionally low ESR.
Capacitance Stability vs. Temperature: Tuning and impedance-matching applications demand capacitance stability across varying temperatures. Any variation in capacitance due to temperature changes can result in circuit detuning or impedance mismatch. RF capacitors are designed to maintain stable capacitance over a broad temperature range, ensuring reliable and consistent performance in dynamic environments.
High Series Resonant Frequency (SRF): SRF is the frequency at which a capacitor's total impedance transitions from capacitive to inductive due to its equivalent series inductance (ESL). Beyond this frequency, it no longer functions effectively as a capacitor and starts behaving more like an inductor. Hence RF capacitors need to have a high SRF because it defines the upper operational limit of the capacitor's effective performance. For some applications, having a high SRF ensures that these capacitors can operate efficiently well below this threshold and maintain optimal performance in high frequencies.
Types of RF Capacitors
Ceramic RF Capacitors: Ceramic RF capacitors use various ceramic materials for their dielectric, offering high stability and low losses at high frequencies. They come in various dielectric compositions to suit different applications. Multilayer Ceramic Capacitors (MLCCs) are one of the most widely used ceramic capacitors. They consist of multiple layers of ceramic dielectric material and metal electrodes stacked together. This multilayer structure increases their capacitance values within a compact size.
Mica RF Capacitors: Mica RF capacitors use natural mica as dielectric material, providing exceptional stability, low loss, and high precision. They are ideal for high-voltage and high-temperature applications.
Film RF Capacitors: Film RF capacitors use plastic films as the dielectric. They offer good performance at high frequencies and are known for their reliability and low dissipation factors. They are non-polarized which means they can be connected in either direction in a circuit.
Electrolytic Film Capacitors: This film capacitor has an anode or positive plate made of a metal, a very thin oxide layer that acts as the dielectric and a solid, liquid, or gel electrolyte covering the surface of this oxide layer, serving as the cathode or negative plate of the capacitor. They are polarized, meaning they have a specific positive and negative lead. Because of their very thin dielectric oxide layer and enlarged anode surface, electrolytic capacitors have a much higher capacitance per unit volume than other capacitors.
Chip capacitors: They are compact surface-mount capacitors made from ceramic or tantalum materials and are mounted directly onto the surface of printed circuit boards (PCBs) without the need for lead wires.
everything RF has developed an RF Capacitor search tool that helps users find capacitors that operate at high frequencies. RF capacitors from the leading manufacturers are listed. Use filters like Capacitance, Capacitor Type, Form Factor, Dielectric Material and various other parameters.
