What are RF Inductors?
An inductor is a passive electrical component with two terminals that store energy in magnetic field form when current flows through it. The inductor opposes any changes (i.e., increase or decrease) in current flow through it due to the production of self-induced emf. The property of resisting changes in current flow is known as inductance, measured in Henry (H). A basic inductor construction consists of a coil wound around a core (e.g., ferromagnetic core or air core). They are used in filter circuits, RF bypass, coupling and decoupling, RF resonant and measuring circuits, tuning circuits, etc.
RF inductors are specialized inductors designed for high-frequency circuits, typically operating from tens of MHz to several GHz. They are widely used in applications like oscillators, biasing networks, impedance-matching circuits, passive filters, Bias Tees, and voltage-controlled oscillators (VCOs). Most RF inductors consist of a copper wire coil wound around a ceramic or air core, as these cores provide high inductance stability, a high Q factor, and low losses, which are the qualities essential for high-frequency applications. Some RF inductor designs can also use ferrite core material that provides higher inductance value for application.
Understanding RF Inductor Selection
To assess and compare the various RF inductor types, It is necessary to understand the specification parameters of inductors such as inductance, tolerance, Quality (Q) factor, self-resonant frequency (SRF), and rated current. Let's understand these parameters.
Type of Inductor by construction: RF Inductors are often classified by their construction. These include Wire wound ceramic inductor, Wire-wound ferrite Inductor, Air-Core RF Inductor, Multilayer ceramic inductor, Thin film inductor, Conical inductor and Tunable RF inductor among others.
Inductance value: Represents the inductance value of the inductor, typically expressed in nH or µH.
Tolerance: Represents ± X % variation in the real inductance value. RF inductors should have tight inductance tolerances (i.e., minimum variation from nominal value), particularly in applications such as oscillators, filtering, and matching circuits.
Self-Resonant Frequency (SRF): This is the frequency at which the inductive reactance (XL) is equal in magnitude to inter-winding capacitive reactance (XC), i.e., XL = XC and both the reactances cancel out each other. At SRF, the impedance of the inductor is purely resistive. Below the SRF, the inductor serves as a true inductor, and beyond the SRF, the inductor acts like a capacitor.
An inductor with a higher SRF is typically more desirable for high-frequency use, as it allows for effective inductive behavior across a broader frequency range.
Quality (Q) Factor: The Q factor measures how close the inductor is to an ideal inductor (which would have no losses). A higher Q factor indicates that the inductor has lower energy losses (i.e., smaller resistance) and performs more efficiently. Mathematically, the Q factor is the ratio between the inductive reactance (XL) and the DC resistance (RDC) of the inductor at a particular frequency.
Note: While DC resistance losses are the primary cause of energy losses in an inductor, other factors—such as skin effect, core losses (including hysteresis and eddy current losses), and radiated energy—also contribute to overall losses, all of which impact the inductor's Q factor value.
DC resistance (RDC or DCR): Represents the conductor resistance of the inductor, typically expressed in Ω or mΩ. The DC resistance is a primary source of power losses in the inductor and is inversely proportional to the Q factor. Higher DCR leads to greater resistive losses, resulting in a lower Q factor.
Rated Current: Represents the maximum continuous current that the inductor can safely handle at the specified operating temperature provided in the datasheet. It is typically expressed in A or mA.
Size: RF inductors come in various sizes, for example, from 0201 up to 1208 (inches). Smaller inductors have lower inductance values and use thinner wire, which results in higher DC resistance (DCR) and a lower Q factor. Therefore, engineers must consider trade-offs among size, performance, and design to select the most suitable RF inductor for their application.
