CHARACTERISTICS AND APPLICATIONS OF RADIO FREQUENCY INDUCTORS

An inductor has to be specially designed to provide inductive impedance at frequencies from few MHz to several GHz. It is used for RF chokes, impedance matching, LC tuning, filtering, and bias isolation. Its impedance over normal operating range is approximately:

Z_L = 2πfL

where f is frequency and L is inductance. Real RF inductors also have winding resistance and parasitic capacitance. For example, for a 100 nH inductor at 100 MHz

│X_L│ = 2π (100 x 10⁶) (100 x 10^-9) ≈ 62.8 Ω

The same inductor will have an impedance of just 62.8 μΩ at 100 Hz.

Parameters of RF inductor

• Inductance: From fractions of a nH to several hundred nH- some may reach µH range.
• Quality factor Q: Higher Q means lower loss and better efficiency.
• Self-resonant frequency (SRF): Above SRF, parasitic capacitance dominates and component behaves as capacitor. At SRF, it becomes purely resistive, with maximum impedance.
• Rated current: Important for bias-feed and RF-choke applications.
• DC resistance, DCR: Determines DC power loss and voltage drop.
• Tolerance and temperature coefficient: Affect frequency stability.
• Parasitic capacitance and coupling: Critical in compact PCB layouts.

Nonmagnetic Cores for RF inductors

Nonmagnetic cores are preferred in RF inductors because they provide lower loss, higher Q,better linearity, and more predictable behavior at high frequency. These cores are typically ceramic, air, or other low-permeability materials, which have relative permeability near unity, and inductance levels are small. Magnetic cores are preferred when high inductance, compact size, filtering, common-mode choking, or energy storage is more important than maximum Q at RF frequencies.

RF inductors commonly use nonmagnetic or ceramic materials since they do not introduce losses and nonlinearity at high frequency. Toroidal cores are often used for their high energy efficiency, low flux leakage and small light weight compared to rectangular or E-I cores. They also help reduce EMI since magnetic flux lines do not leak outside the core.

Nonmagnetic cores have following characteristics:

1) Lower core loss

Nonmagnetic cores almost entirely eliminate frequency-dependent losses arising from hysteresis, eddy currents, magnetic-domain relaxation and residual magnetic effects. At VHF and microwave frequencies, a nonmagnetic core avoids most of these mechanisms.

2) Higher quality factor                

A nonmagnetic core contributes to very low effective loss and higher Q. Thisis important in resonators, impedance-matching networks, oscillators, and RF filters.

3) Better linearity

A ceramic or air core has permeability close to that of free space, so its inductance remains more stable as signal level changes. This reduces distortions in RF circuits.

4) Higher service frequency

Nonmagnetic cores avoid the practical upper-frequency limitations associated with magnetic core loss. At VHF frequencies, required inductance is often sufficiently small  with a short air core or ceramic core.

5) Low parasitic effects

Nonmagnetic RF inductors can be made with small dimensions and carefully controlled geometry. This helps reduce unwanted interwinding capacitance and raises the self-resonant frequency.

Common inductor constructions

1. Wire-wound: Most common for comparatively high inductance and good precision. Typically used for matching networks, voltage-controlled oscillators, filters etc. Stores energy in magnetic field, handles high currents and filters noise in power frequency and radio frequency circuits. Their resistance is low, and quality factor is high.

2. Air core: Inductor is made without any solid or ferromagnetic material. May use air or plastic as core. Zero core loss, no core saturation. Very commonly used, this air core offers low inductance compared to ferromagnetic cores. Commonly used in RF circuits, impedance matching, high-frequency filters, oscillators etc. Very low core losses and high linearity makes these suitable for high-frequency tunes circuits and custom-built coils.

3. Multilayer Ceramic (MLC): Common as chip inductors in mobile and wireless circuits, and RF tuning. These are compact in size and are inexpensive. Stability is excellent. Other advantage is their high Q. Useful in high RF circuits, DC-DC converters

4. Thin Film: These are high precision made with photolithography and vacuum deposition process (similar to semiconductor processing). They have microscopic spiral coils on ceramic, silicon or magnetic substrates, offering ultra-compact sizes, close tolerances and stable high-frequency performance. Ideal for portable and wearable electronics, mobile sets.  Small size, good repeatability   with excellent high-frequency performance makes these suitable for RF IC modules and portable communication equipment. Common in Microwave circuits, communication, Wi-Fi, Bluetooth devices.

Variable RF inductors are used to adjust the inductance for fine-tuning or impedance matching. Adjustments are commonly made by means of threaded magnetic core inside a molded bobbin, while tapped inductors are also in use.

RF inductor self-resonance frequency

An RF inductor always has some parasitic capacitance between turns, pads, and nearby conductors. Its inductance   and parasitic capacitance   form a parallel resonant circuit called the self-resonant frequency (SRF) F_SRF = 1/ 2π√LC_p..

Far below SRF, the coil is predominantly inductive, with impedance increasing with frequency. Below SRF, an RF choke can block RF current because its inductive reactance (X_L = 2πfL) is high. Near SRF, it provides its greatest impedance and therefore often achieves maximum RF isolation. At resonance, inductive susceptance and capacitive susceptance cancel out, and impedance is decided by winding resistance, dielectric loss, radiation loss, and other parasitic effects.

Beyond SRF, coil becomes predominantly capacitive, and impedance drops with increasing frequency.  For a matching network, oscillator, or filter, operation too close to SRF is usually undesirable because the effective inductance, phase, and   become highly frequency-sensitive.

A 100 nH inductor with parasitic capacitance 0.25 pF has an SRF of 1,01 GHz, as can be seen from calculations The component is inductive below approximately  , reaches maximum impedance near this frequency, and becomes capacitive above it.

Selection and design criteria for RF inductor

1. SRF at least 2-3 times above the highest operating frequency.
2. High Q at operating frequency.
3. Rated current higher than RF operational plus DC bias current.
4. Low DCR to limit losses.
5. Inductance tolerance and temperature stability desired.
6. PCB parasitic capacitance at operating frequency.

RF inductors are specifically optimized for high-frequency signal, unlike power inductors (which are designed mainly for energy storage and high-current conversion).

Applications of RF inductors

• Impedance matching
• Oscillators, electronic switches
• Filtering, biasing
• Tuning of circuits

Blocking high frequency noise in communication devices, mobile phones, IoT devices etc.