RF Coils and Tuned Circuits
At radio frequencies, coils are used with capacitors to select frequencies, match impedances, couple stages, reject unwanted signals, and form oscillators. The physical construction of the coil becomes part of the circuit because lead length, turn spacing, nearby metal, and even the circuit board affect inductance and capacitance.
Air-core coils
Air-core inductors avoid magnetic-core saturation and core loss. They are common in RF tuned circuits where inductance values are relatively small and high Q is important. Coil diameter, number of turns, turn spacing, and wire diameter all affect inductance.
Ferrite and powdered-iron cores
A magnetic core provides more inductance with fewer turns. Ferrite materials are widely used at high frequencies, while powdered-iron materials have useful characteristics in tuned and power circuits. Core material must be chosen for the frequency and magnetic level involved.
Resonance
An inductor and capacitor exchange energy between magnetic and electric fields. For the ideal LC circuit:
Increasing either L or C lowers resonant frequency. This is the principle behind many tuned radio circuits.
Q factor
Q describes how low-loss an inductor is at a particular frequency. For a simple series-loss model:
A higher-Q coil can produce a narrower, sharper resonance. Real Q includes winding resistance, core loss, skin effect, proximity effect, radiation, and other frequency-dependent losses.
Self-resonant frequency
A coil's turns form tiny unwanted capacitors between themselves. As frequency rises, that parasitic capacitance eventually resonates with the inductance. Above the first self-resonance, a coil may no longer behave as the inductor the schematic symbol suggests.
Slug-tuned coils
Some coils include a threaded ferrite or powdered-iron slug. Moving the slug into or out of the winding changes magnetic coupling and inductance, allowing a radio or filter to be aligned after assembly.
Shielded RF coils
Metal cans and magnetic shields reduce coupling between nearby tuned circuits. The shield itself can slightly change inductance and losses, so the final component and layout should be adjusted as assembled.
Mutual coupling
Two nearby coils can deliberately or accidentally exchange magnetic energy. Deliberate coupling is used in RF transformers and coupled resonators. Unwanted coupling can cause feedback, detuning, instability, or crosstalk. Changing the distance or orientation between coils changes the coupling.
Practical RF layout
- Keep leads short.
- Avoid large loops unless they are intentional.
- Keep high-Q tuned coils away from metal objects that may detune them.
- Separate coils that should not couple, or orient them at right angles.
- Use the manufacturer's S-parameters or impedance data for demanding RF work.