How do capacitors and inductors behave at RF?
At RF every passive component is a network: a capacitor has equivalent series inductance (ESL) and resistance (ESR) and becomes inductive above its self-resonant frequency SRF = 1/(2π√(ESL·C)); an inductor has inter-winding capacitance and becomes capacitive above its SRF; a resistor has lead inductance and pad capacitance. Use manufacturers’ S-parameter models for accurate design.[1][2]
How should capacitors be chosen?
- C0G/NP0 ceramic: stable, low loss, preferred for tuning, filters and matching.
- X7R/X5R: high capacitance per volume for decoupling; capacitance varies with voltage and temperature.
- Smaller packages (0402, 0201) have lower ESL and higher SRF.
- Use series resonance deliberately: a capacitor at its SRF is a low-impedance short, useful for DC blocks and bypass.
How should inductors be chosen?
- Wire-wound chip inductors: highest Q.
- Multilayer ceramic: smaller, lower Q, low cost.
- Thin-film: tight tolerance at small values.
- Air-core spring inductors: very high Q, used in PA matching.
- Keep SRF well above operating frequency, typically by at least 1.5–2×.
What about layout parasitics?
A pad and via can add tenths of a nanohenry and tenths of a picofarad, comparable to the component itself in the GHz range. Include pads in EM simulation; see grounding and vias.
Frequently asked questions
What is ESR?
Equivalent series resistance, the loss term in a capacitor model.
Why does a 100 nF capacitor not bypass at 2 GHz?
Its ESL dominates above SRF; a smaller value with matching SRF may be more effective.
Are 0201 parts better?
Lower parasitics, but harder to assemble and lower power rating.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “RF Circuit Design, 2nd ed.”. C. Bowick, Newnes. 2008.
- “Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.