What is Q factor in RF circuits?
Q (quality factor) is 2π times the ratio of peak stored energy to energy dissipated per cycle. For a resonator, Q = f0/BW3dB; for a reactive component, Q = |X|/R. Higher unloaded Q gives lower filter insertion loss and lower oscillator phase noise.[1][2]
What is the difference between unloaded and loaded Q?
Unloaded Q (Qu) counts only the resonator’s internal losses. Loaded Q (QL) includes the external circuit: 1/QL = 1/Qu + 1/Qe. In a filter, insertion loss at center frequency rises as QL/Qu approaches 1, so narrow filters need very high-Q resonators.[1]
What Q values are typical?
| Element | Typical unloaded Q |
|---|---|
| On-chip spiral inductor (GHz) | 5–20 |
| Chip inductor (0402/0603) | 20–80 |
| Air-core wound inductor | 100–300 |
| Microstrip resonator | 100–300 |
| Ceramic coaxial resonator | several hundred |
| Metal cavity | thousands |
| Quartz crystal | 104–106 |
Ranges are indicative; component datasheets specify Q at frequency.
How does Q affect matching networks?
Each L-section has a node Q set by the impedance ratio, Q = √(Rhigh/Rlow − 1); higher Q means narrower bandwidth and greater sensitivity to component loss. See L-network matching.[3]
Frequently asked questions
Is higher Q always better?
Not always. High Q narrows bandwidth and increases sensitivity to tolerances; wideband matching needs low Q.
How do I measure Q?
From the 3 dB bandwidth of a resonance on a VNA, correcting for coupling to get unloaded Q.
Does Q change with frequency?
Yes; component Q usually rises then falls as parasitics and losses change.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
- “A Simple Model of Feedback Oscillator Noise Spectrum”. D. B. Leeson, Proceedings of the IEEE, vol. 54, no. 2, pp. 329-330. February 1966.
- “RF Circuit Design, 2nd ed.”. C. Bowick, Newnes. 2008.