What specifications define an RF filter?
An RF filter passes wanted frequencies and attenuates others. It is specified by passband edges, insertion loss, passband ripple and return loss, stopband rejection at stated frequencies, group-delay variation, power handling and size. Designs usually start from a normalized low-pass prototype that is scaled and transformed to the required response.[1][2]
What filter responses exist?
- Low-pass: harmonic suppression after PAs and anti-aliasing before ADCs.
- High-pass: removes lower-frequency interferers.
- Band-pass: channel or band selection, receiver pre-selection.
- Band-stop (notch): removes a specific interferer.
- Multiplexers: several band-pass filters sharing a common port (diplexers and duplexers).
Which technology suits which frequency?
| Technology | Typical range | Unloaded Q |
|---|---|---|
| Lumped LC (chip or wound) | kHz to a few GHz | 20–200 |
| LTCC / IPD lumped | 100 MHz to ~10 GHz | 30–100 |
| Microstrip / stripline | 1–100 GHz | 100–300 |
| SAW | ~10 MHz to ~3 GHz | ~1,000 |
| BAW / FBAR | ~1.5 to ~7 GHz and above | 1,000–3,000+ |
| Ceramic coaxial | 300 MHz to ~6 GHz | several hundred |
| Cavity / dielectric resonator | ~100 MHz to mmWave | thousands to tens of thousands |
Ranges are indicative; vendors continually extend them.
How does resonator Q affect a filter?
Midband insertion loss of a band-pass filter is approximately IL ≈ 4.343·Σgk·(f0/Δf)/Qu dB, summing the prototype element values gk. Narrow fractional bandwidths and more sections demand higher Qu.[1]
Why does group delay matter?
Group delay τ = −dφ/dω. Variation across the passband distorts wideband modulated signals; sharp filters have large delay peaks near band edges. Bessel and linear-phase designs trade selectivity for flat delay.[3]
Frequently asked questions
What is insertion loss?
The reduction in transmitted power caused by inserting the filter, usually quoted as −|S21| in dB at passband.
What is shape factor?
Ratio of the bandwidth at a stopband attenuation level (e.g., 60 dB) to the passband bandwidth.
Do filters need matching?
Filters are designed for specified terminations; mismatched terminations degrade ripple and rejection.
See also
References
Online sources accessed October 21, 2026; printed works cited by edition. See all sources.
- “Microwave Filters, Impedance-Matching Networks, and Coupling Structures”. G. Matthaei, L. Young, E. M. T. Jones, McGraw-Hill / Artech House. 1964; Artech reprint 1980.
- “Microwave Engineering, 4th ed.”. D. M. Pozar, Wiley. 2012.
- “Handbook of Filter Synthesis”. A. I. Zverev, Wiley. 1967.