Definition
Fractional Bandwidth (FBW) = BW₋₃dB / f₀ × 100% where: BW₋₃dB = f_high − f_low (3 dB bandwidth) f₀ = √(f_high × f_low) (geometric mean center frequency) Loaded Q = f₀ / BW₋₃dB = 1/FBW High FBW = wideband (low Q) Low FBW = narrowband (high Q)
Fractional Bandwidth by Application
| Application | BW (MHz) | f₀ (MHz) | FBW | Q_L |
|---|---|---|---|---|
| GPS L1 filter | 20 | 1575 | 1.3% | 78.8 |
| LTE Band 3 RX filter | 75 | 1843 | 4.1% | 24.6 |
| WiFi 2.4 GHz | 100 | 2450 | 4.1% | 24.5 |
| 5G NR n78 (3300–3800) | 500 | 3540 | 14.1% | 7.1 |
| UWB (3.1–10.6 GHz) | 7500 | 5740 | 131% | 0.76 |
FBW and Matching Network Design
L-network minimum Q for given impedance ratio: Q_min = √(R_high/R_low − 1) FBW_max = 1/Q_min = 1/√(R_high/R_low − 1) Example: 10 Ω PA → 50 Ω system Q_min = √(50/10 − 1) = 2.0 FBW_max = 50% → can achieve >50% fractional bandwidth in theory (Bode-Fano limit further restricts for reactive loads)
RF View BW Marker: RF View's BW Marker automatically computes BW₋₃dB, center frequency f₀, and loaded Q from any S21 trace. Fractional bandwidth = BW/f₀ is immediately calculable from these readings. Free on Android.