Definition of 3 dB Bandwidth
BW₋₃dB = f₂ − f₁ f₁, f₂ = frequencies where |S21(f)| = |S21_peak| − 3 dB Center frequency: f₀ = √(f₁·f₂) [geometric mean] Loaded Q: Q_L = f₀ / BW₋₃dB
Method 1: BW Marker Mode in RF View (Fastest)
- Load .s2p file → select S21 dB display
- Activate BW Marker: tap the marker icon → select "BW Mkr"
- RF View automatically:
- Finds peak of S21
- Searches left and right for −3 dB crossing points
- Reports: f₁, f₂, f₀, BW₋₃dB, Q_L
- Note: also works for finding VSWR=2:1 bandwidth (−10 dB crossing)
Method 2: Manual Delta Marker
1. Place reference marker at S21 peak
e.g., Marker 1 at 1815 MHz, S21 = −1.8 dB
2. Activate delta marker (Δ mode relative to Marker 1)
3. Move delta marker left until it reads Δ = −3.0 dB
→ Records f₁ (e.g., 1797 MHz)
4. Move to right side until Δ = −3.0 dB
→ Records f₂ (e.g., 1835 MHz)
5. BW = f₂ − f₁ = 1835 − 1797 = 38 MHz
f₀ = √(1797 × 1835) = 1816 MHz
Q_L = 1816 / 38 = 47.8
Method 3: Export to CSV and Compute Programmatically
import pandas as pd, numpy as np
df = pd.read_csv('filter.csv')
peak = df['S21_dB'].max()
thresh = peak - 3.0
above = df[df['S21_dB'] >= thresh]
f1 = above['Freq_GHz'].min()
f2 = above['Freq_GHz'].max()
BW = (f2 - f1) * 1000 # MHz
f0 = np.sqrt(f1 * f2)
Q = f0 * 1000 / BW
print(f"BW = {BW:.1f} MHz, f0 = {f0*1000:.1f} MHz, Q = {Q:.1f}")
Common Bandwidth Thresholds Used in Practice
| Threshold | dB Level | Use Case |
|---|---|---|
| 3 dB bandwidth | S21_peak − 3 dB | Standard filter bandwidth definition |
| 10 dB bandwidth | S11 < −10 dB | Antenna operating bandwidth (VSWR < 2) |
| 1 dB bandwidth | S21_peak − 1 dB | Amplifier flat gain bandwidth |
| 60 dB bandwidth | S21_peak − 60 dB | Shape factor = BW₋₆₀ / BW₋₃ |
RF View BW Marker: One-tap automatic 3 dB (or custom threshold) bandwidth measurement from any S21 trace. Reports f₁, f₂, center frequency, bandwidth, and Q_L simultaneously. Free on Android.