Definition
Stopband rejection = attenuation provided at a specific stopband frequency
Rejection (dB) = −S21(f_stop) − IL_passband [relative to passband level]
Or simply: Rejection = −S21(f_stop) [absolute, below 0 dB reference]
Example: S21 at passband center = −1.8 dB
S21 at TX band = −52 dB
Relative rejection = −52 − (−1.8) = 50.2 dB
Absolute rejection = 52 dB
Required Rejection by Application
| Application | Stopband | Required Rejection |
|---|---|---|
| LTE RX filter (vs TX band) | TX band (45–195 MHz offset) | >50–60 dB |
| GPS L1 filter (vs cellular) | 1700–2700 MHz cellular | >35 dB |
| WiFi 2.4 GHz (vs 5 GHz) | 5150–5850 MHz | >40 dB |
| Harmonic filter (2nd H) | 2×f₀ | >40–45 dB (FCC compliance) |
| TX diplexer (band separation) | Other band | >50 dB (system level) |
Increasing Rejection
Option 1: Higher filter order N=3 Butterworth: 3×20 = 60 dB/decade rolloff N=5 Butterworth: 5×20 = 100 dB/decade rolloff Option 2: Elliptic response (finite transmission zeros) Notch at specific stopband frequency → >80 dB at the notch Option 3: Two filters in series Rejection doubles (in dB): 50 dB → 100 dB total But IL also doubles: 2 dB → 4 dB total (tradeoff)
Measuring Rejection in RF View
- Load filter .s2p → S21 dB view
- Place single marker at the target stopband frequency
- Read S21 level: e.g., −52 dB at 1750 MHz
- Compare with specification: must be <−50 dB (rejection >50 dB)
- Sweep marker across entire stopband to find worst-case frequency
RF View Rejection Measurement: Single marker at any stopband frequency reads rejection directly from S21 dB plot. Sweep across entire stopband to find worst-case (minimum rejection) frequency. Free on Android.