What Determines Stopband Rejection?
Filter stopband rejection depends on: filter order N (more poles = steeper rolloff), filter type (elliptic > Chebyshev > Butterworth), technology (BAW sharper than LC), and frequency distance from passband edge (more distance = more rejection available).
Method 1: Increase Filter Order
Butterworth filter rolloff: −20·N dB/decade beyond cutoff 5th-order BPF at 1 GHz: 60 dB attenuation 3 decades above cutoff 7th-order BPF: 84 dB at same frequency 9th-order BPF: 108 dB (but higher IL in passband) Tradeoff: higher order → more passband IL (each stage adds loss) Typical: 5th to 7th order for cellular filters
Method 2: Use Elliptic (Cauer) Filter Response
Elliptic filter has transmission zeros (notches) in stopband: At specific stopband frequencies: infinite attenuation Shape factor BW₋₆₀/BW₋₃ < 1.2 (vs >2.0 for Butterworth) Example: 5th-order elliptic at 1.8 GHz (LTE Band 3 RX): At 1710 MHz (TX band): >55 dB rejection ← only 95 MHz from passband! Without elliptic (Butterworth): only 35 dB at same offset
Method 3: Stack Two Filters
Single SAW filter at 1800 MHz: IL=2.0 dB, TX rejection=50 dB Two SAW filters in series: IL=4.0 dB, TX rejection=100 dB Trade-off: insertion loss doubles, rejection doubles (in dB) Used when: single filter inadequate for blocking strong interferer (e.g., strong Bluetooth at 2.4 GHz when WiFi LNA needs 60 dB rejection)
Method 4: Choose BAW Instead of SAW
| Comparison | SAW (1.8 GHz) | BAW/FBAR (1.8 GHz) |
|---|---|---|
| Passband IL | 2.0 dB | 1.2 dB |
| Shape factor | 1.5 | 1.15 |
| Rejection at TX band | 50 dB | 60 dB |
| Out-of-band leakage | Higher at harmonics | Lower |
Method 5: Add Notch Filter for Specific Interferer
If rejection is needed at a specific frequency (e.g., WiFi 2.4 GHz interfering with GPS 1.575 GHz), a narrow notch filter (bandstop) provides very high rejection at the target frequency without affecting the desired passband. Notch depth >40 dB achievable with a single λ/4 shunt stub.
RF View Filter Analysis: Load filter .s2p to measure actual stopband rejection at specific frequencies. Compare multiple filter technologies (SAW vs BAW vs LC) on the same chart. Free on Android.