Definition
Passband Ripple = S21_max − S21_min within the passband [dB, peak-to-peak] Typical specs: Butterworth: 0 dB (no ripple by design) Chebyshev 0.1 dB: ±0.05 dB variation Chebyshev 0.5 dB: ±0.25 dB variation Chebyshev 1.0 dB: ±0.5 dB variation
Intentional vs Unintentional Ripple
| Type | Pattern | Cause |
|---|---|---|
| Chebyshev ripple | Regular, N−1 oscillations across passband | Intentional design for steeper rolloff |
| Fixture reflection ripple | Sinusoidal, period proportional to cable length | S11·S22 interaction; reduce with matched ports |
| Mounting resonance | Local spike or notch | PCB trace stub, pad resonance |
| Temperature gradient | Slow baseline drift | Uneven heating; thermal equilibrate first |
Measuring Ripple in RF View
Load filter .s2p → S21 dB view → zoom to passband Delta marker method: 1. Set Marker 1 at S21 maximum (peak of ripple, highest point in BW) 2. Move Δ marker to S21 minimum within same bandwidth 3. Read |ΔdB| = peak-to-peak ripple FN marker method: 1. FN marker → set ±ripple_spec threshold 2. Marker shows bandwidth where S21 stays within ±threshold 3. Must cover entire specified passband frequency range
Impact of Passband Ripple on System
- 0.5 dB ripple across 20 MHz channel: different subcarriers see different attenuation → OFDM EVM degradation
- 1 dB ripple: typically acceptable for LTE but marginal for 256-QAM
- 3 dB ripple: significant problem for all data modulations
RF View Ripple Measurement: Delta marker quickly measures peak-to-peak passband ripple. Load filter .s2p, zoom to passband, set markers at max and min S21 levels. Free on Android.