Intentional Ripple: Chebyshev Filter Design
Chebyshev filters are designed with equal-ripple passband response — the ripple is intentional and traded for steeper rolloff compared to Butterworth. Common Chebyshev ripple levels:
0.1 dB ripple: small ripple, barely noticeable, good selectivity 0.5 dB ripple: moderate, common for cellular SAW filters 1.0 dB ripple: higher selectivity, more S11 variation in passband 3.0 dB ripple: poor, unusual for modern RF filters
Unintentional Ripple Sources
| Cause | Ripple Type | Fix |
|---|---|---|
| Multiple port reflections | Regular, sinusoidal pattern | Improve S11/S22 match at filter ports |
| Test fixture resonance | Sharp spikes at specific frequencies | De-embed fixture or use better test setup |
| PCB mounting resonance | Ripple related to PCB stub length | Minimize PCB trace length at filter pins |
| Temperature gradient | Slow drift baseline | Thermal stabilization |
| Damaged filter | Irregular, non-repeating | Replace component |
Multiple Reflection Analysis
If source S11 = −15 dB and filter input S11 = −12 dB:
Round-trip reflection: |Γ_source| × |Γ_filter| = 0.178 × 0.251 = 0.045
Ripple amplitude = ±20·log(1 ± 0.045) = ±0.39 dB ripple
Period of ripple = c / (2 × electrical_length_between_reflections)
Short cable: fast ripple pattern
Long cable: slow ripple
Fix: add 3 dB pad between source and filter to improve mismatch
or use VNA with better source match (E-cal calibrated)
Identifying Ripple Source in RF View
- Load filter .s2p → S21 view → observe ripple pattern
- Check if ripple is regular (sinusoidal) → suggests multiple reflections
- Use group delay view → constant GD = standing wave artifact, non-flat GD = filter poles
- Load S11 and S22 → high |S11| or |S22| suggests port mismatch causing ripple
- Compare with manufacturer datasheet: if ripple matches datasheet, it's Chebyshev
RF View Ripple Analysis: View S21 and Group Delay simultaneously to distinguish intentional Chebyshev ripple from fixture/mismatch artifacts. Load S11 to check port match contribution. Free on Android.