Overview
Group delay τ_g = −dφ/dω measures how long different frequency components take to traverse a device. Flat group delay means all frequencies are delayed equally (no dispersion). Group delay ripple causes inter-symbol interference in digital modulation.
Step 1: Load File and Switch to Group Delay View
- Load filter .s2p file into RF View S-Parameter Plot
- Select "Group Delay" from the parameter display dropdown
- RF View numerically differentiates the S21 phase: τ_g = −Δφ/(360°·Δf)
- Set display range: typical 0–500 ns for cellular SAW/LC filters
Step 2: Read Peak Group Delay
Place single marker at group delay peak location:
Typical readings:
SAW filter (cellular): peak τ_g = 100–500 ns at center freq
BAW/FBAR filter: peak τ_g = 1–10 ns
LC bandpass filter (N=5): peak τ_g = 20–100 ns at band edges
LNA (wideband): τ_g ≈ 1–3 ns (flat across band)
50 Ω SMA cable (0.5m): τ_g ≈ 2.5 ns (constant)
Step 3: Measure Group Delay Ripple
Group delay ripple = max τ_g − min τ_g across passband Using delta marker: 1. Set Marker 1 at passband max τ_g (e.g., 1835 MHz, τ_g = 182 ns) 2. Activate Δ mode → move to passband min (e.g., 1815 MHz, τ_g = 165 ns) 3. Read Δτ_g = 17 ns (ripple over 20 MHz passband) Spec check: LTE 20 MHz channel → τ_g ripple < 5 ns required for 64-QAM 17 ns ripple → FAILS spec → need BAW filter or equalizer
Step 4: Compare Multiple Devices
Load multiple filter .s2p files simultaneously. Group delay overlay shows which device has the flattest passband delay — critical for selecting the best component for high-order modulation systems.
Interpreting Group Delay Anomalies
| Group Delay Feature | Physical Meaning |
|---|---|
| Sharp spike at band edge | Filter pole near cutoff — SAW/BAW resonance |
| Constant flat delay | Pure transmission line — propagation delay |
| Increasing delay toward center | Chebyshev or elliptic filter characteristic |
| Flat delay with abrupt jump | Phase discontinuity — unwrapping needed |
RF View Group Delay: Select Group Delay view from the S-parameter display. Delta marker reads passband τ_g ripple. Compare multiple devices for filter selection. Free on Android.