Key Metrics for a Bandpass Filter
A bandpass filter is fully characterized by its S-parameters (S11, S21, S12, S22). The following metrics are extracted directly from the S21 and S11 traces:
| Parameter | Source | Typical Spec (LTE BPF) |
|---|---|---|
| Passband center frequency f₀ | Peak of |S21| | Specified band center |
| Insertion Loss (IL) | −|S21|_max in passband | < 2 dB (SAW), < 3 dB (LC) |
| Passband ripple | Max − min of S21 in passband | < 0.5–1 dB |
| 3 dB bandwidth | Freq span where S21 ≥ (peak − 3 dB) | ≥ channel BW |
| Input return loss (S11) | −S11 in passband | > 14 dB (VSWR < 1.5) |
| Stopband rejection | −|S21| at specified stop freq | > 30–60 dB |
| Shape factor | BW₋₄₀dB / BW₋₃dB | < 1.3 (BAW), < 2 (LC) |
| Group delay ripple | Peak-to-peak τ_g in passband | < 2–5 ns |
Step 1: Load the S2P File
Open the .s2p file in RF View S-Parameter Plot. Confirm port 1 = input, port 2 = output (check file header or component markings). Select S21 display mode with dB scale.
Step 2: Read Center Frequency and Insertion Loss
1. Enable single marker, move to peak of S21 trace
2. Read frequency (f₀) and level (S21_peak)
3. Insertion Loss = −S21_peak
e.g., S21 = −1.8 dB → IL = 1.8 dB
Step 3: Extract Bandwidth
Method 1 (BW Marker in RF View):
Select "BW Mkr" mode → automatically finds −3 dB points left and right of peak
Reads f₁, f₂, BW = f₂ − f₁, and Q = f₀/BW
Method 2 (manual delta marker):
Set ref marker at peak
Move delta marker to −3 dB level on each side
Note frequencies f₁, f₂
3 dB BW example: f₁=1795 MHz, f₂=1835 MHz → BW=40 MHz, f₀=1815 MHz, Q=45.4
Step 4: Measure Stopband Rejection
For an LTE Band 3 RX filter (DL 1805–1880 MHz):
Check S21 at TX band (UL 1710–1785 MHz):
→ Should be < −50 dB for adequate TX-to-RX isolation
Use single marker to read S21 at:
- Lower edge of TX band: 1710 MHz
- Upper edge of TX band: 1785 MHz
- Any known interferer frequencies
Step 5: Check Port Match (S11 and S22)
Plot S11 across the passband Spec: S11 < −14 dB (VSWR < 1.5) across f₁ to f₂ Check both S11 and S22 — asymmetric filter topology may have different port matches Switch to Smith chart view to see impedance locus across band — should stay close to chart center in passband
Step 6: Measure Group Delay
Select Group Delay plot type in RF View. Check:
- Peak group delay at passband center (acceptable: < 100 ns for LC, < 500 ns for SAW)
- Group delay ripple across passband (peak-to-peak, spec: < 5 ns for LTE data)
- Group delay near band edges (often spikes — watch for filter sensitivity)
Step 7: Compare Multiple Filters
Load 2–4 different vendor .s2p files simultaneously in RF View. All S21 traces appear on the same chart with auto color coding. Use markers on each trace to compare IL, BW, and rejection. Export comparison as CSV for documentation.
RF View Filter Analysis: RF View supports multi-file S21 overlay with full marker capability — single, table, delta, BW markers — on S21, S11, Smith chart, and group delay. Compare filter lots on Android without a PC or EDA software.