Low-Pass Filter S-Parameter Characteristics
A low-pass filter passes DC to cutoff frequency f_c and attenuates signals above. S21 shows the frequency response; S11 shows input port matching. The transition band sharpness is characterized by the shape factor.
Key LPF Specification Parameters
| Parameter | Description | Typical LPF Spec |
|---|---|---|
| Cutoff frequency (f_c) | −3 dB frequency | Specified (e.g., 1 GHz) |
| Passband IL | −S21 at DC or low freq | <0.5 dB (LC), <1 dB (stub) |
| Passband return loss | S11 in passband | >14 dB (VSWR <1.5) |
| Stopband rejection at 2f_c | −S21 at 2×f_c | >30 dB (3rd order Butter) |
| Stopband rejection at 3f_c | −S21 at 3×f_c | >50 dB (5th order Butter) |
| Shape factor BW₋₄₀/BW₋₃ | Selectivity ratio | <2.0 (good), <1.5 (very good) |
Step-by-Step LPF Analysis in RF View
- Load LPF .s2p file → select S21 dB view
- Set BW Marker threshold to −3 dB → reads f_c automatically
- Place single marker at 2f_c → reads stopband rejection
- Add delta marker at f_c peak → move to 2f_c → reads relative rejection
- Switch to S11 → verify passband return loss >14 dB for f < 0.9·f_c
- Check Group Delay: Butterworth LPF should show monotonically increasing delay toward f_c
Comparison: LPF Filter Type vs Attenuation at 2f_c (N=5)
| Filter Type | At 2f_c | At 3f_c | Group Delay Flatness |
|---|---|---|---|
| Butterworth (N=5) | −60 dB | −95 dB | Moderate |
| Chebyshev 0.5 dB (N=5) | −80 dB | −130 dB | Poor at f_c |
| Bessel (N=5) | −34 dB | −54 dB | Excellent (flat) |
| Elliptic (N=5, 0.5 dB) | >−100 dB | >−100 dB | Very poor at f_c |
RF View LPF Analysis: Load any LPF .s2p to measure −3 dB cutoff, stopband rejection, passband IL and RL with BW Marker and single marker. Group delay view shows filter type signature. Free on Android.