High-Pass Filter Applications
High-pass filters (HPFs) pass signals above the cutoff frequency and block lower frequencies. Common RF applications:
- DC blocking in bias-T networks (f_c = 100 kHz to 100 MHz)
- Subharmonic suppression (block fundamental, pass 2nd harmonic)
- Diplexer lower-band rejection (block WiFi 2.4 GHz, pass 5 GHz)
- LTE high-band filtering (block low-band leakage)
Key HPF S-Parameter Specifications
| Parameter | Definition | Typical Spec |
|---|---|---|
| Cutoff frequency f_c | −3 dB upper transition point | Specified per application |
| Passband IL | −S21 at frequencies >> f_c | <1 dB for LC, <2 dB for LTCC |
| Stopband rejection at f_c/2 | −S21 at half the cutoff freq | >20 dB (3rd order) |
| Stopband rejection at DC | Blocking at low freq | >40 dB for DC block |
| S11 in passband | Port match above f_c | >14 dB RL |
Reading HPF S-Parameter Plots
S21 for an ideal 3rd-order Butterworth HPF (f_c = 1 GHz): DC to 300 MHz: S21 < −30 dB (stopband) At 500 MHz: S21 ≈ −15 dB (transition band) At 1 GHz: S21 = −3 dB (cutoff) At 2 GHz: S21 ≈ −0.3 dB (passband) At 10 GHz: S21 ≈ −0.1 dB (passband) Shape factor = BW₋₃(high side) / BW₋₃(low side) → not defined the same as BPF For HPF: use "steepness" = Rejection at f_c/2 vs at f_c
HPF Analysis in RF View
- Load HPF .s2p file → S21 dB display
- Place marker at high-freq passband → read insertion loss (e.g., −0.5 dB at 3 GHz)
- Use delta marker → move to cutoff region → find −3 dB point
- Move to stopband (f < f_c/2) → read rejection
- Switch to S11 → verify passband match >14 dB above 1.2·f_c
RF View HPF Analysis: Load HPF .s2p file, place markers in passband and stopband to characterize cutoff, IL, and rejection. Compare multiple HPF variants on one chart. Free on Android.