Bias-T Function
A bias-T (or bias tee) injects DC power onto an RF line without disturbing the RF signal path. It consists of an RF choke (high impedance at RF) from the DC port and a DC block capacitor in the RF path. Used to power LNAs, drivers, and other active components through the RF coaxial line.
Bias-T Port Configuration
3-port bias-T: Port 1 = RF + DC (combined port — connects to active device) Port 2 = RF only (pure RF path — connects to VNA or filter) Port 3 = DC only (DC bias input from power supply) Ideal S-parameters: S21 (RF path): 0 dB, flat from DC block cutoff to RF choke SRF S11, S22: −∞ dB (perfectly matched) S31 = S13: −∞ dB (DC port isolated from RF at all RF frequencies)
Bias-T Frequency Range and Loss
| Property | Low-Freq Limit | High-Freq Limit |
|---|---|---|
| RF signal passband | f_DC_block (DC block C resonance) | f_choke_SRF (RF choke self-resonance) |
| Typical passband | 10 MHz–100 MHz | 3 GHz–18 GHz |
| Insertion loss S21 | Rises as f → f_low | Rises as f → f_high |
| Mid-band IL | −0.2 to −0.5 dB typical | — |
Measuring Bias-T S-Parameters
- SOLT-calibrate VNA including the RF cable that will connect to Port 3
- Terminate DC port with 50 Ω (Port 3 must be terminated for RF S11/S21 accuracy)
- Measure S21 (Port 1 to Port 2): should be <0.5 dB loss across rated band
- Measure S11, S22: should be <−20 dB across rated band
- Check frequency limits: find where S21 drops to −3 dB on each side
RF View Bias-T Analysis: Load .s2p (RF path) or .s3p (full 3-port) file. S21 view shows insertion loss, BW Marker finds −3 dB frequency limits. Compare bias-T units for lab inventory qualification. Free on Android.