How do I select a bias tee for injecting DC power onto an RF line without affecting signal performance?
Bias Tee Selection Guide
Bias tees are critical components in remote-powered RF systems, satellite LNBs, CATV amplifiers, active antenna systems, and fiber-optic RF links where DC power must be delivered through the signal path.
| Parameter | Option A | Option B | Option C |
|---|---|---|---|
| Performance | High | Medium | Low |
| Cost | High | Low | Medium |
| Complexity | High | Low | Medium |
| Bandwidth | Narrow | Wide | Moderate |
| Typical Use | Lab/military | Consumer | Industrial |
Technical Considerations
(1) DC block (series capacitor): passes RF, blocks DC. Requirements: same as standalone DC block (low reactance at the lowest RF frequency, SRF above the highest RF frequency). For 10 MHz - 18 GHz: C = 100-220 pF (C0G/NP0, 0402 package). For 10 kHz - 6 GHz: C = 10-100 nF (may need X7R for the large value, with C0G in parallel for high-frequency coupling). (2) RF choke (shunt inductor from DC port to RF+DC port): blocks RF, passes DC. Requirements: high impedance at the lowest RF frequency (|Z_L| > 500 ohms at f_low for < 0.5 dB IL contribution). For f_low = 10 MHz: L > 500 / (2×pi×10e6) = 8 uH. Use a 10-22 uH inductor. The SRF of the inductor must be below the lowest RF frequency OR the inductor must be de-Qed (lossy at RF) to prevent resonances in the RF band. A resonance in the RF band creates a notch in the RF response (high insertion loss at the resonant frequency). Mitigation: use a ferrite bead or a lossy inductor that absorbs RF without resonating. (3) For broadband bias tees: the RF choke is often a conical (wideband) inductor wound on a ferrite core, providing high impedance from 10 MHz to 40+ GHz without sharp resonances.
Performance Analysis
(1) RF leakage to the DC port: the RF choke attenuates RF on the DC port by 20-40 dB. If the DC supply is sensitive to RF (switching regulator with RF coupling to the feedback loop): add additional RF filtering on the DC supply side (ferrite beads + bypass capacitors). (2) DC on the RF port: the DC block attenuates the DC to the RF port. At very low frequencies near the DC block cutoff: some DC or low-frequency AC appears at the RF port. If the downstream device is sensitive to DC offset: verify the DC block provides sufficient rejection. (3) Group delay: the DC block capacitor introduces a frequency-dependent phase (and group delay) at low frequencies. For narrowband signals well above the DC block cutoff: the group delay is flat and small (< 1 ns). For wideband signals that extend down near the cutoff: the group delay increases significantly at the low end, potentially distorting the signal. (4) Power dissipation: the RF choke carries the DC current, dissipating P = I^2 × R_DC. For a choke with R_DC = 2 ohms and I = 500 mA: P = 0.5 W. This heat must be dissipated. Use a bias tee with a current rating at least 50% above the required current for reliable operation.
- Performance verification: confirm specifications against the application requirements before finalizing the design
- Environmental factors: temperature range, humidity, and vibration affect long-term reliability and parameter drift
- Cost vs. performance: evaluate whether the application demands premium components or standard commercial grades
Design Guidelines
(1) Satellite LNB: the LNB (low-noise block converter) is mounted on the dish. DC power (13/18 V, 300-500 mA) is sent up the coaxial cable from the indoor receiver to power the LNB. A bias tee in the receiver separates the incoming RF signal (950-2150 MHz) from the DC power. Bias tee requirement: f_range = 950-2150 MHz, I_DC = 500 mA, V_DC = 18 V, IL < 0.5 dB. (2) Active antenna: a GPS or GNSS active antenna has an internal LNA that requires 3-5 V DC at 10-30 mA. The bias tee in the receiver provides the DC through the antenna cable. f_range = 1.1-1.6 GHz, I_DC = 30 mA, V_DC = 5 V, IL < 0.3 dB. (3) Photonic link: an RF-over-fiber link uses a laser diode transmitter and photodiode receiver. The photodiode requires reverse bias (5-15 V). A bias tee at the photodiode injects the DC bias without affecting the RF signal from the photodiode. f_range = DC-20 GHz, I_DC = 10 mA, V_DC = 15 V, IL < 0.5 dB.
Frequently Asked Questions
Can I build my own bias tee on my PCB?
Yes. A simple bias tee is just a DC block capacitor in series with the RF path and an inductor from the DC supply to the RF+DC node. However: the inductor selection is critical. A standard chip inductor (0402-0805) will resonate at some frequency within the RF band, creating a passband notch at the resonant frequency. For narrowband applications (where you can place the resonance outside the band): a single chip inductor works. For broadband applications: use multiple inductors of different values (forming a multi-section low-pass network), or use a ferrite bead (lossy inductor that absorbs RF without resonating), or use a conical wideband inductor (commercial bias tees use these). For best results: use a commercial bias tee for broadband applications and design your own for narrowband cost-sensitive designs.
What happens if the RF choke saturates?
If the DC current exceeds the inductor saturation current rating: the inductance drops sharply (the ferrite core saturates). The RF impedance of the choke decreases, allowing RF to leak to the DC port. This causes: insertion loss increase in the RF path (the RF power is diverted to the DC port), RF interference on the DC supply (potentially disrupting the DC-DC converter), and increased NF if the bias tee is before the LNA because the RF choke becomes lossy. Always select an inductor with a saturation current rating at least 1.5× the maximum DC current.
Is there a difference between a bias tee and a diplexer?
Functionally similar: both combine/separate signals on different frequency bands. A bias tee separates DC (0 Hz) from RF (MHz-GHz). A diplexer separates two RF bands (e.g., 900 MHz from 1800 MHz). Structurally: a bias tee uses a simple LC topology (one cap, one inductor). A diplexer uses two bandpass filters (much more complex, higher order). The key distinction: a bias tee cannot separate two closely-spaced RF bands (it only separates DC from everything else). A diplexer cannot pass DC (the filter input is AC-coupled by definition).