Ultra Broadband Inductor 550nH ±20% Micro Wire Resonance Free Bias Tee RF Choke 50MHz - 40GHz

HALT40005 Ultra-Broadband Conical Inductor | 550nH ±20% Micro-Wire Resonance-Free Bias Tee RF Choke (50MHz - 40GHz)

Why 40 GHz and 550 nH Cannot Coexist in a Solenoid

Take a standard cylindrical inductor wound to produce 550 nH of inductance. The number of turns required—given a practical form factor—will be large enough that the cumulative turn-to-turn capacitance resonates with the coil inductance somewhere between 200 MHz and 400 MHz. Above that frequency, the impedance collapses from inductive to capacitive, and the component ceases to function as a choke. This is not a manufacturing defect; it is a direct consequence of uniform-winding geometry.

The HALT40005 escapes this constraint by abandoning the uniform winding. Instead, the turns are wound in a continuously tapered cone—narrow at one end, wide at the other. Because adjacent turns have progressively changing diameters, no two turn pairs have identical facing surface area. The parasitic capacitance that does exist is spectrally distributed rather than concentrated at one LC product. At the narrow apex tip, where the winding connects to the 50-Ω microstrip line, the turn diameter approaches 0.40 mm and the shunt capacitance to the ground plane is negligible. At the wide base, the larger turns accumulate the full 550 nH of blocking reactance, effective down to 50 MHz. Between these two extremes, the impedance transition is smooth and continuous—no self-resonance, no S21 notch.

The HALT40005 is wound from 0.05 mm (50 µm) oxygen-free copper wire with polyimide insulation, rated for 200 mA of continuous DC current with a temperature rise held below 15°C. The self-resonant frequency exceeds 40.0 GHz, confirmed by VNA measurement on a TRL-calibrated microstrip test fixture. An optional 0.08 mm wire variant is available on custom order for applications requiring higher bias current at the expense of approximately 5 GHz of upper bandwidth.

Performance Data Across the Operating Envelope

Specification Reference

The Optoelectronic Bias Tee: One Component Replaces Three

In a conventional 400G optical transceiver bias tee, the DC injection network might use a cascade of three inductors—perhaps 1 µH, 100 nH, and 10 nH—each covering a portion of the spectrum. The junctions between these components form unintended parallel LC tanks that resonate in the 3–8 GHz range, producing the characteristic mid-band S21 dip that degrades PAM4 eye linearity.

The HALT40005 collapses this three-inductor cascade into a single 3.0 mm component. Because the conical geometry provides a continuous impedance gradient rather than three discrete inductive states, there is no junction, no parasitic tank circuit, and no mid-band dip. The measured S21 of a properly assembled bias tee using the HALT40005 is flat to within the fixture measurement uncertainty from 50 MHz through 40 GHz. For optical transceiver designers, this means the choke is no longer the limiting element in the channel budget.

Physical Assembly Requirements

1. Tip-side connection: The narrow cone end solders to the 50-Ω microstrip or coplanar waveguide trace. Mount the coil perpendicular to the board plane. Any deviation from 90° orientation creates asymmetric field coupling that elevates S11 at millimeter-wave frequencies.
2. Lead exposure at the apex: The length of flying lead between the solder fillet and the first turn must be ≤0.3 mm. Each additional 0.5 mm of exposed lead introduces roughly 0.3 nH of parasitic series inductance—enough to measurably shift the input match above 30 GHz.
3. Base-lead DC routing: The wide cone end connects to the DC bias input node. Place a 100 pF ceramic capacitor in parallel with a 10 nF capacitor within 1 mm of this pad to shunt any RF leakage to ground.
4. Mechanical stabilization: Affix the coil body with a single micro-dot (≤0.3 mm diameter) of Epotek H70E low-outgassing epoxy on the winding side. This eliminates microphonic turn-spacing modulation that generates phase noise sidebands.
5. Solder joint thermal budget: Iron tip at 280–320°C. Dwell limit: 2 seconds. The 50 µm conductor reaches soldering temperature in a fraction of a second; extended heating anneals the copper and can permanently deform the precision taper.

Application Scope

• Laser driver and TIA DC bias injection in 100G/400G/800G TOSA and ROSA modules
• GaAs pHEMT LNA gate bias and GaN HEMT drain bias in Ka-band receivers and transmitters
• High-speed photodiode reverse-bias decoupling in coherent detection chains
• PIN diode switch and digital step attenuator DC return networks
• 5G mmWave (FR2) phased-array antenna element bias power distribution
• Electronic warfare and SIGINT receiver front-end bias (0.05–40 GHz)

Questions from the Field

Q: How does the HALT40005 differ from the HALT40005A and HALT40005B?
A: The core inductor—550 nH, 0.05 mm wire, 50 MHz–40 GHz air-core conical—is the identical component manufactured on the same production line. The three variants differ in their application documentation focus: HALT40005A serves general-purpose ultra-wideband choking; HALT40005B provides optoelectronic-specific reliability qualification and wedge-bonding guidance; HALT40005 emphasizes resonance-free bias tee integration with module-level assembly instructions. Choose the variant whose supporting documentation best aligns with your design workflow.

Q: Our laser driver draws 190 mA. The data sheet says 200 mA maximum. Is that sufficient margin?
A: At 190 mA, the I²R dissipation is roughly 72 mW (based on a typical winding DCR of 2.0 Ω). The resulting ΔT is approximately 12°C—comfortably within the 15°C design limit. The 10% current headroom provides adequate margin for unit-to-unit DCR variation and ambient temperature rise within the module enclosure.

Q: What physical orientation gives the best S11 above 30 GHz?
A: Perpendicular mounting (coil axis at 90° to the board plane, apex tip soldered directly to the microstrip) consistently produces the best return loss above 30 GHz. Horizontal mounting with a 90° bend in the apex lead is possible in height-constrained modules but introduces additional parasitic inductance at the bend. If horizontal mounting is required, Hoan recommends 3D EM simulation of the specific lead-bend geometry using the provided .s2p data.