Air Core Conical Inductor 1000nH 25MHz - 40GHz RF Choke Inductor

HALT50005 Ultra-Broadband Conical Inductor | 1000nH (1.0µH) ±20% Micro-Wire Bias Tee RF Choke (25MHz - 40GHz)

Inside the Optical Transceiver: Why the Bias Choke Defines Channel Performance

The bias tee inside a 400G QSFP-DD transceiver occupies perhaps 4 mm² of PCB area. In that space, it must inject DC bias current onto a 53 GBd PAM4 signal path without introducing amplitude ripple that would close the eye diagram. The component that does the heavy lifting—the choke inductor—sits directly in the signal current loop. Its parasitic capacitance loads the transmission line. Its self-resonance creates an S21 notch. Its temperature coefficient shifts the bias point. Every imperfection in this one component translates into deterministic jitter at the receiver.

The HALT50005 was designed from the outset for this environment. Its 1.0 µH of nominal inductance, achieved through a precision 0.05 mm conical winding, provides sufficient low-frequency reactance to block signals down to 25 MHz—covering supervisory channel tones and low-rate telemetry that ride on the bias line in modern pluggable optics. The 250 mA continuous current rating accommodates the laser driver bias requirements of both EML and DML transmitter types, including the higher bias currents drawn by silicon photonics Mach-Zehnder modulators. And the >40 GHz SRF ensures the choke remains genuinely inductive—not capacitive—across the entire modulation bandwidth of current and next-generation optical standards.

Precision Winding at the 50-Micron Scale

Producing a 1.0 µH conical inductor with 0.05 mm diameter wire demands manufacturing precision that exceeds conventional coil-winding capabilities. At this conductor scale, the winding tension, turn placement, and lead formation are governed by surface forces and material elasticity rather than bulk mechanical properties. Hoan’s production line addresses these challenges through optically guided, closed-loop winding systems with 100% automated optical inspection:

• Taper accuracy: The turn-diameter-versus-position profile must conform to the design taper within ±5% along the full 3.0 mm winding length. Profile deviations create narrowband capacitance anomalies that generate S21 ripple.
• Turn spacing regularity: Adjacent turns must maintain uniform gap. Irregular spacing concentrates parasitic capacitance at discrete positions, producing unintended resonant modes within the rated operating band.
• Lead exit alignment: Both the apex and base flying leads are verified to exit along the central cone axis. Off-axis leads reduce automated wedge-bonding throughput and introduce orientation-dependent parasitic coupling.
• Insulation surface quality: The polyimide enamel coating is inspected at 100× magnification for thin spots, abrasion marks, and pinholes. Undetected insulation defects become latent turn-to-turn shorts after extended thermal cycling in the field.

Lot-level statistical process control is maintained with CpK values exceeding 1.67 for all critical dimensions. Each production lot undergoes sample-based VNA acceptance testing on a characterized microstrip fixture to verify SRF, insertion loss, and isolation compliance.

Validated Performance Across the Bias Tee Spectrum

Specification Reference

Optoelectronic Module Assembly Sequence

1. Apex connection: Narrow tip to 50-Ω microstrip. Coil perpendicular to board. Angular error causes S11 asymmetry above 30 GHz.
2. Lead length control: Apex flying lead between solder fillet and first turn ≤0.3 mm. Each additional 0.5 mm contributes ~0.3 nH series inductance, degrading high-frequency return loss.
3. Base to DC supply: Wide cone end to bias pad. Place 100 pF || 10 nF bypass capacitors within 1 mm.
4. Epoxy stabilization: Single micro-dot (≤0.3 mm) of Epotek H70E on the winding side. For hermetic optical packages, Epotek H70E meets ASTM E595 low-outgassing requirements.
5. Solder dwell: 280–320°C tip, 2 seconds maximum. The 50 µm wire reaches thermal equilibrium in milliseconds.

Deployment Scenarios

• EML/DML laser driver DC bias in 100G/400G/800G TOSA modules
• TIA supply bias in ROSA modules
• Silicon photonics MZM bias tees in coherent transmitter subassemblies
• High-speed waveguide photodiode reverse-bias in coherent receivers
• GaN-on-SiC SSPA drain bias feed in X/Ku/Ka-band radar transmitters
• 5G NR FR2 phased-array antenna element DC distribution

FAQ

Q: How does the HALT50005 differ from other 1000 nH conical inductor variants?
A: All HALT50005-series inductors share the identical core specification: 1000 nH, 0.05 mm wire, 25 MHz–40 GHz, 250 mA. The HALT50005 variant is specifically documented for optoelectronic bias tee applications, with assembly guidance for transceiver module integration, outgassing specifications for hermetic packaging, and application notes for laser driver and TIA bias network design.

Q: Our EML-based TOSA draws 230 mA of laser bias. Does the 250 mA rating provide adequate production margin?
A: At 230 mA, the conductor dissipation is approximately 106 mW (based on typical DCR of 2.0 Ω). The resulting ΔT is approximately 14°C—within the 15°C limit. The 20 mA headroom provides margin for unit-to-unit DCR variation and elevated ambient conditions within the module enclosure. No derating is required at this operating point.