SFP-USB a USB to Fiber optic adapter

In this article, I wrote about my SFP-USB project, in which my goal was to build a reliable and affordable adapter using commercially available components that could seamlessly convert USB data signals into optical signals for high-speed transmission. The heart of the adapter was designed around the ASIX AX88772B controller IC and an SFP (Small Form-factor Pluggable) fiber optic transceiver.

🧩 Project Objective and Motivation

The motivation behind this project stemmed from the increasing demand for data transfer solutions capable of operating at higher speeds over longer distances, without suffering significant signal loss. Copper-based USB cables have inherent limitations, including maximum distance and vulnerability to interference.

Therefore, I wanted to create an adapter that:

  • Converts USB data into optical signals.
  • Extends USB connectivity beyond copper cable limitations.
  • Utilizes standard components, enabling affordability and ease of assembly.
  • Operates reliably at 1 Gbps speeds with full-duplex communication.

🛠️ Components Selected for My Adapter

Careful selection of components was pivotal. After thorough research, I selected the following:

1. SFP Transceiver Module (Agilent HFBR-5701L)

  • Purpose: Transmitting and receiving optical signals.
  • Features: Hot-swappable, LC connector, 100 Mbps (100Base-SX), capable of distances up to 275 meters.

2. USB to Ethernet Controller (ASIX AX88772B)

  • Purpose: Essential interface converting USB signals into Ethernet format compatible with fiber optics.
  • Key features:
    • USB 2.0 high-speed operation (up to 480 Mbps).
    • Integrated MAC and PHY, versatile MAC and PHY operation modes.
    • Reliable performance with built-in Ethernet features like flow control and duplex operation.

3. Voltage Regulator (LM117)

  • Purpose: Provides a stable 3.3V power supply derived from USB’s 5V line.
  • Features: Compact SOT-23 package, thermal and short-circuit protection.

4. EEPROM (Microchip 93C66LC)

  • Purpose: Configures AX88772B IC to operate specifically in fiber mode instead of the default copper mode.
  • Benefits: Low power consumption, small size, programmable via EZP2019+ programmer.

5. P-Channel MOSFET (BSS138)

  • Purpose: Monitors Loss of Signal (LOS) from SFP module, alerting the AX88772B IC appropriately.

6. Fiber Optic Cable (Digitus D2533024 LWL Multimode)

  • Specifications: Multimode, 50/125µ diameter, 2m length, LC connectors.
  • Role: Connects SFP transceivers for data transmission.

7. Mini USB Connector

  • Function: Facilitates data transmission and power delivery from the host computer to the adapter.

8. Crystal Oscillator (25MHz)

  • Purpose: Ensures accurate timing and synchronization for Ethernet communication via AX88772B.

📐 Circuit Design and PCB Layout

I began by thoroughly examining ASIX's reference circuit schematics for the AX88772B IC to ensure optimal component placement and minimize signal interference. Utilizing EAGLE PCB software, I meticulously designed my schematic, prioritizing signal integrity and compactness.

To reduce potential noise and crosstalk:

  • Signal traces carrying high-speed data were shortened and carefully routed.
  • Ground planes were integrated on both sides of the PCB.
  • Small decoupling capacitors were strategically placed close to the power pins and critical signal lines.

The final PCB layout featured a clear, compact arrangement to support reliability and manufacturing ease.

🔍 Assembly and Detailed Testing Process

With my PCB professionally fabricated, the assembly phase involved precision soldering of each component using a soldering oven, carefully ensuring proper placement and avoiding solder bridges or shorts.

Once assembled, thorough testing was paramount. My testing approach involved:

  1. Visual Inspection: Using microscopes to identify potential soldering defects or component misalignments.
  2. Electrical Testing: Employing multimeters to verify connectivity and absence of short circuits.
  3. Signal Integrity Checks: Oscilloscopes ensured correct data signal waveform and frequency stability.
  4. Fiber Optic Verification: A fiber optic power meter confirmed that optical transmission met expected parameters.

🧪 Challenges Encountered

Despite rigorous design and testing processes, engineering projects seldom proceed without hurdles. My adapter encountered one major issue during the testing phase:

  • Data Reception Issue: The adapter successfully transmitted optical signals, recognized by both host computers and fiber optic testing equipment. Windows OS, WireShark software, and power meters confirmed data transmission. However, the receiving end failed to interpret these signals at the USB port level, preventing data from reaching the host computer's application layer.

I systematically performed extensive debugging to pinpoint this elusive issue. Efforts included:

  • Double-checking EEPROM configuration settings (ensuring fiber mode was correctly enabled).
  • Re-examining PCB traces and component interconnections.
  • Cross-verifying component datasheets to confirm pin assignments and functionalities.
  • Evaluating potential driver or software-level conflicts.

Despite considerable effort and rigorous troubleshooting, the exact source of the problem remained unresolved within the project timeframe.

🎯 Conclusion and Personal Reflections

This project presented a tremendous opportunity to delve deeply into high-speed fiber optic communication, advanced PCB design, and meticulous debugging. While the unresolved reception issue was disappointing, I gained invaluable experience in component selection, system-level integration, precision hardware design, and exhaustive troubleshooting strategies.

Building this USB to Fiber Optic Adapter reinforced the importance of patience, methodical analysis, and thorough documentation in engineering. Although the adapter didn't achieve complete operational success, the practical experience provided critical insights into real-world engineering challenges.

Looking forward, potential follow-up investigations could include deeper firmware/software explorations or consultation with component manufacturers for further insights into the unresolved issue.

Through this journey, I've grown considerably as an engineer, better equipped for future challenges and innovations.

Thank you for reading about my journey creating this USB to Fiber Optic Adapter!

— Devi Surya Teja Chilukuri

Share article on