Optical module bit error rate meter coaxial cable Tx level

Bit error rate (BER) testing of optical modules over coaxial cables requires careful Tx level adjustment and signal equalization to compensate for cable losses and ensure accurate measurements.Understanding BER Testing

Bit error rate (BER) is a measure of signal integrity, representing the fraction of transmitted bits received incorrectly. BER testing involves sending a predefined data stream, often a pseudorandom binary sequence (PRBS), through the optical module and coaxial cable, then analyzing the received signal to calculate errors relative to transmitted bits. Typical acceptable BER values range from 10^-9 for telecom systems to 10^-13 for high-speed data transmission .

Role of Coaxial Cables

Coaxial cables introduce attenuation and frequency-dependent losses, which can degrade the signal and increase BER. To mitigate this, modern BERTs provide Tx de-emphasis and continuous-time linear equalization (CTLE). Tx de-emphasis reduces high-frequency overshoot to compensate for cable loss, while CTLE at the receiver boosts attenuated high-frequency components, improving signal fidelity .

Tx Level Adjustment

The transmitter (Tx) output level is critical for BER testing. Setting the Tx level too low can result in insufficient signal amplitude, increasing errors, while excessively high levels may cause distortion or reflections in the coaxial cable. BERT instruments allow programmable Tx amplitude and de-emphasis settings to optimize the signal for the specific cable length and type .

Practical Setup Tips

• Use a high-quality coaxial cable with known loss characteristics.
• Configure the BERT to match the data rate of the optical module (e.g., 10 Gb/s, 25 Gb/s, 30 Gb/s).
• Apply Tx de-emphasis and CTLE to compensate for cable loss.
• Select an appropriate PRBS pattern (e.g., PRBS7, PRBS31) to stress the link and reveal potential errors.
• Monitor BER over a sufficient number of bits to ensure statistical confidence.

Recommended Equipment

• MATRIQ BERT 1001/1005 series: Supports up to 30 Gb/s, programmable 7-tap Tx de-emphasis, and CTLE for coaxial cable compensation .
• OptoBERT OPB-BERT-400G-P8: 8-channel BERT with input equalization algorithms for coaxial cables, suitable for R&D and manufacturing .
• PXI BERT modules: Compact multi-channel solutions with integrated Tx de-emphasis and CTLE for high-speed optical transceivers . By carefully adjusting the Tx level and using equalization features, BER testing over coaxial cables can accurately characterize optical module performance, ensuring reliable high-speed data transmission.