N4917BSCB Optical Receiver Stress Test Solution

Introduction

Complete optical receiver stress test solution for 400GbE optical transceivers with automated stress eye calibration and performance compliance testing.

Optical Receiver Stress Test for 400 Gb/s Ethernet

The telecommunications industry represented by the IEEE decided to address the steadily increasing need for more bandwidth at a lower cost for the intra and inter data centers by combining the spectral efficient PAM-4 modulation with the mature direct modulation/direct detection technology. The shift from NRZ to PAM-4 modulation effectively doubles the line rates, as compared to optical 100 Gigabit ethernet transceivers, while maintaining modulation speed at 26.56125 Gbaud and enabling continued use of some of the existing 100 G components.

Consequently, the compliance test procedures defined for next-generation 400GBASE transceivers are similar to those adopted in IEEE 802.3ba for NRZ-based 100GBASE transceivers. But there are noticeable differences:

• A new TDECQ metric is employed to characterize the quality of a transmitted/received signal instead of the traditional eye mask analysis.
• A digital reference equalizer is required to compute various signal metrics during transmitter performance testing or during stress signal calibration for receiver stress testing.
• Because of the significant sensitivity penalty resulting from the shift from NRZ to PAM4, the optical transceiver is not expected to operate error-free under the stress conditions defined by the standards or during typical use, while forward error correction (FEC) is typically performed outside the transceiver module. In addition, some flavors like 400GBASE-DR4 are based on 53.125 Gbaud, increasing the requirements for test and measurement equipment. Therefore, achieving accurate, stable and repeatable stress signal calibration, to ensure reliable transceiver performance test and qualification, has become even more challenging. Optical receiver stress test procedures, defined by the IEEE, are performed using several instruments such as a bit error ratio tester, digital sampling oscilloscope, optical reference transmitter and tunable laser source. The purpose of the test is to generate a stable and repeatable stressed optical signal with specific characteristics and send it to the receiver under test to measure the resulting bit error ratio. However, achieving this is not a trivial task as the combination of different stress factors (inter symbol interference, jitter, sinusoidal interferences, Gaussian noise, optical power level) gives rise to complex dependencies on the target metrics.

Keysight’s N4917BSCB software enables a complete test solution from instrument configuration and

control to automated stressed signal calibration and system performance test, according to IEEE

802.3bs/cd and db specifications (clauses 121, 122, 124, 138, 140 and 167) for following standards:

• 200GBASE-FR4/-LR4/-DR4, 400GBASE-FR8/-LR8/-DR4 (IEEE 802.3bs)
• 50GBASE-FR/-LR/-SR, 100GBASE-DR (IEEE 802.3cd)
• 100/200/400GBASE-SR/Sr2/SR4 (IEEE 802.3db)

Automation of the stress signal calibration and conformance tests results in considerable time savings. In addition, the N4917BSCB optical receiver stress test solution provides:

• Automated calibration of the optical stressed eye according to IEEE 802.3bs/cd/db clause 121, 122, 124, 138, 139 and 140
• Adjustable target values for outer extinction ratio (OER), transmission and dispersion eye closure (TDECQ), optical modulation amplitude (OMA) enabling user-defined stress signal calibration
• Repeatable and stable calibration of optical stressed PAM-4 eye
• Unified instrument control and setup
• Customized device testing and reporting via DUT control and scripting interface

Typical Setup for 200GBASE-LR4/-FR4 Optical Stress Test

The N4917BSCB optical receiver stress test solution consists of a M8040A BERT plus an arbitrary waveform generator for electrical signal and stress generation; an electro-optical converter that modulates the optical signal and a digital sampling oscilloscope which is required for calibration of the stressed eye.

An example setup for 200GBASE-LR4/-FR4 using four 50 Gb/s lanes on four wavelengths in the O-band is shown in Figure 2, which assumes the use of a 200 GAUI- 8 electrical interface. The IEEE 802.3bs standard establishes two ways to provide a clock signal to the digital sampling oscilloscope:

1. Using the ‘clean clock’ of the pattern generator or
2. Extracting it from the stressed signal using an external clock recovery

Refer to the configuration guide section for the detailed setup.

Optical Receiver Stressed Test Challenges

The IEEE 802.3 standard describes the optical specifications for optical receivers. The following tables contain the key characteristics 1 of the stress signal measured at TP3 (see Figure 2) to perform stressed receiver sensitivity and jitter tolerance compliance tests. Note that these values are subjected to change in future amendments of the standard. They can be adjusted using the “debug mode” of the Configure tab.