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N4917BSCA Optical Receiver Stress Test Solution

Data Sheets

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 400 GBase transceivers are similar to those adopted in IEEE 802.3ba for NRZ-based 100 GBASE 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 400 GBASE-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 N4917BSCA 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 specifications (clauses 121, 122 and 124) for following standards:

200 GBASE-FR4/-LR4/-DR4 IEEE 802.3bs

400 GBASE-FR8/-LR8 IEEE 802.3bs

400GBASE-DR4 IEEE 802.3bs and 400G-FR4 100G lambda MSA

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

The N4917BSCA 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.

SECQ/TDECQ

The Stress Eye Closure Quaternary (SECQ) metric is identical to TDECQ but refers to the stress signal used for the receiver stress test, while TDECQ is a metric for the transmitter. For stress signal calibration, the SECQ/TDECQ measurement should be performed using a SSPRQ test pattern captured by a sampling scope with a specific bandwidth and after digital equalization. Details can be found in IEEE802 clause 121.8.

Setting-up a stressed eye compliant with the standard’s specifications can be a very time consuming task because stressed eye parameters are interdependent and therefore several iterations of the optimization cycle are required to converge on the solution. In addition, it is important that the setup is repeatable and remains stable from initiation of the stressed eye calibration to the end of the DUT measurement.

The N4917BSCA optical receiver stress test solution provides a repeatable and stable measurement in a fraction of time compared to manual setup of the stress signal. This not only results in a major time saving during daily measurements, but also speeds-up development of a standard compliant test solution, when compared to a self-made solution.

N4917BSCA User Interface

The N4917BSCA user interface is structured to follow the generic workflow of an automated test application (Figure 5).

1. Set Up tab

Check connection to instruments (USB, LAN or GPIB connections are supported) and specify the standard to be checked. This step sets the default values for the stress signal metrics and performance targets listed in the Configure tab. You can deactivate the connection check of a particular device by selecting ‘not used’ in the corresponding Channel or Slot field. This lets you use the internal laser of the reference transmitter instead of the tunable laser source or deactivate one of the interference sources.

2. Select Tests tab

Select the actions or tests you want to perform. For example, you can perform a signal calibration, load settings from a previous calibration, measure characteristics of the current optical signal or perform automated performance measurements. These tests are performed one by one in the order they are listed. Additional functionalities, such as optimization of the reference transmitter bias and optical power adjustment, are available.

3. Configure tab

Specify key instrument settings (de-emphasis, max-min voltage, active ports) as well as the target value for the calibration metrics. The debug mode enables you to modify the original standard specifications, such as the TDECQ and ER of the stress signal or the jitter profile to be tested (see Figure 6.). It is also possible to adjust the calibration conditions to your own setup by deactivating the optical power control or accounting for additional loss present in the optical link to the DUT.

4. Connect tab

Displays the hardware connection diagram before the start of a test. This optional step allows the user to check the physical connections between the devices to ensure compliance with the standards.

5. Run and Automate tabs

Run the selected tests and measurements or use your own commands sequence implemented with a python script

6. HTML Report and Results tabs

Displays high-level and detailed measurement results. Some tests return a pass/fail value and others return detailed measurement results (e.g. jitter tolerance measurement).

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