“For a long time, engineers have not worried about synchronizing the trigger clock when conducting optical interface testing (especially optical module testing), either from the pattern generator (Pattern Generator), or using the clock recovery unit (Clock Recovery) from the tested clock. recover the clock on the signal. The former has a great cost advantage in production testing, while the latter is mostly used for research and development, pursuing the ultimate test effect. However, with the continuous improvement of the speed of optical interfaces, especially the multi-channel optical interfaces above 25GBps.A re-timer unit is added to each signal path, that is to say, the signals of different paths in the same optical interface (optical module) are actually different.
For a long time, engineers have not worried about synchronizing the trigger clock when conducting optical interface testing (especially optical module testing), either from the pattern generator (Pattern Generator), or using the clock recovery unit (Clock Recovery) from the tested clock. recover the clock on the signal. The former has a great cost advantage in production testing, while the latter is mostly used for research and development, pursuing the ultimate test effect. However, with the continuous improvement of the speed of optical interfaces, especially the multi-channel optical interfaces above 25GBps. A re-timer unit is added to each signal path, which means that different paths of signals in the same optical interface (optical module) are actually from different sources. That is to say, the synchronous clock and signal from the pattern generator (Pattern Generator) are not necessarily synchronized, which is especially obvious in the current PAM4 optical module test. After the actual comparison test, it is found that the test results using the clock recovery unit (Clock Recovery) are better than using the pattern generator (Pattern Generator) synchronous clock as a trigger. However, when engineers need to test at higher speeds, the price of clock recovery equipment can be very high, greatly increasing the cost of testing.
Figure 1: A typical multi-channel 25GBps optical transceiver, each channel has a retimer
As we all know, the inconvenience of sampling oscilloscopes is the natural advantage of real-time oscilloscopes: real-time oscilloscopes use an internal time base when capturing signals, and no external synchronization trigger clock is required. But real-time oscilloscopes always give the impression of low bandwidth, high noise, and large quantization errors. There is more terrible is: only supports electrical input. Fortunately, with the continuous innovation of technology, real-time oscilloscopes have undergone earth-shaking changes. Bandwidth is no longer a problem, noise is getting lower and lower, and the most important thing is that high-bandwidth optical probes with real-time oscilloscopes have appeared. Therefore, engineers can try to use high-performance real-time oscilloscopes to test the new generation of optical interfaces.
The combination of real-time oscilloscope + optical probe completely does not need to consider the external synchronization trigger signal, and directly completes the signal capture. The clock recovery is implemented in software, and the bandwidth of the phase-locked loop is more accurate and flexible (the bandwidth of the phase-locked loop can be arbitrarily set in the research and development situation). And the test connection is extremely simple, requiring only one fiber and no cables.
Figure 2: New real-time optical interface test solution
Figure 3: Simple configuration interface of real-time analysis software
Perfect test item support: In addition to supporting the conformance test items required by IEEE, it also supports many R&D test items: such as jitter, bit error location and bit error rate estimation.
Figure 4: In addition to the conformance test items required by IEEE, many R&D analysis items are also supported
Figure 5: Error detection and location
Comparison of test results:
The test results were very consistent.
Figure 6: 53GBd PAM4 test results using a real-time oscilloscope
Figure 7: PAM4 test results using a sampling oscilloscope
Discussion & Conclusion:
Real-time oscilloscope solutions have emerged, showing positive results both in terms of ease of testing connections and correlation of test results.
Real Time Vs Sampling: Pros and Cons?
Real Time Vs Sampling: Which Test Results Are More Realistic? Or can it better reflect the receiving effect of a real receiver? This is a question that every engineer is very concerned about.
Those who feel that the real-time solution is unreliable are basically worried about the vertical resolution and noise floor of the oscilloscope. I think the real-time solution is good mainly based on trigger jitter and more accurate clock recovery settings.
However, Tektronix’ new ATI oscilloscopes use asynchronous timing interleaving architecture to achieve 70 GHz and 200 GS/s (5 ps/sample) real-time acquisition performance. This patented symmetrical structure inherently has far superior noise than conventional bandwidth interleaving methods. The DPO70000SX offers the lowest noise, highest fidelity and maximum performance. The figure shows a 60 GHz sine wave jitter analysis applied to the ATI input. Results show clean eye diagram, random jitter RJ
Formally based on the high performance of Tektronix ATI oscilloscopes, ATI oscilloscopes have been widely used for complex optical modulation analysis, jitter and noise analysis for high-speed serial signaling and frequency, and phase and modulation analysis for broadband RF signals. For example, the Sino-German Green Photonics Research Center of Changchun Institute of Optics and Mechanics recently used Tektronix ATI oscilloscopes to conduct research on “low-energy VECSEL 200+ Gbps optical interconnection”.
The image below is from Gunter Larisch’s paper “Energy-efficient VCSELs for 200+ Gb/s optical interconnects”
Figure 8: Real-time oscilloscope helps research on low-cost, low-power 50GBps laser devices
In this paper, the real-time oscilloscope is used for the research of multi-mode low-cost, low-power laser devices. The DUT rate in the figure is 50GBps and has been transmitted over a 2-meter-long OM5 multimode fiber. Excellent signal integrity and flexible and comprehensive jitter analysis capabilities were the reasons Dr. Larisch chose a real-time oscilloscope.