High-speed, all-round analysis based on an oscilloscope

When debugging high-speed signals, engineers must first debug and verify whether their designs meet physical layer specifications. At this stage, signal integrity (such as eye diagrams and jitter) is a key issue, and much of this verification and debugging is accomplished by using pseudo-random code sequences (PRBS) or cyclic test codes in combination with oscilloscopes and serial data eye diagram and jitter analysis software provided by oscilloscope manufacturers.

After ensuring that there are no problems with the quality of the physical layer signal, the serial signal changes from the test code to the 8b/10b encoded character sequence. At this time, the system-level problem becomes the focus of debugging. The problem may occur in the physical layer-link layer domain (involving the intersection of signal integrity and data integrity). At this time, it is necessary to decode and analyze the physical layer signal.

For modern high-speed serial systems, coordination between systems becomes more prominent, and any conflict between protocols will cause problems in the entire system. Therefore, it is often not enough to analyze the physical layer and link layer. The protocol layer of the system must also be analyzed, and a dedicated protocol analyzer is often required. This article will focus on the solutions of LeCroy oscilloscopes for the physical layer, link layer, and protocol layer of high-speed serial signals.

Analysis of the transmission process of high-speed signals

In order to ensure good signal transmission quality, high-speed serial data signals often need to be encoded before transmission. The following figure 1 shows a simple transmission process of a serial signal. The signal at the transmitting end is first scrambled and 8b/10b encoded. After the processed signal is transmitted through the transmission link and enters the receiving end, it needs to be decoded by 10b/8b and scrambler. We need to observe the signal on the transmission link, so what we observe is the scrambled signal and 10b signal after encoding.

Figure 1 Simple transmission process of serial signal

8b/10b encoding is a very common encoding method currently used by most high-speed serial signals. For example, SATA and PCIE GEN1/2 all use 8b/10b encoding. Using 8b/10b encoding can ensure the DC balance of the circuit (keeping the density of 0 level and 1 level balanced), so that the system can more accurately recover the ideal clock from the data, and can also effectively reduce inter-code interference jitter, and minimize the probability of system errors. In addition, usually 8 bits represent a data bit. If all bits are used to represent data, there will be no extra bits for code synchronization. Therefore, another benefit of 8b/10b encoding is that it can provide extra bits as synchronization codes, such as the common K28.5, K28.3 and other code types. Figure 2 is an example of 8b/10b:

Figure 2 8b/10b decoding

The number of data bits is increased from 8 to 10, and the level characteristics that appear less frequently in the original data bits are increased after encoding.

Figure 3 shows the decoding process of the signal collected on the signal transmission link. Using an oscilloscope, you can directly observe the physical layer waveform at the top of Figure 3. If you want to observe 10b decoding information and Scramble decoding information, you need to use the dedicated high-speed serial signal decoding and analysis software provided by the oscilloscope manufacturer.

Figure 3 Serial signal decoding example

Further analysis can obtain the information of the protocol layer, as shown in Figure 4.

Figure 4 Protocol layer information of serial data

Comprehensive testing solutions

Physical layer test analysis (1) The real-time serial data analyzer (real-time oscilloscope) with up to 45GHZ bandwidth, 120GS/S sampling rate, and 768MS analyzable storage depth is not only suitable for all current serial data standards, but also provides sufficient bandwidth and sampling rate guarantees for the test solutions of the next generation of high-speed serial data standards.

(2) SDAII professional signal integrity analysis software package, long storage data eye diagram test speed is extremely fast (20M data eye diagram analysis only takes 1-2 seconds), and provides Spectrum and NQ-Scale two jitter algorithms, of which NQ-Scale algorithm is a patented algorithm of LeCroy, which can realize the separation of inherent jitter hidden in random jitter. This type of jitter is often caused by crosstalk caused by high-order harmonics and long pseudo-random codes. Other algorithms in the industry usually treat this type of jitter as random jitter. SDAII provides engineers with very powerful debugging functions for high-speed serial signals.

(3) The Qualify conformance test software package can test whether the physical layer characteristics of high-speed serial signals meet the specification requirements, automatically issue reports in multiple formats, and complete all tasks with one click. [page]

(4) EyedoctorII professional signal integrity analysis software can realize a full range of analysis functions for high-speed signals, including channel simulation, fixture de-embedding, pre-emphasis, equalization, virtual detection, etc.

Link layer and protocol layer analysis (1) LeCroy's professional decoding and analysis software package based on a real-time oscilloscope. As shown in Figure 5, the information decoded by the oscilloscope can distinguish different code types with different colors, and the data bits and command bits are clear at a glance. The interface display is very clear. The decoded information is not only displayed in the form of data, but also in the form of a list, which is easy to find. It supports the decoding of commonly used high-speed serial signals such as 8B/10B, SATA, SAS, PCIE, XAUI, and also supports the decoding of the latest USB3.0. The decoded information can be exported as a CSV file for offline analysis. It can decode and analyze four signals at the same time.

Figure 5 LeCroy oscilloscope decoding example

(2) LeCroy's dedicated protocol layer analysis software package based on real-time digital oscilloscope. Since LeCroy has protocol analyzer products for high-speed signals, its protocol analysis software has powerful protocol analysis and error correction capabilities. In order to allow engineers to observe relevant information of the protocol layer while performing physical layer tests, LeCroy has specially developed a protocol analysis function for LeCroy's high-end oscilloscopes. This function can synchronize the oscilloscope with the protocol analyzer software, and send the signal collected by the oscilloscope to the protocol analysis software to perform protocol analysis of serial signals.

The specific implementation method is to install the software ProtoSync and the corresponding serial signal decoding software such as PCIE_D, USB3_D, USB2_D in the LeCroy oscilloscope, and then install the analysis software of the LeCroy protocol analyzer in the oscilloscope to realize the protocol analysis of high-speed serial data. Figure 6 shows the protocol layer analysis results of the PCIE signal by the LeCroy oscilloscope:

Figure 6 LeCroy oscilloscope's analysis of the PCIE signal at the protocol layer

summary

This article briefly introduces the test solutions and features provided by LeCroy's high-speed serial data oscilloscopes, especially its integration into the protocol analysis software, which makes the oscilloscope more comprehensive. It can not only conduct in-depth analysis of the physical layer signal, but also analyze the deeper protocol layer of the serial signal. This provides engineers with a more comprehensive and faster test solution for debugging high-speed serial signals.