With the rise of third-generation I/O technology, we have entered an era of high-speed data transmission. As systems become faster and more complex, maintaining signal integrity has become a critical challenge, especially when working with high-speed serial buses like PCI Express or SATA. This article explores the fundamentals and practical approaches to signal integrity verification through real-world examples.
In most electronic system designs, several key components are essential: power supply, clock signals, reset functions, and various bus interfaces. These signals act as the backbone of the system, playing a crucial role in ensuring its stability and overall performance. The quality of these signals directly reflects the reliability of the entire design. This is the core of signal integrity research, which focuses on analyzing and optimizing signal behavior to ensure robust system operation.
As technology continues to evolve, engineers face increasing challenges in handling high-speed signals. The development of advanced data buses brings new testing complexities that require deep technical knowledge and precise methodologies.
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Third-generation I/O technologies such as PCI Express have revolutionized data transfer by breaking through the bandwidth limitations of older standards, enabling more flexible and powerful system designs. While PCISIG provides detailed testing procedures, real-world systems often differ from theoretical models, requiring engineers to thoroughly understand each bus specification and tailor their testing accordingly. Similar challenges exist for other high-speed protocols like Fibre Channel, Infiniband, Gigabit Ethernet, 1394b, and USB.
For any bus or signal test, understanding the underlying technical specifications is crucial. An experienced engineer can quickly identify the right testing approach just by reviewing the specs. However, this requires not only knowledge but also access to appropriate instrumentation.
One of the first steps in testing is measuring the eye diagram. To do this, engineers must find the corresponding "eye mask" defined in the technical specifications. Each protocol has its own standard for this. Engineers can use pre-defined templates available from instrument manufacturers, but they should always verify that the template matches the actual standard. Sometimes, the provided templates may be incorrect, leading to unnecessary delays.
Once the eye mask is set, the next step is capturing the signal. This can be done by connecting the test signal directly to the instrument, using a dedicated test fixture, or accessing it through the device's output interface.
High-speed serial buses now commonly use encoding schemes like 8B/10B, along with features such as pre-emphasis and embedded clocks, making signal integrity even more critical. Understanding these concepts is essential for accurate testing.
In some cases, capturing the required signal during testing can be straightforward, such as with PCI Express or Fibre Channel, where compliance patterns are automatically generated. However, for other protocols like Ethernet or 1394, the process is more complex and often requires support from chip manufacturers. Users must know how to configure the chip’s registers to enter test mode and generate the necessary test patterns.
Probes play a vital role in signal capture, but selecting the right ones depends on factors like bandwidth, rise time, sensitivity, and sampling rate. Although many instrument manufacturers offer solutions, users need to understand these basics, as high-end test equipment can be costly.
Modern test instruments often come with user-friendly applications that allow users to run tests with a single click and generate reports automatically. However, over-reliance on these tools can be risky, as they are not free and may not cover all necessary parameters. At times, manual measurement and analysis are still required, highlighting the importance of an engineer’s expertise.
To illustrate, let’s look at a Fibre Channel signal test. The signal runs at 2.125 Gbps, so a 7 GHz bandwidth oscilloscope with a 20 GSPS sampling rate, such as the Tektronix TDS7704B, is suitable. A differential probe like the P7350 with 5 GHz bandwidth is used. It’s important that the oscilloscope includes a CDR (Clock Data Recovery) module, as high-speed serial buses embed their clocks within the data stream. Additionally, real-time serial data analysis software like Tektronix RT-eye is needed to process the data.
This software can perform a wide range of tests, including eye diagrams, jitter, rise and fall times, eye height, eye width, Jitter@BER, Dj, Rj, eye opening@BER, rate, UI, and more. It can also generate bathtub curves, histograms, and other visualizations. Figures 2 and 3 show test results for a 2.125 GHz Fibre Channel signal and a 2.5 GHz PCI Express bus signal, respectively.
When conducting such tests, there are several important considerations: the pattern length, the type of pattern used, the bandwidth of the phase-locked loop filter, and the amount of data required for BER (Bit Error Rate) measurements. All these factors contribute to the accuracy and reliability of the test results.
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