How to use an oscilloscope to collect high-speed serial bus signals?

The characteristics of the measurement equipment may cause a normally working DUT to not meet the conformance requirements. The acquisition system, including probes, cables, and oscilloscopes, must allow sufficient signal energy to be acquired. The following are the key factors for a good acquisition system:

Bandwidth

●Sufficient input channels

Sampling rate

● Sample point record length

Bandwidth requirements

Many first-generation serial bus architectures have a rate of 1.5Gbps to 3.125Gbps, and the fastest clock frequency is 1.56GHz. An oscilloscope with a bandwidth of 4 to 5GHz can be used for measurement. However, signal fidelity testing and accurate eye diagram analysis require an oscilloscope with a higher bandwidth. Many standards have realized the importance of this.

Many specifications require the oscilloscope to accurately capture the 5th harmonic of the signal fundamental frequency at the signal transmitting end chip pin. For the first generation of PCI Express, this means that a 6.25GHz oscilloscope is required with a sampling rate at least twice the bandwidth. Similarly, for a 3.125Gbps XAUI signal, an oscilloscope bandwidth of 7.8GHz is required. Many standard organizations, especially PCI-SIG, directly require that the bandwidth of the test instrument meet the principle of the 5th harmonic of the signal.

As the second generation of serial buses develops, the bandwidth requirements for oscilloscopes are also increasing. Table 1 lists the current and future requirements for oscilloscope bandwidth for serial bus compliance testing.

Table 1: Oscilloscope bandwidth requirements

Capturing the 5th harmonic is critical

Oscilloscope bandwidth is the key factor affecting eye diagram testing. The Fourier expansion of a square wave shows that the signal is composed of a fundamental wave and harmonics of various orders, as shown in Figure 1. The change in the amplitude characteristics of the signal illustrates the contribution of different harmonics to the signal level.

Figure 1: Contribution of each harmonic order to the synthetic square wave

An oscilloscope with insufficient bandwidth will cause the attenuation of the 5th harmonic amplitude of the signal when acquiring the signal. The 5th harmonic contributes to the increase in the eye height test because the data judgment is performed at the 50% position of the UI.

Figure 2 illustrates the importance of bandwidth for eye diagram analysis. Using two oscilloscopes with bandwidths of 20 GHz and 13 GHz to measure the same 6.25 Gbps signal, the eye diagram test is significantly different, and the test results will be reported in very different ways.

Figure 2: Effect of oscilloscope bandwidth on eye diagram analysis

Bandwidth and rise time

Usually, the bandwidth requirements of the standard are more based on the rise time of the signal. Table 2 lists the oscilloscope bandwidth requirements required to perform high-precision tests on signals with different rise times.

Table 2: Bandwidth and rise time measurement accuracy

The rise time of the oscilloscope and probe will affect the measurement of the signal rise time. The following formula shows the mathematical relationship between the oscilloscope, probe rise time and the signal's true rise time and the measurement result. The influence of the probe and oscilloscope on the signal rise time test must be removed from the test results.

For example, if the rise time index of the test system 20%-80% is 65ps, and the rise time of the measured signal is 75ps, then the measurement result is 99ps. This is a test error that cannot be ignored. Therefore, in order to more accurately test the rise time of the signal, an oscilloscope and probe system with a faster rise time should be selected.

Multi-channel acquisition

For example, HDMI, a high-speed, low-cost medium and multi-channel serial bus architecture, is very prone to crosstalk and time deviation between channels. Oscilloscopes provide the ability to simultaneously collect signals in real time, reducing the complexity of testing and improving the efficiency of verification and debugging.

Capturing signals on multiple channels in a time-correlated manner makes it easier to understand the cause of a fault. For example, in the case of HDMI, recording all serial signals simultaneously not only captures the fault completely, but also reveals the impact of the fault on the entire link.

Sampling rate and record length

In addition to requiring the oscilloscope to have sufficient bandwidth to acquire signals, the oscilloscope must also be able to record enough waveform information at a high enough sampling rate for analysis. Each standard specifies the minimum amount of data storage. Of course, the longer the waveform recorded, the longer the time interval that can be analyzed for signal anomalies. High-performance oscilloscopes have high bandwidth, faster sampling rates, and longer recording memory, all of which will benefit high-speed serial system analysis.

Nyquist sampling theorem states that to perform alias-free sampling, the sampling rate must be twice the highest frequency component of the analog signal. As signal rates exceed 20Gbps, devices with sampling rates exceeding 40GS/s can meet test requirements.

Advanced oscilloscopes with large storage depth, combined with jitter analysis software, can reveal more details in the signal and provide reliable assurance for the design. For example, for the PCIe1.0 standard, a minimum of 250 UIs must be collected for consistency testing. Advanced oscilloscopes capture and store 1M continuous UIs, and any 250 continuous UIs can be analyzed, providing a better assessment of signal quality.

For multi-channel serial signal acquisition, the oscilloscope must perform high sampling rate and long storage for each channel to ensure that the most signal details are acquired.