N8833A and N8833B Crosstalk Analysis Application for Real-Time Oscilloscopes
Increased data communication speeds as well as the increased circuit density of mobile devices has led to very high serial data rates on multiple lanes, placed very close together. This combination of higher bit rates, spaced tightly together, leads to an increased amount of crosstalk distortion between serial data signals. As a result, crosstalk is becoming a more important problem to diagnose and quantify. Integrated circuit power supplies are also increasingly susceptible to crosstalk at these higher serial data rates. Simultaneous switching noise (SSN) and ground bounce from serial data signals and switching power supplies can create perturbations on the supplies that distort the data lanes they drive in the form of noise and jitter.
The need to troubleshoot and characterize crosstalk is not new, but the legacy methods of measuring crosstalk in digital communications systems has relied on the process of selectively disabling some channels while enabling others. This necessarily requires measuring the crosstalk effects in the system while operating in special test modes, which means measuring them under abnormal conditions. Worse yet, some systems cannot even operate the necessary special modes.
Keysight Technologies, Inc. has developed a crosstalk analysis application to assist in the diagnosis and quantification of crosstalk that does not require the system under test to operate in any special test modes. The application works by constructing optimal models of the crosstalk mechanisms between multiple measured signals. The user, for example, can acquire up four signals simultaneously of a running system and then configure the application to calculate the optimal crosstalk models between them. Once these models are known, various victim signals can be displayed with and without the crosstalk from each aggressor. In this way, noise, jitter and eye-diagram measurements can be made on the victim with and without crosstalk to quantify the amount each aggressor contributes to each of these measurement values.
Features of the crosstalk analysis application include:
In another scenario, the user could move around their running system by probing various suspected aggressor signals while monitoring their victim signal until they identify which suspected aggressor signals in their system are corrupting their victim signals.
The application not only detects and quantifies the presence of crosstalk, but it can also determine the relative magnitude of error each aggressor imparts to the victim. It can also go one step further by actually removing the crosstalk from the victim so you can visually compare the original waveform with the clean waveform side-by-side. You can compare the “before” and “after” waveforms directly on the scope display or by comparing the results from other scope analysis tools such as real-time eye diagrams or jitter analysis. This approach gives you a direct way of quantifying the amount of improvement you can expect by mitigating the different sources of crosstalk.
The crosstalk analysis application can provide a lot of valuable insight into your design. For instance, it can help engineers determine the margins the design would recover without the crosstalk. It can also help determine if a signal that fails design specification can now pass without the crosstalk. This approach can lead to important design decisions on whether it is worth the time and effort to improve the crosstalk effect and where in the board to make improvements.
Types of Crosstalk
Transmission line crosstalk
Parallel transmission lines are prime candidates for crosstalk. The mutual inductance and capacitance between them allows energy to couple from one lane into another. The voltage generated by capacitive coupling creates a current that travels in both directions, whereas the inductive coupling creates a current that travels only in the reverse direction. When added together, the two currents reinforce each other in the reverse direction but cancel each other out (at least to some extent) in the forward direction. The reverse traveling wave returns back to the transmitter end and is called “near-end” crosstalk (NEXT), while the forward traveling wave arrives at the receiver end and is called “far-end” crosstalk (FEXT). The magnitude and shape of these waveforms are quite different from each other.
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