Understanding the effects of crosstalk in high speed communication circuitry and pin pointing the root causes have been extremely hard for designers, until now. Keysight’s industry leading N8833A and N8833B software not only identifies the crosstalk but also allows the user to determine the sources. In addition, it can remove the effects of the crosstalk and determine the recovered margin.
First let’s discuss the many types of crosstalk and their origins. Crosstalk has become an important issue as data rates have increased, and more and more lanes are being packed into smaller and smaller spaces. The amplitude interference of crosstalk impacts the signal fidelity of a communication eye diagram, essentially causing the eye to become more closed. The majority of crosstalk is caused by capacitive or inductive coupling between multiple transmission lines and/or power delivery networks. Prominent sources are near end crosstalk, far end crosstalk, power supply induced jitter, voltage dependent amplitude noise and simultaneous switching noise. Transmission line crosstalk is the result of electromagnetic interference between electrical components and is mainly caused by capacitive or inductive coupling. Forward traveling or far end crosstalk travels the same direction as the aggressor signal and its energy grows and it travels down the transmission line resulting in amplitude bulge in one area of the eye pattern. Reverse traveling or near end crosstalk is constantly moving away from the aggressor edge and is spread somewhat evenly over the transmission line resulting in a smearing of the entire eye pattern. Power supply aggressor crosstalk is created by noise on the power rail supply and caused phase noise changes or jitter. Voltage dependent amplitude noise crosstalk adds noise to the voltage and ground bus and causes non-linear effects on each logic level. A power supply can also be a victim of crosstalk due to simultaneous switch noise on serial lines and this is caused parasitic inductances lying between board and system ground and is also known as ground bounce. The above crosstalk origins, effects and sources may seem overwhelming at first but Keysight’s N8833 application greatly simplifies both the user knowledge and effort needed to get to root cause.
Legacy methods of determining crosstalk digital communications systems has relied on the process of selectively disabling some channels while enabling others. This process usually took significant time and effort. Power supply noise adds yet another analysis hurdle creating a non-linear transfer on the serial data timing called the Time Interval Error and has been difficult to solve and correlate. Past troubleshooting methods also required special design and test modes to analyze the crosstalk. Another challenge is that many times crosstalk aggressor signals are created within a package or in a system that is not accessible to probing.
The new Keysight crosstalk analysis application meets all of the above challenges by:
1) identifying the sources of crosstalk affecting the victim,
2) quantifying how much each aggressor is disrupting the victim,
3) removing the effect of crosstalk from the victim signal for analysis and
4) checking how much design margin is recovered when crosstalk is removed from victim.
Other features of the Keysight software are:
1) analyzes up to four signals (aggressors or victims) at once,
2) requires no crosstalk simulation or model,
3) identifies and reports the amount of crosstalk present on victims from each aggressors,
4) plots waveforms without crosstalk,
5) compares them with the original waveforms using scope tools such as eye diagram and jitter separation to see how much margins can be recovered.
The key types of crosstalk that can be analyzed are:
1) transmission line aggressors: Near-End Crosstalk (NEXT) and Far-End Crosstalk (FEXT),
2) power supply aggressors: Power Supply Induced Jitter (PSIJ) and voltage-dependent amplitude noise,
3) power supply victim: Simultaneous Switching Network (SSN).
Finally the software can report the results in different ways:
1) Inter-Symbol Interference (ISI) magnitude of the victim on itself,
2) crosstalk magnitude of the transmission line aggressors on the serial data victim,
3) crosstalk magnitude and jitter of the power supply aggressors on the serial data victim,
4) crosstalk magnitude of the transmission line aggressors on the power supply victim.
The Keysight crosstalk application N8833A/B is the most comprehensive solution in the market enabling a closed loop design cycle, saving designers both time and money. This application effectively solves the challenges mentioned above by enabling designers and engineers to: 1) identify which signals are coupling into your victim signal, 2) quantifying how much error each aggressor signal adds to your victim signal, 3) see what the victim signal would look like without the crosstalk and how much eye margin can be recovered without the crosstalk, 4) determine if the existing crosstalk justifies a design change and where to improve the circuit or system design.