June 9, 2016
As data rates continue to grow and circuits continue to shrink, signal channels behave more like complex transmission lines. For this reason, vector network analyzers (VNAs) have become a staple in signal integrity analysis, however, not all VNAs are created equal. Signal integrity (SI) engineers working on high-frequency designs, including those for 5G and IoT, should look for new capabilities in processing and visualizing data that are beneficial when conducting channel diagnostics and model validation of high-speed digital circuit designs.
An ‘Eye’ on a Solution
One challenge SI engineers often face is the ability to find correlation between VNA measurements and bit error rate (BER) performance. While S-parameters provide an overall indicator of transmission quality through insertion loss and return loss measurements, they do not automatically translate into BER performance. For this reason, engineers commonly use an eye diagram to combine the S-parameter measurement in the form of a SnP file and add other data transmission effects, such as bit rate speed, anticipated jitter of the system, and noise level of the system, to ultimately provide a qualitative measure of data transmission. The shape of the eye diagram, therefore, provides a means to quickly approximate BER without the need to use a separate bit error rate tester (BERT).
Another objective for the SI engineer is to quickly move between S-parameter measurements and eye diagram measurements, so he can tune the transmission channel while monitoring both the RF performance and eye diagram. Typically, VNAs do not show eye diagrams on the instrument display. Engineers must take a SnP file and transfer it to an external software program to evaluate the performance. This is a very cumbersome and time-consuming process.
Some lower frequency VNAs can display the eye diagram on the instrument but they still use file-based rather than trace-based procedures. This means that the engineer must save the SnP file, store it in memory, and call it up in the eye diagram display software - still adding unwanted time to the measurement process. Plus, their frequency coverage limitations make them ill-suited for many emerging designs operating in the microwave and millimeter spectrums.
To solve this quandary, the Anritsu VectorStar® VNA can provide a trace-based eye diagram display and have the ability to supply all key parameters, such as an eye diagram, time domain reflections, and S-Parameters, on the same channel (figure 1) while continuously sweeping. Using a VNA with this capability eliminates the need to constantly store the S-parameter data in a file and then recall the file to update the eye diagram. The result is a significant timesaving advantage, as the engineer has the ability to tune discontinuities while monitoring eye diagram performance without switching instruments or relying on an external software program.
Advanced Time Domain
Another tool in VNAs that SI engineers can take advantage of is an Advanced Time Domain (ATD) capability, such as one available as an option in the ShockLine Performance VNAs. It is particularly useful when building circuit models and conducting validation measurements on them, as well as troubleshooting SI issues. ATD in a VNA can allow engineers to plot eye diagrams, determine single-ended or differential near-end crosstalk (NEXT) and far-end crosstalk (FEXT), and apply various equalization techniques.
With ATD, engineers can select among several IEEE and OIF specifications and compare the power sum of coupled noises, insertion loss crosstalk ratio (ICR), insertion loss, insertion loss deviation (ILD), and integrated crosstalk noise (ICN). Additionally, they are able to convert S-parameter data into an impedance profile, a time domain reflection (TDR) with an open end or a TDR/time domain transmission (TDT) with matched terminations.
VNAs will continue to integrate testing tools to help SI engineers confidently verify their high frequency designs for emerging commercial and military applications. To learn more about these tools and which VNA is best for your application, visit this VNA page or download this VNA literature.