December 21, 2016
Higher data rates and more complex modulation schemes are requiring digital engineers to consider the analog and RF performance of the channels to a much greater degree than in the past. Moreover, increasing demands are requiring digital engineers to move from oscilloscopes and TDRs to vector network analyzers (VNAs) because the latter test instruments are very useful in determining the actual cause of a signal integrity issue. Certain attributes are necessary, however, when selecting a VNA to ensure the most accurate measurements on these high-speed designs.
VNAs are used by design engineers to get a better understanding of the cause of eye closures in high data rate systems. For the most accurate results, VNAs with wide frequency bandwidths should be used. For example, Anritsu offers a VectorStar® broadband VNA system that can conduct channel characterization over a frequency range of 70 kHz to 145 GHz. This wide bandwidth helps build accurate models by including multiple harmonics and providing a low-end frequency for better DC extrapolation. It also provides very accurate resolution in the time domain for locating defects in the channel.
Improved PCB Testing
One signal integrity application in which VNAs are becoming essential is to understand the physical structures and their imperfections. For example, no manufacturing process is perfect when it comes to printed circuit boards (PCBs). VNAs allow engineers to analyze real-world channel defects, such as exceeding tolerances on PCB artwork, plating, and dielectric thickness variations. To better evaluate connector performance, construction and how well they are mounted, as well as to analyze multilayer PCB stack ups to find imperfect vias or ground plane issues, engineers are turning to a VNAs, such as the ones shown in figure 1.
Most signal integrity channels use test fixtures during their characterization. VNAs have capabilities that make it easier for engineers to accurately determine the effects of these fixtures. Network extraction creates a model that is used to minimize the effect of the fixture. De-embedding applies the model to remove the effects of the fixture on the measured results. Fixture embedding/de-embedding allows network element effects to be inserted to verify performance.
The VectorStar architecture provides advanced tools in these scenarios. Hundreds of calibration combinations due to hybridized calibrations and seven network extraction methods are possible. Additionally, a full circuit and file-based embedding/de-embedding engine is standard in the VNAs to better match calibration and de-embedding methods to DUTs and fixture structures to more successfully de-embed fixtures and probes.
In addition, VNA results can help correlate simulations to measured results. Many design engineers are using EDA tools to simulate their channel before they are built. This can speed the time to market for a design. Verifying simulations can provide insight into the expected performance of your design.
Frequency Range Matters!
The bit rate increase places greater emphasis on the upper frequency limit capability of a VNA to more accurately evaluate backplane and interconnect transmission characteristics. Higher speeds basically translate into higher test frequencies being required to perform measurements to the 3rd or 5th harmonic of the NRZ clock frequency. For example, a 28 Gbps data rate equates to either 42 GHz or 70 GHz stop frequency for an S-parameter sweep.
Attenuating the harmonics of the clock frequency will distort the signal and hence the need to characterize the frequency response of transmission media to higher frequencies. Today, many engineers are working with 56 Gigabit NRZ types of modulations. The 5th harmonic at 56 GB/s NRZ signal is 140 GHz (Figure 2). Anritsu’s broadband millimeter wave system – the VectorStar® ME7838D – has a single sweep range of 70 kHz to 145 GHz to characterize these channels with a high degree of accuracy.
It is important to remember that accurate measurements to the lowest possible frequency are also very important for signal integrity applications. Often the accuracy of a model can be improved by measuring down to as close to DC as possible. Consider the case in which the measured S-parameter data for a backplane is fed into a software model to estimate the impact of that backplane on the eye pattern. Figure 3a shows what the eye pattern estimate will look like when the low frequency data has some error.
The error may be from poor dynamic range at lower frequencies or from extrapolation errors when the data doesn’t exist. In this example, it was found that a 0.5 dB error injected at a lower frequency (<10 MHz) on transmission could take an 85% open eye to a fully closed eye. Since mid-band (10 GHz) transmission uncertainty may be near 0.1 dB, depending on setup and calibration – and higher at low frequencies – this eye distortion effect cannot be neglected. Figure 3b shows the resulting eye pattern if the low frequency measurement data is of good quality and extends down to 70 kHz, as is the case with VectorStar VNAs. This prediction correlates very well with the actual eye pattern measured using an oscilloscope as shown in Figure 3c.
Since the non-transitioning parts of the eye-diagram are inherently composed of low frequency behavior, the sensitivity of the calculation to the low frequency S-parameter data makes sense. Because the low frequency insertion losses tend to be small, a large fixed-dB error (which is how VNA uncertainties tend to behave) can be particularly damaging.
Superposition vs. True Mode Stimulus
Another signal integrity application in which VNAs are invaluable is to characterize balanced/differential transmission lines. There are two approaches to performing these measurements and the selection of the best method depends on what you need to measure.
The “superposition technique” relies on the inherent linear nature of a transmission line and mathematically derives the differential and common-mode transmission line characteristics through superposition while stimulating just one side of the differential transmission line at a time. For situations where the transmission signal is propagated through a non-linear active device, stimulating one side at a time will not produce accurate measurements. The alternative uses two sources to create actual differential and common-mode stimuli, hence the shortened name of “true mode stimulus” (TMS).
Most signal integrity measurements are of passive components and the superposition technique is widely accepted to generate the required differential and common-mode responses from a differential device. Even active devices, if kept in their linear region, can be accurately tested using the superposition technique. For active devices that are driven into compression or saturation, the TMS technique is required.
A Guide to Making RF Measurements for Signal Integrity Applications is a new white paper that provides a great deal more detail on the benefits of VNAs to verify high-speed designs. You can also learn more by visiting this Signal Integrity page.