VNAs With Wide Frequency Coverage Help Overcome High-Speed Interconnect Challenges

By Bob Buxton, Marketing Manager, General Purpose Test, Microwave Measurements Division, Anritsu Company

Cloud computing, smartphones, and LTE services are creating a significant increase in network traffic. To support this added traffic, speed of IT equipment – such as those used in high-end services in data centers – must be increased, creating challenges for signal integrity engineers that must be met by advanced test instruments, such as vector network analyzers (VNAs) like the one shown in figure 1.

Figure 1: The VectorStar Broadband ME7838A system with 3743A millimeter-wave module

Cost/Performance Trade-Offs

Higher data rates introduce new design challenges (such as conductor skin effects and dielectric losses on PC boards), along with design trade-offs related to vias, stackups, and connector pins. Evaluating a selection of backplane materials and the impact of various structures requires accurate measurement in both frequency and time domain. Accurate measurements provide the confidence to make cost/performance trade-off decisions. The aim is to evaluate the impact of interconnects on eye closure. Figure 2 shows an example of backplane impact on the eye pattern.

Figure 2: An example of backplane impact on the eye pattern

Some problems are caused by vias, stackup issues, and connector pins. However, frequency domain data alone is not enough to locate the position of particular problems. It is necessary to transform that data into the time domain. Passive components, as well as near- and far-end points between daughter boards, must be measured in the frequency and time domains to assure that the transmission characteristics at each measurement point meet the standards. Using the best resolution capability improves the ability to locate discontinuities, impedance changes, and crosstalk issues. In addition, many of today’s structures are electrically large and put pressure on the measurement solution’s alias-free range.

Accurate models help accelerate a design cycle. However, models are only as good as the data fed into them. Poor causality results in reduced confidence in simulations, potential convergence problems, and inaccuracies. Conversely, poor low-frequency information leading to DC extrapolation errors also degrades model accuracy and leads to poor agreement with 3-D EM simulators.

There are many situations where it may not be possible to connect directly to the device under test (DUT). In these cases, it is necessary to de-embed the DUT from the surrounding test fixtures. The opposite is sometimes required: It may be useful to take a device and assess its performance when it is surrounded by other networks. However, many passivity and causality problems are due to poor calibration and de-embedding methods. In addition, high fixture loss may affect the accuracy and repeatability of de-embedding. Fortunately, the latest VNA technology can provide a solution to these challenges.

Maximizing Frequency Range

The lower and upper frequency limits of an S-parameter characterization of a backplane or other interconnect impact the quality of the data and any subsequent modeling, but for different reasons. The upper frequency range is what usually comes to mind first, and many people perform measurements to the 3rd or 5th harmonic of the NRZ clock frequency. For a 28 Gbps data rate, this means either a 42 GHz or 70 GHz stop frequency for an S-parameter sweep. Another way to think about the requirement for the upper measurement frequency is causality. When S-parameter data is transformed into the time domain for use in further simulation, causality errors can arise.

While massaging the frequency domain data can reduce these problems, there are potential issues related to distorting the actual physical behavior of the device. It is often safer and more accurate to use as wide a frequency range as possible – up to the point where repeatability and related distortions (e.g., the DUT starts radiating efficiently, making the measurement very dependent on the surroundings) obscure the results. The desire for wider frequency range data becomes more compelling as faster and more complex transients are being studied in the higher-level simulations.

Figure 3: This is what the eye pattern estimate will look like where the low-frequency data has some error.

The lower frequency of the sweep is just as important. Model accuracy generally improves the closer data is acquired to DC. For example, consider the case where 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 3 shows what the eye pattern estimate will look like where the low-frequency data has some error. In this example, it was found that a 0.5 dB error distribution at 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 4: This is what the resulting eye pattern will look like if the low-frequency measurement data is of good quality and extends down to 70 kHz.

Figure 4 shows what the resulting eye pattern will look like if the low-frequency measurement data is of good quality and extends down to 70 kHz. This prediction correlates very well with the actual eye pattern measured using an oscilloscope, as shown in figure 5.

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.

Optimizing Time Domain Resolution

The time domain performance of a VNA is critical when trying to locate defects. In general, the wider the frequency sweep, the better the time and spatial resolution. Figure 5 shows the differences in time domain resolution for three different frequency spans: 40, 50, and 70 GHz. Resolution is maximized when low-pass time domain mode is used. Low-pass mode, which also permits characterization of impedance changes on the backplane, requires a quasi-harmonically related set of frequencies that begin at the lowest frequency possible. A DC term is extrapolated to provide a phase reference, so the true nature of a discontinuity can be evaluated. Hence, the lower frequency that the sweep can commence, the better the extrapolation of the DC term.

Figure 5: The differences in time domain resolution for three different frequency spans: 40, 50, and 70 GHz

Conclusion

Higher data rates require accurate measurements to provide the confidence to make performance/cost decisions. Measurement tools must help shorten design times and ensure stable signal integrity in mass production. VNAs play a key role in helping the signal integrity engineer meet the challenges of increasing data rates, make appropriate cost/performance trade-offs, achieve correlation between simulations and measurement, and extract the effect of fixtures. When selecting a VNA, consider characteristics such as upper and lower frequency limits, performance in time domain, and a wide selection of advanced calibration and de-embedding techniques.