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Simplifying HDMI 2.0 Source Impedance Measurements with a VNA-Based Methodology

Parent Category: 2014 HFE

By Yoji Sekine

The HDMI 2.0 specification, released by the HDMI Forum on September 4, 2013, increases the maximum per lane throughput from 3.4 Gbit/s to 6 Gbit/s. The result is a maximum total throughput of 18 Gbit/s, which can support 4K image transmission with a 4:4:4 full-color format. While this bodes well for consumers, it also presents a number of measurement challenges. One key challenge stems from the fact that the transmission rate is nearly doubled and yet it is still necessary to support existing HDMI cables. This raises interoperability issues due to poor signal integrity. 

One reason for the poor signal integrity is impedance mismatch of active devices. Impedance matching is essential in the design of high-speed applications and many modern high-speed digital standards specify limits for impedance and return loss. For HDMI 2.0, the source and sink differential impedance requirements are detailed in section HF1-9 and HF2-4 of the Compliance Test Specification. Most of the standards require that devices operate during measurements, because device characteristics will differ between the power-on and the power-off states. Depending on the device design, the impedance may also differ for different data rates as well (Figure 1). To obtain an accurate representation of the impedance, it is therefore crucial to evaluate the impedance of active devices under actual operating conditions. Fortunately a new methodology based on a Vector Network Analyzer (VNA) is now helping to dramatically simplify this task.

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Figure 1 • These graphs illustrate the source impedance and return loss with power off (red), power on at 1333 Mbps (blue), and power on at 334 Mbps (green).

 

Measuring Source Impedance

The impedance measurement of active devices in the powered-on and operating state is called Hot TDR. Hot TDR measurements are difficult to make because the signal from the source causes measurement errors. The key measurement challenge here is how to avoid the effects of the source output signal and provide stable measurements.

Generally, TDR oscilloscopes have been used to measure Hot TDR. VNAs can also be used for this purpose. Due to instrument architectural differences, however, VNA-based solutions provide significant advantages over the traditional solutions based on TDR oscilloscopes. 

Importance of Impedance Matching

The eye diagram is a key metric for signal integrity engineers. One factor impacting the eye opening is signal reflection due to impedance mismatch. When there is more than one impedance mismatch in the link, multiple reflections occur and degrade signal integrity. A portion of the transmitted signal is reflected from the sink due to non-ideal impedance match, as shown in Figure 2. If the source is not impedance matched, the signal is re-reflected back again into the channel and causes eye closure when it reaches the sink. This effect becomes more critical for multi-gigabit systems, such as HDMI 2.0. Impedance matching on the source and sink is therefore, essential to improving signal integrity and opening up the eye diagram.

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Figure 2 • Multiple reflections between the source and sink are shown in this diagram.

 

The eye diagrams in Figure 3 depict simulation results for the purpose of comparing different termination conditions. The eye diagram on the left was computed using the return loss extracted from an actual source device that is not impedance matched. The eye diagram on the right was computed assuming a perfectly terminated source. Obviously, the eye diagram on the right has a wider eye opening, verifying that impedance matching of the source can in fact dramatically improve eye opening.

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Figure 3 • This eye diagram compares simulation results without (left) and with (right) ideal source termination.

 

Effects of the Source Signal on Measurement

Both the TDR oscilloscope and VNA work by applying a stimulus to the Device-Under-Test (DUT) and measuring the response. To measure the response, the TDR oscilloscope uses a wideband receiver up to the maximum bandwidth of the instrument, typically 20 GHz. The VNA, on the other hand, uses a narrowband receiver; typically on the order of 10 kHz. 

As can be seen in the frequency domain plot in Figure 4, the impact of the data signal from the source is dramatically different, depending on whether the TDR oscilloscope or VNA is employed. The data signal is represented by a number of line spectra, or spurious, in the frequency domain. Since the TDR oscilloscope uses a wideband receiver that captures all of the signal energy, including source spurs, the measurement result is highly noisy. To reduce the noise, extensive averaging (on the order of 1000 times) is necessary. In contrast, the VNA sweeps across the desired frequency range acquiring data on discrete frequency points. The narrowband receiver used in the VNA  filters out the unwanted source spurs and, in many cases, averaging is not necessary. The result is a significant speed advantage for the VNA-based solution.

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Figure 4 • Shown here are the Hot TDR measurement principles for TDR oscilloscopes and VNAs.

 

When the receiver sampling points coincide with the source spur frequency, the source signal energy cannot be minimized by averaging. This typically results in an excessive amount of noise and ripple on the measured impedance profile, and spikes in the frequency domain response. In these cases, the sampling points must be adjusted to avoid the effects of source spurs. This is done by adjusting the TDR repetition rate on the TDR oscilloscope. As the ideal setting is related to the harmonic relationship of the repetition rate and the DUT’s source signaling rate, the ideal repetition rate setting is unique to each DUT. The process for finding the ideal setting is determined by trial and error.

A similar situation occurs for the VNA as well. Although VNAs avoid the data signal by using a narrowband receiver, the source spurs can coincide with the measurement points during the frequency sweep. Consequently, the measurement points must be adjusted to avoid the source spur frequencies. This can be accomplished by setting the appropriate start and stop frequencies, and number of points. Some modern VNAs, such as the Keysight E5071C ENA Option TDR, have the ability to automatically set up the optimum frequency sweep to minimize the effects from the source spurs based on the data rate input. Its Avoid Spurious feature provides a one-click operation for Hot TDR measurements.

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Figure 5 • Keysight Technologies’ ENA Option TDR automatically minimizes the effects of source spurs.

 

An example of how the ENA Option TDR minimizes the effects of spurious signals from the source is shown in Figure 5. Note that the effects of the source spurs in the left image are minimized when the Avoid Spurious feature is activated.

Summary

Impedance measurement of active devices or Hot TDR, is essential when designing with high-speed digital standards like HDMI 2.0. The increase in bit rates due to HDMI 2.0 means that the impedance of active devices must be properly evaluated to provide new insight into signal integrity issues. While a TDR oscilloscope can be used for this purpose, the VNA with its range of functionality and features like Avoid Spurious, offers a much more viable solution—one that offers many advantages over the traditional TDR oscilloscope solution.

For more information on the ENA Option TDR, visit www.Keysight.com/find/ena-tdr.

About the Author

Yoji Sekine is a marketing engineer at Keysight Technologies. During his 14 years with the company he has held various positions, including as an R&D engineer designing various products, such as vector network analyzers, signal source analyzers and LCR meters. Sekine holds a BSEE degree from the University of California at Davis. Agilent Technologies Electronic Measurement Group is now Keysight Technologies Inc.

 

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