Topological modeling facilitates accurate characterization of the electrical behavior of HDMI cables, reducing design time and compliance costs.

The high-definition multimedia interface (HDMI) is an emerging consumer electronics standard that offers the first industry-supported, all-digital audio/video, one-cable interface. The HDMI interface allows data rates as fast as 5Gbit/s through a single connector instead of several cables as used in the past. High data rates used in the HDMI cables require careful design and analysis techniques to ensure that the product passes required compliance tests.

The time-domain reflection and transmission (TDR/T) measurement of the HDMI cable allows users to locate and model discontinuities caused by the geometrical features of a connector, and by the frequency-dependent losses of the cable itself. Topological models can be consequently built for each part of the HDMI cable assembly, verified with the measurement data, and then used to predict time- and frequency-domain responses for the longer HDMI cable. The topological modeling methodology allows an accurate approximation of the electrical behavior of the device under test (DUT) before the longer prototype is manufactured.

In this article a 3-meter long cable was used to build a topological model from TDR data, then the model was scaled up and electrical performance was predicted for a 10-meter long prototype. HSPICE simulation, linked with IConnect modeling software, reveals excellent correlation between the prediction and the actual TDR/T measurement for a 10-meter long HDMI cable assembly for both in time- and frequency-domains. The model, generated from TDR measurements, allowed obtaining an eye diagram to compare with an eye diagram generated from TDT measurements for the fabricated HDMI cable prototype. Such eye diagrams can be generated directly from TDT measurements, but the TDR-only approach works best when only one side of the DUT is accessible. The result of an eye mask test from model prediction agreed with the test performed for the fabricated prototype. The presented technique allowed the designer to quickly accomplish interconnect modeling and analysis tasks, resulting in faster design time and lower design costs.

TDR measurements

The TDR measurement instrument was a very wide bandwidth equivalent sampling oscilloscope with an internal step generator. The TDR sends a step stimulus to the DUT, and, based on reflections from the DUT, the designer can deduce a lot of information about its properties such as location of failures, DUT impedance and time delay, and can generate an eye diagram for the system¹. An engineer can also use a time domain transmission (TDT) measurement to measure crosstalk or to characterize lossy transmission line parameters such as rise time degradation, insertion loss, skin effect and dielectric loss. Frequency dependent behavior of the system can be obtained from the time domain (TD) measurements using IConnect software that employ a so-called time domain network analysis technique (TDNA)².

TDR measurements are visual and intuitive to the digital designers because of the transient nature of this technique. As the incident step propagates through the discontinuities in the DUT, it causes reflections that indicate the exact locations of discontinuities and their sizes. The fast TDR rise time provided by the TDS8200 oscilloscope from Tektronix³ ensures that a wide range of frequencies is captured during this broadband measurement. The generalized diagram of the TDR/T measurement setup is shown in Figure 1. Any of these measurements can be performed in a differential or single-ended fashion. The differential, common or mixed mode measurements require at least two synchronized sources and a four-port measurement setup, as shown in Figure 2.

The TDR/T response of the DUT allows users to not only observe different discontinuities and characterize the HDMI interconnects, but also enables an engineer to quickly create topological models. The topological models capture the distributed nature of high-speed interconnects and allow the determination of a precise impact of each individual discontinuity on the overall performance of the DUT. IConnect modeling software utilizes the TDR/T modeling techniques to generate and analyze topological models of various interconnects, including the HDMI cable assemblies.

Differential impedance modeling of the HDMI cable

The HDMI standard uses transition minimized differential signaling (TMDS) technology, which provides differential signals with nominal amplitude transitions of 500mV. If just the differential signaling is considered then the interconnect can be reduced to a two-port structure, and the model can be built using just a differential TDR voltage waveforms. Moreover, if the HDMI test fixtures are not available, a differential probe connected to the desired channels at the reference plane can be used. In this modeling example a differential probe P80318, shown in Figure 3, is used to obtain the differential TDR response of a 2.5-meter long HDMI cable. If the second probe is available the differential TDT can be acquired as well, and insertion loss and eye diagram of the cable assembly can be obtained directly from the measurement without resorting to the modeling process. The combination of two models, a connector model and a lossy cable model, is then used to predict both S-parameters and an eye diagram of the interconnect.

The modeling process begins from modeling losses for the HDMI cable. IConnect software uses two approaches to extract the losses, “matched” and “open.” In a matched approach both TDR and TDT data are used to generate a lossy line model. The open approach can be handy in cases when it not possible to acquire a transmission waveform because it allows using a reflection data with the other port kept open to generate an accurate model. The losses in this approach can be extracted and optimized based on the information from the signal’s rise time degradation and on the slope of the TDR voltage in the DUT’s region.

The TDR data for open-ended configuration is acquired using a TDS8200 instrument and is then loaded into IConnect’s lossy line modeling tool (Figure 4). After the extraction is activated, IConnect extracts the RLGC losses for the DUT. Figure 5 shows the results for such extraction in both time- and frequency domains. Note that the correlation is excellent in terms of the rise time degradation and modeling of the TDR’s slope, however, the connector’s reflections are not modeled in this case. This is also observable in the frequency domain correlation shown on the right of the Figure 5; the depth of the modeled resonances is smaller than the depth of the measured ones. This behavior is accurately captured when the connector’s reflections are modeled using a single line modeler tool of IConnect.

The single line modeler tool utilizes a true impedance profile to generate a model for each discontinuity in a connector-cable transition of the HDMI cable assembly. To generate such model, only the TDR data is required. Figure 6 shows both the true impedance profile for the connector-cable area and the model’s correlation with measurements in terms of reflected voltages. The discontinuities of the connector-cable area are modeled using sections of the ideal transmission lines; however, lumped element topologies can be selected as well. After the model’s parameters are adjusted, the single line model can be combined with the lossy line model to represent both losses and reflections, and to predict S-parameters as well as the eye diagram for the cable under test.

A model for the connector and the lossy line model can be combined in the composite modeler window. The length of the lossy line model can be scaled down using the “scale parameters by” feature of the lossy line model. This feature allows adjusting the length of the lossy model to fit connectors’ models. There is no need to create another model for the connector-cable transition because these areas are identical for both sides of the cable. Hence, the model can be reused by interchanging the port’s direction in the sub-circuit’s netlist. HSPICE circuit simulation of the final assembly model reveals excellent correlation in both time and frequency domains (Figure 7).

Once an accurate model is created, it can be used to predict an eye diagram and S-parameters for the cable under test. In order to extract a two-port S-parameter and an eye diagram, both TDR and TDT responses of the HDMI cable model are required. By the definition of S-parameters, the response is measured when all ports are terminated with matched terminations⁴. This is simply done by changing the termination impedance in the composite modeler and simulating the response. The HSPICE simulated TDR/T waveforms are then used to compute S-parameters. The eye diagram is produced by using a symmetric coupled lossy line modeler and by loading the measured reference and simulated transmission waveforms (Figure 8).

Fully coupled modeling of HDMI cable

Although a differential model can be efficiently used in the system’s simulations, a fully coupled model provides a more accurate representation of a device’s performance. Signals that propagate differential lines can be decomposed into even and odd mode components. Therefore, if the model is capable of accurately capturing these two modes of propagation, any signaling can be accurately represented in circuit simulations. When coupled models are built with IConnect they can be simulated with a linked simulator and the results can be compared with measurements in both modes of propagation.

A process of building coupled models is similar to the one described in the previous section. Separate models for the reflections and losses need to be created first, and then combined into one model assembly and compared with the actual measurements data. The measurements should include both even and odd mode responses and the models can be built based on TDR only, or based on both TDR and TDT measurements. In the example described in this section, the model was built assuming the minimum availability of the measurement equipment that is differential TDR capability only.

TDR data was acquired for both odd and even modes of propagation. The DUT connection was performed according to the Figure 4, with the reference plane set as an open at the end of the fixture or probes. The odd mode was excited by setting opposite polarity steps and acquired by using the difference between two channels. The even mode was excited by using the same polarity signals, and acquired by summing the TDR responses of channels one and two.

A coupled lossy line model correlation is shown in Figure 9a. After accuracy of the models for both reflections and losses is verified, they can be combined into one model assembly (Figure 10). The model assembly shows excellent correlation for both even and odd modes of propagation in terms of RLGC losses and reflections.

Model-based prediction of an eye diagram

The eye diagram test is another key measurement required by the HDMI signaling standard. The measurement of the eye diagram captures the deterministic jitter in the interconnect, which is caused by losses and inter-symbol interference. Since the transfer characteristics of a cable assembly contains all the information required to re-construct this deterministic jitter, the eye diagram computed from the TDT measurements using a TDR oscilloscope is as valid and accurate as the eye diagram obtained using a pattern generator and a sampling oscilloscope.

Modeling tools of IConnect can be used to estimate an eye diagram of a long cable from the measurements of a short one, thus enabling a designer to predict interconnect performance before even manufacturing it. This is done by creating an accurate model for the short interconnect using a topological modeling approach described in the previous sections, and scaling up the lossy line model to represent the longer cable assembly. In this section we used a 3-meter long HDMI cable to predict an eye diagram of a 10-meter long cable. Then the actual measurements of a 10-meter long cable were used to verify the prediction.

To build an eye diagram with IConnect, reference and transmission waveforms are required. The reference waveform can be acquired from the open configuration with the DUT disconnected, while TDT waveform can be obtained from simulation of the scaled model using the reference waveform as a source signal. The TDT response is then saved and loaded into a new lossy line modeler tool, and the eye diagram is generated according to the compliance specifications.

Figure 11 shows a time domain correlation of the scaled model originally built using measurements of a 3-meter long HDMI cable with the real measurements of a 10-meter long cable assembly. Both TDR and TDT responses show excellent correlations. The circuit model allows users to generate a transmission waveform that can be used to generate an eye diagram. Figure 12 shows a comparison of the modeled and measured eye diagrams for a 10-meter long cable. The scaled model provides a reasonable estimation of the actual 10-meter long cable performance. 

The IConnect software efficiently perform the analysis of HDMI cable assemblies. Accurate two-port differential models can be quickly built from only TDR measurement data. The designer can also build coupled models from even and odd mode TDR measurements, and then use those models to predict both insertion and return losses of the cable assembly. Finally, the topological models obtained with IConnect can be scaled up to accurately predict performance of the longer cables, which was demonstrated in the last section of this application note.

Eugene Mayevskiy is an applications engineer at Tektronix.  He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..

REFERENCES

1. “Eye Diagram Measurements Using TDR Oscilloscope Transmission Data,” Application Note, Tektronix Inc., WebID: 3059.
2. “S-parameters, Insertion, and Return Loss Measurements Using TDR Oscilloscope,” Application Note, Tektronix Inc., WebID: 3058.
3. Tektronix Inc., Beaverton, OR, 97077, USA.
4. Simon Ramo, John R. Whinnery, Thodore Van Duzer, “Fields and waves in communication electronics,” 3^d ed., John Wiley &Sons Inc., 1993.

• Prev
• Next