High definition multimedia interface (HDMI) cable modeling: topological modeling facilitates accurate characterization of the electrical behavior of HDMI cables, reducing design time and compliance costs.
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. 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.
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 system (1). 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 he 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 Tektronix (3) 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.
[FIGURES 1-2 OMITTED]
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.
[FIGURE 3 OMITTED]
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.
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.
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.
(1.) "Eye Diagram Measurements Using TDR Oscilloscope Transmission Data," Application Note, Tektronix Inc., WeblD: 3059.
(2.) "S-parameters, Insertion, and Return Loss Measurements Using TDR Oscilloscope," Application Note, Tektronix Inc., WeblD: 3058.
(3.) Tektronix Inc., Beaverton, OR, 97077, USA.
The complete article by Eugene Mayevskiy can be found in PCD&M's online addition, at pcdandm.com/cms/ content/view/3219
EUGENE MAYEVSKIY is an applications engineer at Tektronix. He can be reached at firstname.lastname@example.org.
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|Title Annotation:||MODELING CABLE CONNECTIONS|
|Publication:||Printed Circuit Design & Manufacture|
|Date:||Feb 1, 2007|
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