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HDMI Cable Modeling
And Analysis
by Eugene Mayevskiy, Tektronix
To analyze HDMI cable assemblies, couple time-domain measurements with
modeling software.
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. It supports data rates as fast as 5 Gb/s
through a single connector instead of several cables as used in the past.
Consequently, HDMI cables require careful design and analysis techniques to
ensure that they pass required compliance tests. The time-domain reflection and transmission (TDR/T) measurement of the HDMI
cable helps locate and model discontinuities caused by the geometrical features
of a connector and frequency-dependent losses of a cable itself. In cases where
it is not possible to measure TDT data directly, an accurate prediction of the
transmission behavior can be made. Moreover, topological models can be built for each part of the HDMI cable
assembly, verified with the measurement data, and then used to predict time- and
frequency-domain response for a longer HDMI cable. The topological modeling
methodology can accurately approximate the electrical behavior of the DUT even
before the longer prototype is manufactured. A 3-meter-long cable is used to build a topological model from TDR data. The
model is scaled up and electrical performance predicted for a 10-meter-long
prototype. The SPICE simulation linked with IConnect modeling software1 shows
correlation between the prediction and the actual TDR/T measurement for a
10-meter-long HDMI cable assembly in the time and frequency domains. The circuit
model generated from TDR measurements allows an eye diagram to be obtained,
which then is compared with an eye diagram generated from TDT measurements for
the fabricated HDMI cable prototype. Such an eye diagram 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 agrees with the test performed for the
fabricated prototype. This technique allows a designer to quickly accomplish
interconnect modeling and analysis tasks, resulting in faster design time and
lower design costs. TDR Measurements
The TDR measurements normally are done with a time-domain sampling oscilloscope
such as Tektronix TDS8200 or similar. This instrument is a very wide bandwidth
equivalent-time sampling oscilloscope with an internal step generator. The TDR sends a step stimulus to the DUT. Based on reflections from the DUT, a
designer can deduce a great deal of information about the DUT’s properties such
as location of failures, DUT impedance, and time delay, and he can generate an
eye diagram for the system.2 An engineer also can use TDT measurements to measure crosstalk or characterize
lossy transmission line parameters such as rise-time degradation and insertion
loss as well as skin effect and dielectric losses. The frequency-dependent
behavior of the system can be calculated from the time-domain (TD) measurements
using the time-domain network analysis technique (TDNA).3 The TDR measurements are visual and intuitive to the digital designers due to
the transient nature of this technique. As the incident step propagates through
the discontinuities in the DUT, it causes the reflections that indicate the
exact locations of discontinuities and their sizes. The fast TDR rise time
provided by the sampling oscilloscope ensures that a wide range of frequencies
is captured during this broadband measurement. A generalized diagram of a TDR/T measurement setup is shown in
Figure 1a. 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 1b.
Figure 1a. General TDR and TDT Block Diagram |
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Figure 1b. Block Diagram for Measurements of Coupled Interconnects |
The TDR/T response of the DUT not only allows an engineer to observe different
discontinuities and characterization of the HDMI interconnects, but it also
enables him to quickly create topological models. The topological models capture
the distributed nature of the high-speed interconnects and allow determination
of a precise impact of each discontinuity on the overall performance of the DUT.
Modeling software uses 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 that provides differential signals with nominal amplitude transitions
of 500 mV. 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 differential TDR voltage waveforms. 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 is used to obtain the
differential TDR response of a 3-meter-long HDMI cable. If the second probe is
available, the differential TDT can be acquired as well, and insertion loss and
the 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, then is used to predict both
S-parameters and an eye diagram of the interconnect. The modeling process starts 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 data and TDT data generate a lossy-line model. The open approach can be handy in cases when it is not possible to acquire a
transmission waveform because it uses 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 the slope of the TDR voltage in the DUT’s region. The TDR data for the open-ended configuration is acquired using a time-domain
sampling oscilloscope and loaded into the software’s lossy-line modeling tool.
The software then extracts the RLGC losses for the DUT. 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 also is observable in the frequency-domain correlation; 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 pieces of the ideal transmission-line models or lumped elements. The approach to model reflections is best used when the true impedance profile
generates a model for each discontinuity in a connector-cable transition of the
HDMI cable assembly. To generate such a model, only the TDR data is required.
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, this model can be combined
with the lossy-line model to represent both losses and reflections and predict
S-parameters as well as the eye diagram for the cable under test. Models for the connector’s reflections and the cable’s losses can be combined in
one composite circuit. The length of the lossy-line model can be scaled down by
adjusting loss parameters of the model to fit the connector models. There is no
need to create another model for the connector-cable transition because two
connectors normally are identical for both sides of the cable. For this reason, the model can be reused by interchanging the port direction in
the subcircuit’s netlist. The SPICE circuit simulation of the final assembly
model reveals excellent correlation in both the time and frequency domains shown
in Figure 2.
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Figure 2. Time- and Frequency-Domain Correlation of the Model
Assembly
Click here to see larger image |
Once an accurate model is created, it can be used to predict an eye diagram and
S-parameters for the cable under test. To extract two-port S-parameters 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 terminations.4 This is simply done by changing the
termination impedance in the composite model and simulating the response. The
SPICE-simulated TDR/T waveforms then are used to compute S-parameters.
Fully Coupled Modeling Of the HDMI Cable
Although a differential model can be used efficiently in the system’s
simulations, a fully coupled model provides more accurate representation of a
device’s performance. Signals that propagate on differential lines can be
decomposed into even- and odd-mode components. So, if the model is capable of
accurately capturing these two modes of propagation, then any signaling can be
accurately represented in circuit simulations. Separate models for the reflections and losses must be created first, then
combined into one model assembly. After the coupled models are built, they have
to be compared with measurements in both modes of propagation. The measurements should include both even- and odd-mode responses, and the
models can be built based on TDR only or on TDR and TDT measurements. In this
example, the model is built assuming the minimum availability of the measurement
equipment, that is, differential TDR capability only. The TDR data is acquired for both odd and even modes of propagation. The odd
mode is excited by setting opposite polarity steps and acquired by using the
difference between two channels. The even mode is activated by using the same
polarity signals and acquired by summing the TDR responses of channels one and
two. After accuracy of the models for both reflections and losses is verified, they
can be combined into one model assembly shown in Figure 3. The model assembly
shows excellent correlation for both the even and odd modes of propagation in
terms of RLGC losses and reflections.
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Figure 3. Time-Domain Correlation and Fully Coupled Model Topology
for HDMI Cable Assembly |
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 interconnects caused by interconnect crosstalk, losses, and reflections.
Since the transfer characteristics of a cable assembly contain all the
information required to reconstruct 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 the software can be used to estimate an eye diagram of a long
cable from the measurements of a short one, 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 and scaling up the lossy-line model to represent the longer cable
assembly. In this section, a 3-meter-long HDMI cable is used to predict an eye diagram of
a 10-meter-long cable. Then the actual measurements of a 10-meter long cable
verify the prediction. To build an eye diagram from the time-domain measurements, both reference and
transmission waveforms are required. The reference waveform can be acquired from
the open configuration with the DUT disconnected while the TDT waveform can be
obtained from the SPICE simulation of the scaled model using the reference
waveform as a source signal. The TDT response is saved and used to generate a transfer function required for
the eye diagram. It then is generated according to the compliance
specifications. Figure 4 shows the modeled eye diagram for a 10-meter-long cable. The measured
eye diagram is almost identical. The scaled model provides a reasonable
estimation of the actual 10-meter-long cable performance. The eye diagrams are
generated at a 1.65-Gbs data rate and 200-ps 20% to 80% rise time.
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Figure 4. Correlation of an Eye Diagram Obtained From the Scaled
Model With the Eye Diagram Obtained From the Actual Measurements of
the 10-Meter-Long Cable
Click here to see larger image |
Conclusion
Time-domain measurements coupled with modeling software can be efficiently used
to perform an analysis of the HDMI cable assemblies. Accurate two-port
differential models can be quickly built from TDR-only measurement data. The
designer also can build coupled models from even- and odd-mode TDR measurements
and then reuse those models to predict both insertion and return losses of the
cable assembly. Finally, the topological models can be scaled up to accurately
predict performance of the longer cables. References
1. Tektronix.
2. “Eye Diagram Measurements Using TDR Oscilloscope Transmission Data,”
Application Note, Tektronix.
3. “S-Parameters, Insertion, and Return Loss Measurements Using TDR
Oscilloscope,” Application Note, Tektronix.
4. Ramo, S., Whinnery, J.R., and Van Duzer, T., “Fields and Waves in
Communication Electronics,” Third Edition, 1993. About the Author
Eugene Mayevskiy is an applications engineer with the electrical optical product
line at Tektronix. He holds B.S.E.E and M.S.E.E. degrees from Oregon State
University where he completed research on measurement-based modeling and
analysis of passive devices fabricated for radio-frequency integrated circuits.
Mr. Mayevskiy also has published a number of papers and given seminars on
measurement-based modeling for high-speed designs. Tektronix, 14200 SW Karl
Braun Dr., P.O. Box 500, Beaverton, OR 97077, 800- 835-9433.
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