HDMI1.3 is the most significant upgrade yet in thespecification that has become the de facto standard interface forhigh-definition (HD) devices that include DVD players, HDTV and thenewest gaming devices. HDMI 1.3 defines the latest HD A/V technologythat began hitting the consumer market late 2006, and continues to rollout this year.
To ensure reliable information transmission and interoperability,industry standards specify requirements for the network's PHY. HDMICompliance Test Specifications (CTS) define an array of compliancetests for the HDMI PHY. Last March, HDMI.Org published an updated CTS(CTS 1.3b) document that incorporates new tests for the capabilitiesdefined in HDMI 1.3.
|Figure1: These are the logical links of HDMI TMDS signaling and core testsrequired by CTS specifications|
High-quality test and measurement instruments, and solutions forhigh-speed serial data are fundamental to ensure compliance with theHDMI CTS 1.3b and the development of successful products that implementthe HDMI 1.3 specification.
Figure 1 above shows themajor elements of the HDMI transmission system – source, cable andsink. While it is recommended to perform as many tests as possible, thecore electrical tests are critical for compliance.
Per HDMI CTS 1.3b specifications, the complete compliance testsolution includes real-time oscilloscopes, sampling oscilloscopes,signal generators, differential probes and test fixtures. However, thekey is how to choose the right test platform to simplify the latestHDMI designs and to ensure compliance with the latest testrequirements. Engineers need to integrate several instruments into acomplete, flexible and cost-effective solution.
A capable, integrated solution enables fast eye-diagram renderingand jitter verification, required to accumulate huge waveforms withinas short a time period as possible.
Also, signal generators need to operate in a closed loop toautomatically perform the needed complex cable and sink tests using”one-button” compliance test software resident on an oscilloscope.
|Figure2: Shown are HDMI source compliance test results.|
A complete test solution for HDMI CTS 1.3b includes high-performancehardware and comprehensive software for design and test engineers inconsumer electronics, semiconductor and cable manufacturers.
In terms of the key instrument, to test HDMI 1.3 signals need aminimum 8GHz real-time oscilloscope in which HDMI compliance testsoftware is running. Figure 2 above isan example of HDMI 1.3 source test results using HDMI compliance testsoftware.
Meanwhile, to ensure adequate representation of signalcharacteristics, the CTS 1.3b specifies a minimum oscilloscope recordlength to acquire the data signal.
This ensures that at least 400,000 unit intervals (or Tbit) areaccumulated for building the eye diagram. With 16M/20M record length,at least 400,000 unit intervals (UI) can be captured forlower-resolution signals, and over 2.6M UI for higher resolutiondevices. Considering the bit rate up to 3.4Gbps, the minimumoscilloscope record length is above 16M with high-speed sampling rate(above 10GSps).
At the nerve center of any transmission system is the clock jitter.The jitter test checks to ensure that the clock signal is not carryingexcessive jitter – e.g. duty cycle jitter is an excellent method ofassessing deterministic jitter.
The CTS also defines the margin to be +10 percent from the nominal50 percent duty cycle. It's important that the variance in duty cycleis measured over a large number of acquired signals.
|Figure3: This clock jitter test used DPO technology.|
As per the CTS, a minimum of 10,000 triggered waveforms are requiredfor test purposes. Trigger re-arm rates of the oscilloscope then takecenter stage. Nominally, oscilloscope trigger re-arm rates are of theorder of about 100wfms/s.
This can mean unacceptably long acquisition and test times.Fortunately, there are sophisticated techniques such as FastAcq onDigital Phosphor Oscilloscopes (DPO) that enhance the trigger re-armrates and deliver over 300,000wfms/s.
Figure 3 above demonstratesthe clock duty cycle test using the FastAcq technology. Notice therichness of information that ensures clear and compelling measurements.
One of the most critical characteristics of a sink device is itstolerance to specified levels of jitter in the signals. The standarddefines the limit as 0.3*Tbit. Specified amounts of jitter are injectedin steps (from low to high jitter) into the transmitted Transition Minimized Differential Signaling(TMDS) signal until the sink device fails to recover thesignal. The amount of jitter that the sink device is able to tolerateis compared against the limits for compliance.
Several measurements are carried out by injecting a specified amountof jitter. Three measurements are performed over two test cases: (a)data jitter frequency at 500kHz and clock jitter frequency at 10MHz,and (b) data jitter frequency at 1MHz and clock jitter frequency at7MHz.
When selecting the right equipment for the HDMI CTS test setup, itis important to understand the necessary aspects that must beaddressed. TMDS signal generation plays a pivotal role in the Sinktests.
|Figure4: Shown is an integrated sink testing system based on a closed-loopmechanism|
The key challenge for a signal generator is to provide a fullcomplement of highly accurate signals and the ability to preciselycontrol their parameters.
For performing minimum differential sensitivity tests, a resolutionof 20mV is required. The intrapair skew test requires precise delaysettings down to sub-picosecond resolution.
The jitter tolerance test assumes larger challenges, since bothclock and data jitter need to be varied. Generating jitter frequencieson the order of 10MHz requires a combination of signal generators.
<>Since margins are tight, precise control is required on jitteramplitude. With various parameters to be adjusted and tight margins,this test tends to be extremely complex and can take a very long timeto complete. Figure 4 above shows an example of an automated sink test system.
The DPO connects to the DTG5334 (TMDS signal generator) using a GPIBcable and to the AWG/AFG (jitter insertion signal generator ifrequired) using a GPIB USB-B cable.
HDMI design and test engineers can control signal generators in aclosed-loop mechanism to automatically perform the needed complex cableand sink tests using “one-button” compliance test software resident onthe oscilloscope. The closed loop mechanism shrinks test time greatlyand eliminates nonlinearities of test setup.
Differential probing system
It's also critical to have a flexible, versatile probing system withhigh performance. Meanwhile, many of today's high-speed serial datastandards use differential signaling on multiple lanes that arechallenging to measure simultaneously on a single oscilloscope.
What is needed is an oscilloscope that can simultaneously acquire upto four high-speed differential signals with the use of fourdifferential probes. As an added benefit, the inputs on the probesconnect to high quality 50 ohm terminations that offer industry-leadingreturn loss, a critical specification in compliance testing asfrequencies increase.
This differential probe provides a common-mode DC voltage input tothe termination network. The termination voltage can be supplied eitherexternally by the user or internally by the oscilloscope. In addition,there is also an automatic mode that senses the common mode voltage ofthe input signal and automatically sets the termination voltage tomatch.
Differential transmission lines used in achieving fast data rates arevery sensitive to impedance matching. Consequently, impedancecharacterization is a very crucial test in HDMI compliance testing. Thethrough-connection impedance has a limit of 15 percent variance to its100 ohm specification. The impedance at termination needs to be tighteras the margins are only 10 percent of its characteristic value of 100ohm.
Meanwhile, engineers need to verify cable key characteristics, likeintra- and interpair skew and crosstalk. Time Domain Reflectometry(TDR) is a powerful and accurate tool for measuring impedance andlength in interconnects.
While fundamental concepts of TDR are relatively simple, a number ofissues must be considered to make accurate measurements, the foremostbeing the ability to perform true-differential TDR.
Evan Sun is Market Development Manager, Asia Pacific Region, Tektronix Inc.