MHL signal test method

       For MHL devices, the equipment required for testing mainly includes high-bandwidth oscilloscopes, probes, MHL fixtures, and MHL automatic test software. MHL testing requires an 8GHz bandwidth oscilloscope and probe. The oscilloscope model recommended by Agilent is DSA90804A. If the user already has this or a similar oscilloscope for HDMI testing, it can be directly upgraded on this basis. The typical connection diagram of MHL testing is as follows.

        During the test, the MHL signal to be tested is led out with a test fixture, and different test connection methods are selected according to different test items. The MHL signal test items are mainly divided into the following categories:

1. Single-ended signal measurement: mainly tests the high and low levels, standby levels, line pair delay and other parameters of the two MHL+/- signal lines.
2. Differential measurement: mainly tests the rise and fall time, swing and other parameters of the signal after subtracting the two signal lines MHL+/-.
3. Common mode measurement: mainly tests the rise and fall time, swing, jitter, duty cycle and other parameters of the common mode clock signal.
4. Signal eye diagram measurement: that is, measuring the eye diagram of the differential data signal relative to the extracted common-mode clock signal.

 

        According to the requirements of the MHL specification, the test of the MHL Source device needs to be tested at TP1 using a test fixture, which is the connector for the signal output of the Source device. The test fixture is used to lead out the test signal, and the fixture needs to have corresponding circuits to detect the common-mode clock signal and the differential-mode data signal respectively. The test fixture should meet the MHL requirements for differential and common-mode impedance, that is, the common-mode impedance Zct = 30Ω, and the differential-mode impedance Zdt = 100Ω. In addition to providing corresponding signal matching, the test fixture also has corresponding single-ended, differential-mode signal and common-mode clock extraction circuits to facilitate the extraction of single-ended, differential-mode signals and common-mode clocks for signal testing. In order to enable the DUT to output the MHL signal, the test fixture is also connected to a dedicated C-BUS control to control the DUT to generate the test signal. When there is no C-BUS controller, it can also be connected to the MHL to HDMI dongle device. The figure below is the MHL test fixture.

 

        Since there are many MHL test items, different tests require different connections and settings. Without guidance, manual operation is prone to errors, and different test item setting parameters may also result in different test results. In order to improve test efficiency and ensure the consistency of test results, Agilent is the first in the industry to provide N6460A MHL consistency test software.

 

        After selecting the corresponding test item in the test software, the test software will have graphical settings to guide how to connect the probe, fixture and DUT. When the DUT generates the required test signal, the test software will automatically capture the signal for testing and analysis, and save the test results as a test report. The following figure shows the actual situation when the software is running.

 

After the software is finished running, it will automatically generate a corresponding test report, listing whether the test items have passed and the detailed test results. The following figure is the final test report generated.