Feature sponsored by Test & measurement
CONFORMANCE TEST FAILED. WHAT NOW?
By Guido Schulze, product manager oscilloscopes, Rohde & Schwarz
ROOT CAUSE ANALYSIS OF SIGNAL INTEGRITY PROBLEMS ON HIGH SPEED DIGITAL BUSES Conformance tests are performed on serial data interfaces such as USB, HDMI and PCI Express to ensure interoperability between electronic devices and accessories. In cases where signal integrity problems are encountered, the R&S RTP oscilloscope supports root cause analysis by providing powerful tools such as eye diagrams, jitter and noise separation as well as time domain reflectometry.
AUTOMATIC CONFORMANCE TESTS FOR HIGH SPEED DATA INTERFACES Conformance tests represent an important milestone during product development. The relevant standardisation committees have published detailed test specifications for many interfaces such as USB and Ethernet. Specialised test labs offer complete testing services for such interfaces, including documentation and certification. For users who need to perform these tests on their own, the R&S RTP provides automated test solutions for all common interface standards. These solutions are equipped with graphical measurement configuration tools and off-shelf test sequences. No matter how the testing is performed: If results do not comply with the standard, time-consuming debugging is required. During root cause analysis, the R&S
RTP oscilloscope provides support with analysis tools such as eye diagrams with mask tests or separation of jitter and noise components. Time domain reflectometry is also available for verifying the transmission characteristics of passive signal path components such as connectors, cables and signal lines on the printed board.
CONFORMANCE TESTING FOR USB 3.2 TRANSMITTERS USB 3.2 transmitter conformance testing focuses on the eye diagram (transmitted eye) for verifying the eye opening, signal levels and jitter components. This test is performed directly on the device output (short channel) as well as with a simulated signal path (long channel). For long channel tests, the USB Implementers Forum (USB-IF) has
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Fig. 1: Conformance test for USB 3.2 Gen 1 with the R&S RTP164
oscilloscope (16 GHz).
published files with S-parameters for various cable and signal trace lengths. During the tests, the oscilloscope acquires clock and data signal sequences with a length of 200 μs. These sequences are then checked for compliance with the standard using the SigTest USB-IF analysis software. Depending on the test mode, each USB device must generate the compliance patterns on its own: For USB 3.2 Gen 1, this means patterns CP0 (data) and CP1 (clock) and for USB 3.2 Gen 2, patterns CP9 (data) and CP10 (clock). Switching to the next CP pattern involves sending short LFPS sequences to the receiver in the USB device. The R&S RTP supports conformance
testing for USB 3.2 Gen 1 (13 GHz model required) and for Gen 2 (16 GHz model) (Fig. 1). The SigTest analysis software is integrated into the R&S RTP- K101 USB 3.2 conformance testing option, and the corresponding test sequence is automated. The option provides convenient graphical support to guide the user through the measurement. With the integrated two- channel 100 MHz generator option, switching between the individual test patterns takes place automatically. Simultaneous testing of the short and long channels is another simplification. The trace from the short channel setup is processed using embedding filters generated on the basis of USB-IF S- parameter files to produce a long channel trace (Fig. 2). Complete test results are compiled in a detailed report.
Fig. 2: Acquisition of the “short channel” trace for a USB 3.2 Gen 1
device on channels 1 and 3 with real-time calculation as a differential signal (Diff 1). Calculation of the “long channel” signal through embedding of the S-parameters from the USB-IF (Math 1).
Fig. 3: The USB 3.2 Gen 1 conformance test report reveals abnormally high jitter in the results table (top) and in the eye diagram.
September 2022 Instrumentation Monthly
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