Feature sponsored by Test & measurement
Fig. 8: Jitter separation results for the faulty USB 3.2 Gen 1 device.
Fig. 10: Jitter results after switching off the interference source.
Fig. 9: Periodic and random jitter are produced by injecting signals from the R&S RTP generator into the 5 V supply voltage of the USB device.
TESTING THE SIGNAL PATH WITH TDR In addition to analysing the active signals, it is also important to check the signal paths in case of signal integrity problems. Here, the focus is on the transmission losses as well as the impedance response and stability along the signal path. Depending on the signal, the bandwidth of the signal paths on the printed board, the connectors, the cables, etc. requires appropriate design and selection. Impedance steps should also be avoided due to the reflections they can cause. The relevant measurements are usually performed using network
analysers. The R&S RTP with integrated time domain reflectometry (TDR) provides a useful alternative. The differential 16 GHz pulse source is used as a stimulus; its reference outputs allow measurement of the reflected signals with the oscilloscope channels. The application software provides support during setup calibration as
well as during the measurement. TDR can be used to measure the impedance and reflection coefficient along the signal path. Fig. 11 shows measurement of a USB test fixture. The differential pulse source was connected to the SMA connectors. The USB type A connector was left open so the supplied signal pulse would be fully reflected. The impedance and the reflection coefficient can be displayed vs. time as well as vs. distance, allowing easy correlation to local sections of the device under test. We can clearly see the impedance step at the transition from the SMA connectors to the printed board, the constant impedance along the signal trace, and the reflection at the USB connector. Time domain transmissometry (TDT) is another useful measurement capability. Here, a fast pulse is also fed into the signal path. The output is connected to the oscilloscope channel to allow determination of the transmission losses. The TDT result shows the pulse shape that arises due to transmission losses. The rise time measured in the example in Fig. 12 suggests a bandwidth of about 3.2 GHz (BW = 0.35/trise).
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Fig. 11: TDR measurement on the USB test fixture: impedance (top), reflection coefficient (bottom).
Fig. 12: TDT measurement on the USB test fixture: The pulse rise time at the output is 108 ps.
SUMMARY Conformance tests on serial bus interfaces include important measurements when it comes to ensuring interoperability between electronic devices and their accessories. When errors are encountered, appropriate T&M equipment is the key to rapidly determining the root causes. Along with software options for performing automated conformance tests, the R&S RTP oscilloscope provides a number of very useful tools for debugging signal integrity problems.
Rohde & Schwarz
www.rohde-schwarz.com September 2022 Instrumentation Monthly
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