FEATURE TEST & MEASUREMENT
window. In the upper trace, a pattern occurring approximately every 25ms can be identified. In the lower trace, the signal is zoomed by a factor of 1,000. R&S has used the built-in annotation tool of the RTB2000 to indicate the additionally identified spikes, occurring approximately every 15µs. We are therefore looking at two periodic events. Both periodic events are visible in one
screen because the 10Msample per channel standard acquisition memory of the RTB2000 makes it possible to retain a
high sampling rate. In this application, it means the complete acquisition time of 120ms is captured with a sample rate of 62.5Msample/s. In other words, events in the nanosecond range can be identified, allowing signals such as the fast periodic events to be reliably detected. In this article, the focus is on the larger, slower periodic event with the higher amplitude change in order to find the root cause. The R&S RTB2000 is a mixed signal
oscilloscope and optionally supports up to 16 digital input channels as well as
Figure 3: Low- and high- speed coupled events are captured using long memory
serial triggering and decoding of CAN bus signals. One of these digital channels is used to capture the CAN bus telegrams. This protocol is decoded using hardware acceleration and a colour scheme to identify write/read addresses, data and all other bits of a CAN bus message. The screenshot in Figure 4 shows the digital channel as well as the decoding of the CAN bus telegram along with the DC supply voltage. The slow-speed repeating pattern on the DC voltage (25ms) can be immediately linked to the CAN bus telegram. Whenever the FPGA starts to transmit CAN bus data, it loads the DC power supply and causes a ripple. Looking at the DC voltage change in the zoom window, the main ripple seems to be coming from bit switching, but there is noise overlaid, which makes it difficult to quantify the pure influence of bit switching. In this example, by triggering on a specific CAN address and/or data, and utilising the DUT’s capability to send repeating CAN messages, we can find the ripple caused only by bit switching. The R&S RTB2000 is set to trigger on a recurring CAN bus telegram and averages several acquisitions. The result is shown in Figure 5. Averaging removes any noise not related
Figure 4: Simultaneous display of analogue DC voltage and CAN bus protocol as digital and decoded signal
Figure 5: Using
averaging to remove the part of the DC voltage ripple not caused by bit switching
to bit switching. Now the DC ripple caused by transmitting CAN signals is isolated and measured as 49.20mV. Using an economy class scope with 300MHz bandwidth and 10-bit A/D converter, R&S has demonstrated how the optimisation of vertical and horizontal settings can provide detailed insight into the root causes of ripple on a DC power supply. Acquisition memory is also highly important, since most coupled events are by nature much slower in speed than the signals at the DUT. Moreover, the capability to trigger on specific serial data telegrams makes it possible to isolate root causes and perform precise ripple measurements. The initial measurement of the ripple of the DC power voltage was approximately 180mV. Optimising the vertical settings revealed that the ripple was only in the range of approximately 68mV. Finally, the transmission of CAN data was identified as the main root cause of the ripple. This was only possible using long memory and capturing CAN bus signals. By triggering on specific CAN data and averaging, the ripple on the DC power supply voltage caused by bit switching was measured to be approximately 49mV, which is approximately one per cent of the nominal voltage.
Rohde & Schwarz
www.rohde-schwarz.com/uk/
30 NOVEMBER 2017 | INSTRUMENTATION
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