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Digital trigger system with low trigger jitter

Posted: 26 Dec 2013 ?? ?Print Version ?Bookmark and Share

Keywords:oscilloscope? trigger? oscilloscope? Rohde & Schwarz? digital converters?

The trigger sensitivity of conventional oscilloscopes is typically limited to a minimum of 1 div to 2 div (vertical) and the noise reject mode is used to select a larger hysteresis in order to achieve stable triggering on noisy signals.

In contrast, R&S RTO oscilloscope's digital trigger system allows the trigger hysteresis to be set flexibly between 0 div and 5 div in order to optimise the sensitivity based on the individual signal characteristics (figure 2). In auto hysteresis mode, the oscilloscope uses a hysteresis that depends on the vertical scaling. Using the manual hysteresis mode, the hysteresis can be increased manually to achieve stable triggering, even for signals with a very high noise component. On the other hand, setting the hysteresis to null provides the greatest trigger sensitivity for very small signals or for precise triggering on steep edges.

Figure 2: Optimised trigger sensitivity: Adjustable hysteresis for stable triggering on noisy signals. Setting the hysteresis to null provides the greatest sensitivity.

In principle, a digital trigger can validate every acquired sample against the trigger definition. In addition, the R&S RTO allows precise triggering down to 1 mV/div without bandwidth restrictions.

Capturing even the smallest pulse widths

An important criterion for trigger system performance is the smallest pulse width that can be captured, i.e. the smallest pulse width that the oscilloscope can capture and trigger on. The oscilloscope supports stable triggering on pulses, glitches, intervals, and rising/falling signal edges of 50 ps (R&S RTO1044 C 4GHz bandwidth model). For example, a trigger can be set for small signal disturbances (glitches), such as occur as a result of crosstalk from adjacent lines when transmitting signals with steep edges.

Flexible filtering of trigger signals
The ASIC in the R&S RTO oscilloscope was designed specifically for acquisition and triggering, and therefore supports flexible setting of the cut-off frequency for a digital lowpass filter in the realtime signal path. The filter settings can be used for both the trigger signal and/or the test signal. Applying the lowpass filter to only the trigger signal suppresses high-frequency noise and permits stable triggering, while simultaneously allowing the unfiltered test signal to be both displayed and analysed.

Figure 3 shows an example. This example uses a runt trigger, but here a steep signal overshoot makes it difficult to set the trigger threshold. The solution is to use the lowpass filter only on the trigger signal. This makes it possible to analyse the original, unmodified signals.

Figure 3: Flexible filtering of trigger signalsa steep overshoot (yellow trace) is suppressed by using a lowpass filter on the trigger signal (white trace).

A completely hardware-based digital trigger, such as used in R&S RTO oscilloscopes, offers clear advantages over analogue trigger systems: The digital trigger works directly on the samples from the A/D converter, providing consistent timing for the acquisition data and the trigger data. One benefit is very low trigger jitter. The high trigger sensitivity over the full bandwidth and the adjustable digital filter for the trigger signal result in very accurate measurements.

About the authors
Guido Schulze is Product Manager for Oscilloscopes at Rohde & Schwarz.

Markus Freidhof is Head of R&D for Oscilloscopes at Rohde & Schwarz.

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