Digital trigger system with minimum trigger jitter
The trigger is an essential oscilloscope function. Triggers capture individual signal events to allow detailed analysis and also provide a stable window for observing repetitive signal sequences. Conventional, analog triggers still have some limitations as a result of the separate signal paths for the test signal and the trigger signal. Rohde & Schwarz has implemented a hardware-based digital trigger in its R&S RTO oscilloscope that offers several advantages such as improved accuracy and acquisition sensitivity, and expanded functionality made possible by realtime digital data processing.
In an analog trigger system, the trigger path runs parallel to the signal acquisition path. The different characteristics of the paths result in a time and amplitude offset in the signal display at the trigger point. This causes measurement inaccuracies (trigger jitter), which software-based postprocessing can only partially correct. The scope’s all-digital trigger architecture (the first in the world) is designed so that the trigger data and the measurement data now share a common signal path (Fig. 1) and therefore a common time base. This means that the digital trigger system uses the same signal that was captured by the A/D converter and processed as a waveform. The result is precise allocation of the waveform to the trigger point in realtime, and hence very low trigger jitter of less than 1 ps (RMS).
Fig. 1: Block diagram of the digital trigger in R&S RTO oscilloscopes.
In addition to trigger jitter, analog trigger systems have a problem with the long rearm times required by the trigger circuit to return to the ready state after analyzing valid trigger events (for example, pulse width comparison). During the rearm time, the system does not respond to further trigger events, and signal characteristics that users want to trigger on are missed. Digital trigger systems, on the other hand, do not have a rearm mechanism and are ready again immediately. Every sample can be used as a valid trigger event, and no events are lost. The R&S RTO oscilloscope’s digital trigger system can evaluate individual trigger events in time-to-digital converters (TDC) within intervals of 400 ps at a resolution of 250 fs, independent on the actual time base setting of the oscilloscope.
The real challenge for a digital trigger is the realtime signal processing required to seamlessly display the test signal. The R&S RTO oscilloscope’s digital trigger is implemented in a high-performance ASIC. It works in the realtime path between the fast A/D converter (10 Gsample/s) and the acquisition memory. Multiple, parallel paths allow the ASIC to handle data rates of 80 Gbit/s (8-bit A/D converter).
A digital trigger based solely on the samples from the A/D converter would not be sufficient, however, because intersections with the trigger threshold value might be missed. Therefore, the time resolution is first increased to 20 GSample/s by upsampling with an interpolator. A subsequent comparator then compares the sample against the defined trigger threshold.
The biggest challenge in capturing a valid test signal at any given time is fulfilling the sampling theorem (Nyquist criterion: sampling rate of at least twice the maximum signal frequency). The R&S RTO uses polyphase filters that can calculate the test signal at any time with a signal-to-noise ratio (SNR) of > 90 dB. The intersections between the test signal and trigger threshold are calculated in realtime at a resolution of 250 fs.
Optimized trigger sensitivity
The trigger sensitivity is determined by two conflicting requirements. Stable triggering on noisy signals requires a specific hysteresis around the trigger threshold. On the other hand, a broad hysteresis limits the sensitivity of the trigger system for signals with a low amplitude or steep edges.
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 optimize the sensitivity based on the individual signal characteristics (Fig. 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.
Fig. 2: Optimized 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 – 4 GHz 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 analyzed.
Fig. 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 analyze the original, unmodified signals.
Fig. 3: Flexible filtering of trigger signals — a 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 analog 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.