Digital trigger system with low trigger jitter

One of the vital oscilloscope functions is the trigger. These triggers capture individual signal events to allow detailed analysis and also provide a stable window for observing repetitive signal sequences. Conventional, analogue 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 is touted to offer several advantages such as improved accuracy and acquisition sensitivity, and expanded functionality made possible by realtime digital data processing.

In an analogue 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 (figure 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).

In addition to trigger jitter, analogue trigger systems have a problem with the long rearm times required by the trigger circuit to return to the ready state after analysing 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.

Realtime processing
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 (8bit 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 poly-phase 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.

Optimised 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.