Oscilloscope Performance & Digital Triggering

Wed, 07/13/2011 - 10:59am
Michael Schnecker, Business Development Manager, Rohde & Schwarz
RTO ScopeTriggering is a critical factor in the measurement performance of oscilloscopes. Triggering enables an oscilloscope to acquire a signal at a specific point in time on the signal waveform. Originally, the trigger function allowed the user to view a signal on the oscilloscope without “rolling” on the screen, but as trigger circuits have become more advanced, so-called “smart” triggers have enabled users to view a variety of signal features such as pulse width, runt, glitch and many more. The one feature that has not changed is that the trigger remains a separate circuit from the acquisition function, which is universally performed by an A/D converter in modern oscilloscopes. Thus, the trigger function has remained a primarily analog circuit.

The new RTO Series of oscilloscopes from Rohde & Schwarz feature a digital trigger, which has a number of advantages over conventional designs. The digital trigger is implemented in hardware using an ASIC to process the digitized samples from the A/D in real time to provide the trigger function. Because the signal path for the digital trigger and the acquisition is the same, the new scopes can deliver a number of distinct benefits which will be outlined here.

Trigger jitter is a significant problem in conventional analog triggers. This phenomenon is caused by the separate physical paths that the signal must take between the acquisition and trigger circuits. The noise and delay over frequency of the two circuits results in uncertainty between the time when the trigger condition is met and when the specific feature in the signal under test is acquired. Trigger jitter can be seen as lateral movement of the signal on the instrument screen as the display is updated. Another contributer to this form of jitter is the noise in the oscilloscope front end. Noise adds to the signal and, at the signal transitions, causes jitter as a result of the signal’s finite slew rate. For example, a signal with a slew rate of 2V/ns plus additive noise of 3mV RMS will contribute (.003) / 2ns or 1.5ps of jitter on top of the existing trigger jitter. A digital trigger, on the other hand, theoretically contributes no jitter to the signal, thus meaning that all of the observed jitter would be from noise as a result of the signal slew rate. This is typically less than 500 femto-seconds RMS for a 1GHz sine-wave signal.

Another issue encountered in trigger circuits that are separate from the acquisition system is that of sensitivity. Oscilloscopes have a specified bandwidth and rise time, but the trigger circuit has its own bandwidth characteristics. Trigger specifications generally list the sensitivity in vertical divisions at certain frequencies, and the upper frequency limit of the trigger is usually less than that of the scope. This can lead to some confusing results when, for example, a user is viewing a signal that is 2 divisions high, but at the signal frequency the trigger sensitivity is 3 divisions. In this case, the scope will not trigger even though the signal appears to be well within the instrument’s range. Since the digital trigger shares its signal path with the acquisition, any signal viewable on the screen can be triggered.

Digital filters are commonly used in oscilloscopes to remove noise or other effects that are outside of the frequency range of interest. Conventional trigger circuits generally include simple filters with limited adjustment range, further limiting their ability to trigger on the observed waveform. A digital trigger can share the same filter with the viewable waveform, thereby enabling the trigger to benefit from the flexibility of digital filters.

Digital triggering provides significant benefits in both the usability and accuracy of oscillscope measurements. Usability is enhanced by providing the ability to trigger precisely on the viewable waveform, greatly simplifying setup. The virtual elimination of trigger jitter allows for more precise timing measurements, such as rise time, and also enables quick and accurate jitter measurements without the need for special jitter measurement software. Finally, the use of digital filters as well as threshold hysteresis provides a much higher degree of noise rejection, enabling stable triggering on noisy signals.

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