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View noisy signals with a stable scope trigger

Posted: 29 Aug 2014 ?? ?Print Version ?Bookmark and Share

Keywords:Noise? test equipment? oscilloscopes? ADC? trigger?

A digital triggering system performs the trigger evaluation on the digitized data after the ADC. This is same exact data that is used by the memory system. By putting the DSP filter ahead of the trigger circuit, we can apply the same filtering to both the trigger system, and to the displayed acquisition data. Although significantly different in implementation, it functions similar to the hardware filters described above with two useful exceptions. The first exception is that we have more filter steps to use. By having a variable set of low-pass filter cut-off values, we can intentionally reject just the right amount of noise in our signal. We aren't just beholden to the 20MHz and 200MHz values in a hardware filter. As figure 5 demonstrates, we can filter down to 1MHz, which removes all of the high-frequency noise on the ripple signal, but allows for a stable trigger and clean signal.

The second useful difference between a DSP filter used in conjunction with a digital trigger and a hardware filter involves choice. A hardware front-end filter simply rejects all signal content to the oscilloscope above the cut-off frequency, effectively making it a 20MHz or 200MHz oscilloscope. The DSP filter is simply operating on the data, so the oscilloscope retains full frequency. If used in a flexible architecture that allows selectively applying the DSP filter independently to the trigger and/or the acquisition/display system, we can have the best of both worlds. Figure 5 was rejecting all frequencies above 1MHz to both the triggering system and the acquisition/display system. Choosing to have the trigger see the filtered signal, but not the acquisition/display produces a stable trigger. The original signal still includes all of the inherent noise (figure 6). This ends up with an identical picture for this signal as the variable hysteresis, but utilising a different technique.

Figure 5: A 1MHz filter produces a stable trigger on a power-supply ripple signal.

Figure 6: A DSO filter produces stable Trigger because it's applied to the trigger only. The original signal is still displayed.

An additional potential benefit of utilising a variable DSP Filter and digital trigger system can be for debug or design prototyping if a signal was unexpectedly noisy. By alternating between the filtered and non-filtered acquisition/display, you could use variable DSP steps to determine the right amount of filtering needed to clear up a noisy signal path. This might speed up the re-design efforts for the next phase of your project.

A variety of techniques can be deployed on signals that contain noise or are operating in a noisy environment. Ensuring a stable trigger in noise isnt a one size fits all technique, but utilising a methodology of different techniques and some more advanced features on an oscilloscope can wrestle down the problem.

Utilising the same example for the three different techniques, they may seem interchangeable. But they operate in different ways on the signal so each has benefits and trade-offs.

About the author
Dave Rishavy is with Rohde and Schwarz.

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