View noisy signals with a stable oscilloscope trigger

August 18, 2014 // By Dave Rishavy
View noisy signals with a stable oscilloscope trigger
Dave Rishavy of Rohde and Schwarz considers how to deal with noise on a signal which poses a triggering challenge for test equipment such as oscilloscopes.

Noise on a signal creates a triggering challenge for test equipment, especially oscilloscopes. Because the instrument itself also contributes noise, small signals in the millivolt range need proper instrument settings prevent noise from overwhelming the signal of interest. Even with larger-ampltude signals, noise can create a condition where a stable trigger is difficult to achieve.

Oscilloscope have built-in features to help deal with the noise. These features can sometimes be buried in menus, or not well known by infrequent oscilloscope users.

You should distinguish between simply suppressing and/or dealing with the displayed noise, and actually delivering a less noisy signal to the trigger circuit. Only the latter will create a stable trigger in these environments. Because oscilloscopes often route a small portion of the incoming electrical energy to a separate analog trigger circuit, any noise suppression techniques need to occur on the incoming signal, not the ADC processed or displayed signals. By triggering on post-ADC data, additional techniques for creating a stable trigger in noise become possible.

Suppressing noise
Common techniques for dealing with noise utilize averaging and/or using High Resolution mode. Averaging, which works on repetitive data only, is effective at combining data points from multiple acquisitions to reduce the displayed noise. Because this is a displayed data technique, it won't suppress noise to the trigger circuit, and thus won't create a stable trigger. Averaging won't work on a single-shot event.

Many oscilloscopes have a high-resolution mode that can be useful for averaging out noise even on a single-shot capture. This method takes advantage of the fact that many signals don't require the oscilloscope's full sample rate. If, for example, you look at a 10-MHz signal with a 1-GHz oscilloscope sampling at 5Gsamples/s, you're acquiring 500 samples for each signal period. Most oscilloscope vendors recommend 5-10 samples per period for adequate signal reconstruction, so this is about 50X more than needed.

High Resolution mode utilizes these extra samples within

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