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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 

Noise on a signal gives way to 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 analogue 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 utilise 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 10MHz signal with a 1GHz 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.

Figure 1: Raw Simulated Power Supply Output Ripple contains noise.

High Resolution mode utilises these extra samples within a trace to average them into a less noisy signal reconstruction. Because it is done post-ADC on the incoming signal, it can suppress noise. Again, this is after the ADC, so therefore not delivered to the trigger circuitry, and it won't create a stable trigger on the oscilloscope. An additional consideration is that it can only be used on lower speed signals, so effectively it will limit the bandwidth of signal the oscilloscope can view.

Create a stable trigger
No one technique will work across the board for gaining a stable trigger. Often the task of obtaining a stable trigger is a trial-and-error process. Three techniques below can be tried to see if the trigger stabilises the display. Usually one of these three will achieve the desired result. The signal we will use as a test case (figure 1) is the simulation of an output of a switch-mode power supply ripple. The output of switched mode power supplies carry high-frequency noise and can be difficult to trigger. That's because the signal we want to measure or view is a small ripple on top of a DC offset signal. This ripple is often small (mV) and in the presence of high-frequency noise and much larger noise generated by the switched-mode supply. Simply viewing the ripple isn't possible due to the lack of a stable trigger.

Using the hardware filter
Techniques that begin to create a potentially stable trigger include using hardware low-pass filters supplied on most oscilloscopes. These bandwidth filters are often at defined pointsmost typically 20MHz and/or 200MHz, limiting the bandwidth almost immediately after the incoming signal enters the channel path. Although the bandwidth is limited, the signal is filtered before the trigger system.

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