Noise on a signal creates a triggering challenge for test equipment, especially oscilloscopes. Becuase 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 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 stabilizes 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.

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

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 points—most typically 20 MHz and/or 200 MHz, 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.

Depending on your signal and the frequency of your noise, filters can be an effective method for creating a stable trigger. Figure 2 shows the effects of a 20-MHz filter. Much of high-frequency noise is reduced, but we still haven't achieved a stable trigger because there's still noise present and the ripple is far below the filter's cutoff frequency. Because this filter didn't work, we know that the 200-MHz filter also won't. This means we should move on to technique #2. (Typically, the use of a 20-MHz filter on a higher-speed signal will filter out too much signal content and create a poorly reconstructed sinusoidal-looking waveform.)

Figure 2. With a 20-MHz low-pass filter, there's still too much noise to get a stable trigger.

Hysteresis/Noise reject Oscilloscopes commonly come with a noise-reject feature on the trigger input. Also called a trigger-hysteresis band, the filter works by rejecting signal movements within a certain tolerance band of the trigger level (Figure 3 ). The noise-reject option is often located within an oscilloscope's trigger menu.

Figure 3. A hysteresis filter requires that a signal cross through it in both directions to be considered noise.

Trigger hysteresis can be an effective way to “ignore” the noise going into the trigger circuit, thus allowing a stable trigger. The hysteresis band must be large enough to reject the noise carried on your signal. This can also create a trial-and-error case to understand if the trigger hysteresis supplied by the noise reject is enough for your signal. By understanding your equipment and the signal in question, you can make trigger hysteresis work.

Some oscilloscopes have a variable trigger-hysteresis band, which lets you “tune” the hysteresis band to match the level of noise on the signal. This can be particularly useful because it will reject high-frequency noise, but still allow for a higher frequency signal to be viewed amongst noise. An example of our power-supply ripple with a larger hysteresis band enabled by this oscilloscope having a variable trigger hysteresis function is used to capture a stable trigger (Figure 4 ). The hysteresis band is overlaid on the signal and we can adjust it to just the right amount. With this stable trigger, we can now perform measurements. Furthermore, we also see the original noise on the signal so that we know just how much noise is present.

Figure 4. A variable hysteresis band enables a stable Trigger on the ripple signal.

Variable DSP and digital triggers While post-processed DSP (digital signal processing) filters are fairlycommon on oscilloscopes today, the fact that they are processed onacquisition data stored in the memory, after the ADC, prevents them frombeing used to create a stable trigger. Unlike low-pass hardwarefilters, a traditional analog trigger circuit is viewing a copy of theanalog data that was picked off before it is digitized. To assist withthings like stable triggering, some oscilloscopes offer a digitaltriggering system.

A digital triggering system performs the trigger evaluation on thedigitized data after the ADC. This is same exact data that is used bythe memory system. By putting the DSP filter ahead of the triggercircuit, we can apply the same filtering to both the trigger system, andto the displayed acquisition data. Although significantly different inimplementation, it functions similar to the hardware filters describedabove with two useful exceptions.

The first exception is that we have more filter steps to use. Byhaving a variable set of low-pass filter cutoff values, we canintentionally reject just the right amount of noise in our signal. Wearen't just beholden to the 20 MHz and 200 MHz values in a hardwarefilter. As Figure 5 demonstrates, we can filter down to 1 MHz,which removes all of the high-frequency noise on the ripple signal, butallows for a stable trigger and clean signal.

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

The second useful difference between a DSP filter used in conjunctionwith a digital trigger and a hardware filter involves choice. Ahardware front-end filter simply rejects all signal content to theoscilloscope above the cutoff frequency, effectively making it a 20 MHzor 200 MHz oscilloscope. The DSP filter is simply operating on the data,so the oscilloscope retains full frequency. If used in a flexiblearchitecture that allows selectively applying the DSP filterindependently to the trigger and/or the acquisition/display system, wecan have the best of both worlds. Figure 5 was rejecting all frequenciesabove 1 MHz to both the triggering system and the acquisition/displaysystem. Choosing to have the trigger see the filtered signal, but notthe acquisition/display produces a stable trigger. The original signalstill includes all of the inherent noise (Figure 6 ). This ends up with an identical picture for this signal as the variable hysteresis, but utilizing a different technique.

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 utilizing a variable DSP Filterand digital trigger system can be for debug or design prototyping if asignal was unexpectedly noisy. By alternating between the filtered andnon-filtered acquisition/display, you could use variable DSP steps todetermine the right amount of filtering needed to clear up a noisysignal path. This might speed up the re-design efforts for the nextphase of your project.

A variety of techniques can be deployed on signals that contain noiseor are operating in a noisy environment. Ensuring a stable trigger innoise isn’t a one size fits all technique, but utilizing a methodologyof different techniques and some more advanced features on anoscilloscope can wrestle down the problem.

Utilizing the same example for the three different techniques, theymay seem interchangeable. But they operate in different ways on thesignal so each has benefits and tradeoffs.

Also See De-embedding improves measurement accuracy DSO fault triggers reveal what went wrong Reduce time-base errors in sampling oscilloscopes Scope basics: Multiple-domain measurements Calibrate scope jitter using a transmission-line loop Use the right data-acquisition mode Understanding the impact of digitizer noise on oscilloscope measurements Perform five common debug tasks with an oscilloscope 10 tricks that extend oscilloscope usefulness

“Hi Dave,nMost oscilloscopes offer high and low frequency reject trigger modes. High frequency reject inserts a low pass filter in the trigger path; while low frequency reject inserts a high pass filter. Most set the cutoff frequencies around 50 kHz.”

“Thanks for reading the article and taking the time to comment Phil. This is an area of subtle distinction. You are correct, trigger coupling modes can have the effect to limit the noise to the trigger path. Their effectiveness would rely on the noise a

“Sometimes it is helpful to use an external trigger source. E.g. if the ripple has a mains synchronous component, the mains frequency can be used as trigger source. If the source of the ripple is a inverter or a switch mode power supply, it is possible to

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