Oscilloscope Basics: External, Line and Fast Edge "Triggers"

An oscilloscope trigger synchronizes the oscilloscope timebase to the input signal so that the displayed trace is stable. In digital storage oscilloscopes, while the digitizer runs continuously converting analog voltage/current inputs to digital values, it is the trigger event that defines the “acquisition window,” marking the point where data is stored to acquisition memory, locking the signal data for display, measurement and further processing. 

Figure 1: The trigger setup showing the possible choices for the trigger source.

Triggers are set to fire based on the state of a trigger source waveform. What are commonly known as External, Line and Fast Edge "triggers" are not really different trigger types, per se, but alternative trigger sources.  Figure 1 shows the typical setup options for an Edge trigger, the most commonly used trigger type.  With Edge triggering, the oscilloscope is triggered when the source waveform crosses a user-defined threshold level and slope.  Usually, the source will be analog input channel C1-Cn. However, three other sources can be used to initiate an Edge trigger: an Ext(ernal) input, the Line (mains) power and, on some oscilloscopes, the built-in Fast Edge signal. 

Measuring Clock Jitter Sensitivity to Power Rail Noise, Pt. 2

Figure 1. 400 mVpp oscillation on the power trace
is due to 48 MHz clock noise.

In Part 1, we used a function generator to create a power source with a known perturbation. Seeing that the noise on the power rail and the clock period were synchronous when we observed both traces together using a WavePro HD oscilloscope, we knew that there was a clear relationship between the two to be further investigated. Now, we're ready to examine more closely how the clock jitter responds to voltage variations on the power rail.

Measuring Clock Jitter Sensitivity to Power Rail Noise, Pt. 1

Figure 1. Voltage variations on the power rail
shown in the same grid as the clock period track
(jitter track). These waveforms are the basis of
the clock jitter sensitivity measurement. The
inverse relationship between the jitter track and
the power trace shows that the clock
is sensitive to variations in rail voltage.

In a previous post, we described A Robust Method for Measuring Clock Jitter with Oscilloscopes as variation in a clock signal’s period. Clock jitter is characterized by the standard deviation (sdev) of the clock period measurement. The track function of the clock period sdev shows us the variations in jitter over time, synchronous with the waveform source. 

In this post and the next, we’ll show how to make use of the clock period track function to match jitter variations to possible sources of jitter, in particular to voltage variations on the clock power rail. The offset voltage of a function generator powers a clock signal source. By creating a known variation in the function generator output, we can match that to the resulting clock jitter to calculate the clock jitter sensitivity to rail voltage changes. A known clock jitter sensitivity value can help you predict how a design will respond to rail voltage changes.

As in our previous post, the clock is a 5-stage ring oscillator based on the the 74AC14 hex inverter, powered by a 5 V rail. The test instrument is a WavePro HD 12-bit, 4-Ch, 8 GHz, 20 GS/s, 5 Gpts oscilloscope with 50 ps time resolution and 60 fs intrinsic sample clock jitter, which is used to measure a square wave clock signal between 10 and 66 MHz. For this experiment, we also use a 5 V DC clean power source to test what our clock jitter is with an “ideal” rail, and a function generator to generate a perturbing signal that will put noise on our 5 V power rail so we can test how the clock jitter responds to it.

Setting Up Your Oscilloscope for EFT Testing

Figure 1: The typical EFT test signal consists of
multiple exponential pulses arranged as pulse bursts.

A third type of electromagnetic compatibility (EMC) testing deals with how devices respond to electrical fast transients (EFT). EFTs are a series of fast, high frequency pulses, often occurring in bursts. These transient events are the result of electrical arcing. EFT pulse bursts occur when a power connection is made or broken, equipment is powered down or circuit breakers are switched. They also occur when inductive loads such as relays, switch contactors or heavy-duty motors produce bursts of narrow high-frequency transients on the power distribution system when de-energized.

The typical compound waveform used for EFT testing is shown in Figure 1.

Figure 2: Acquisition of two EFT bursts at
1.25 GS/s, zoomed to show several timing epochs.

The pulse waveform for EFT testing is defined by a risetime of 5 ns and a pulse width of 50 ns. These pulses are combined into bursts of 5 to 50 pulses spaced at from 10 to 100 µs (10 to 100 kHz). The bursts are typically spaced as much as 300 ms apart. Figure 2 shows an EFT test signal, with two EFT bursts captured by an oscilloscope on channel 2 sampled at 1.25 GS/s.

The two EFT pulse burst are shown in the upper left-hand trace (the two pink blocks). A series of zoom-on-zoom traces are opened to show several timing epochs. Trace Z2 (second from top left) shows a single burst. The zooms keep expanding the horizontal scale until finally at Z8 (bottom right) we see a single EFT pulse.

Following are six, important things to do to make sure you get the best EFT test measurements from your oscilloscope.

Setting Up Your Oscilloscope for Surge Testing

Figure 1: A typical EMC surge test waveform with
1.2 µs rise time and 50 µs half amplitude time.
Zoom trace Z1 shows the details of the rise time.
Click any image to expand it.

Electrical surges result from phenomena like lightning strikes and switching transients.  Electronic devices are subjected to simulated surges to confirm that they continue to operate properly following a surge, just as they are tested against electrostatic discharge. 

Surge pulses are similar to ESD pulses in that they have a very fast rise time, but the fall time is much, much slower. Figure 1 shows a typical surge pulse waveform. Surge testing involves similar measurements to those made for  ESD pulse testing—such as rise time, pulse width and max—but surge testing also requires some additional measurements, such as area under the pulse curve and transmitted charge. 

Figure 2: To verify the surge generator waveform, the
oscilloscope is connected to the generator through an attenuator.

As with ESD pulse tests, oscilloscopes are primarily used to “test the tester,” confirming the output of the surge generator. So, before a surge generator is hooked up to the device under test, it's required to hook it up to an oscilloscope and conduct a series of measurements to make sure the surge pulse is within specification. Figure 2 shows a typical surge test setup.

Following are three, important things to do to make sure you get the best surge measurements from your oscilloscope.

Setting Up Your Oscilloscope for ESD Pulse Testing

Figure 1: An ESD calibration test setup. 
The ESD gun discharges its waveform
into a properly attenuated current target. 

Electrostatic discharge (ESD) pulse tests are a type of conducted immunity testing done to confirm that a device can withstand a sudden transient electrostatic discharge. It is done by using an ESD gun to shoot a pulse of the required voltage at a device while testing that the DUT continues to operate properly. The ESD pulse shape simulates a person, carrying a static charge, touching a device. When their fingertip first touches the device, there is a leading edge with a high peak and fast decay, often visible in the real world as a spark flying from the fingertip. This is followed by a second edge due to the charge in the rest of the human body that propagates toward the fingertip with a time delay.

Oscilloscopes are most often used to “test the tester” in ESD pulse test setups, confirming that the pulse from the ESD gun is the right shape and meets the requirements of the standard to which the device is being tested. A typical calibration test setup is shown in Figure 1. The pulse from the ESD gun is fired directly into a current shunt target connected to the oscilloscope through an attenuator required to keep the signal within the limits of the oscilloscope’s 50 Ω input, which is used for this testing. Then, key parameters of the ESD pulse are measured per one of several standards, such as IEC 61000-4-2.  

ESD standards require a range of measurements. The most common are the initial edge 10% to 90% rise time, peak amplitude, pulse width, amplitude and current levels at specified times from the initial edge (e.g., T1 and T2), and time to half value. 

Following are four, important things to do to make sure you get the best ESD pulse measurements from your oscilloscope.

Oscilloscope Basics: When to Use Trend to Graph Oscilloscope Measurements

Figure 1: Applying the Trend operator to the
same input waveform illustrates how the
Trend is asynchronous to the input waveform.  

In a previous post, we described the characteristics of the Track math function and two key applications of using Tracks to graph oscilloscope measurement data: anomaly detection and waveform demodulation. In this post, we'll discuss the characteristics and uses of the Trend function.

To illustrate an important distinction between Tracks and Trends, the Trend math operator in Figure 1 is now applied to the same signal as was the Track in our previous post without first reacquiring the input waveform. 

Note that unlike a Track, the Trend is not time-synchronized to the input waveform. Only the order of events, and not the timing of events, is retained. The underlying shape of the Track may be displayed in the Trend because the same measurement values from a single acquisition are displayed in the same sequence—however, the timing information of when each of the values has occurred is not retained in the Trend. Therefore, unlike the Track, the Trend does not point to the location of an anomaly. Without time scaling, the Trend does not have the frequency information needed to demodulate an input waveform.