Monday, September 17, 2012

Overview of Oscilloscope Triggering

In this post we will take a look at Oscilloscope triggering. We will cover the basics and look at some of the advanced triggering capabilities found in modern digital scopes. Triggering is often the least understood but one the most important capabilities of a scope. You can think of oscilloscope triggering as “synchronized picture taking”. And one waveform picture actually consists of many individual and consecutive digitized samples. When monitoring a repetitive input signal the oscilloscope performs repetitive acquisitions (or repetitive picture taking) to show a “live” picture of your input signal. This repetitive picture taking of the oscilloscope must be synchronized to a unique point on the input signal in order to show a stable waveform on the scope’s display.

Although some scopes have various advanced triggering modes to choose from, the most common type of triggering is to trigger the scope when the input signal crosses a particular voltage threshold level in either a positive or negative direction. We call this “edge triggering”. In other words, the scope triggers (takes pictures) when the input signal changes from a lower voltage level to a higher voltage level (rising edge trigger) or when the input signal changes from a higher voltage level to a lower voltage level (falling edge trigger). A photo finish of a horse race is analogous to oscilloscope triggering. To accurately record the finish of the race, the camera’s shutter must be synchronized to when the lead horse’s nose crosses the finish line in the forward direction.

Edge Triggering Examples
Let's look at two examples of oscilloscope edge triggering. In the screen-shot below, the scope’s trigger level is set above the waveform. In this case the input signal never crosses the trigger threshold level in any direction. The scope is taking asynchronous pictures of the input signal and we observe what appears to be an unstable waveform. This is actually an example of not triggering – or unsynchronized picture taking.

Untriggered (unsync'd picture taking)
In the below screen-shot, the scope is setup to trigger on rising edges of the input signal with the trigger level set at +2.01 V. In this case, we can see a rising edge of the input signal at exactly center-screen. 

Triggering on Rising Edge @ +2.01 V
Although the default trigger localization on all digital oscilloscopes is at center-screen (horizontally), you can re-position the trigger location to the left or right by adjusting the horizontal delay knob – sometimes called the horizontal position knob. Older technology analog scopes are only able to trigger at the left side of the screen. This means that analog oscilloscopes are only capable of showing portions of waveforms that occur after the trigger event – sometimes called “positive time data”. But DSOs are able to show portions of waveforms both before (negative time or pre-trigger data) and after (positive time data) the trigger event. Observing pre-trigger data can be useful for analyzing waveform data that may have led up to a specific error trigger condition.

Edge triggering is by far the most common method used for scope triggering. Modern digital scopes also include many advanced triggering methods such as Pulse Width, Pattern, and Runt. Let's take a quick look at a few advanced triggering methods in modern scopes.

Advanced Oscilloscope Triggering – Rise/Fall Time
The below screen shot is an example of signal parametric violation triggering. In this case we are showing an example of triggering on a rising edge that fails to meet a specified rise time of 100 ns. This scope can also trigger on falling edge violations, as well as edge speeds that are faster than a user-specified time.

Triggering on rising edges if slower than 100 ns
Advanced Oscilloscope Triggering – Setup & Hold Time
In the below screen-shot we showing an example of a setup & hold time violation. The scope has been setup to trigger on a rising edge of a clock signal (channel-1, yellow trace). The green trace shows the channel-2 waveform, which is a data signal represented as an eye-diagram. When writing data into a storage device, shift register, or latch, the data signal must be stable for a minimum amount of time before the arrival of the clock signal. This is called “setup time”. In addition, the data signal must remain stable (high or low) for a minimum specified time. This is known as “hold time”. 

Edge Triggering Reveals Data Edge Shift
In this particular example we can see that the data signal occasionally shifts in the positive time direction closer to the clock edge. We know that it is an occasional or infrequent timing shift as evidenced by the dimmer intensity of the trace (assuming that the scope has waveform intensity gradation capability). This is a violation of the device’s setup time.

In this post we looked at oscilloscope triggering, from basic to advanced. If you have anything to add to this post use the "Comments" section below and if you have any questions send me an email.

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