How do I extract the desired information from my acquired measuring signal? Suitable triggers help to detect and visualize one-time or recurring events.
The first part of our introduction to the world of digital oscilloscopes was about the basics of an oscilloscope: how are the signals sampled, stored and displayed on the screen? In the second part, we will show you how to capture the desired signal shape and evaluate the results.
One of the most difficult tasks for the user of an oscilloscope is to capture the desired event from the multitude of data accurately. We are looking for one-time or repetitive events in the data stream. Various triggers that can be used for analog and digital channels help here.
There are three different modes to trigger a trigger: The “Normal-Trigger” continuously triggers events, while the “Single-Trigger” only triggers the trigger once. The “Auto-Trigger” triggers either when the required event occurs in the data stream, but at the latest after 100 ms automatically.
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Hardware and software triggers in comparison
Modern oscilloscopes have a variety of trigger options. Here, a distinction must be made between hardware and software triggers. The hardware trigger is fed directly with the analog data acquired by the preamplifier. In this case, the event to be checked is evaluated directly in the hardware, usually an FPGA. This has the advantage that trigger results are available quickly and without large coverage gaps between the trigger times.
The user has to decide what type of available trigger he wants to use. Simple edge and pulse width triggers to complex “smart” or “measurement triggers” are available depending on the oscilloscope. Even serial busses (specific messages, errors or bit combinations) can be easily and quickly trigged. It is also possible to cascade trigger events and compare trigger levels across multiple channels.
Evaluate and record different events
After properly triggering and capturing the desired event, it is important to be able to evaluate the data directly in the device. A simple tool for this is the use of “Cursor.” With “Cursor” you can quickly analyze all measuring points visible on the screen. A horizontal or vertical “cursor” and a combination of the two types can measure and check signals on the screen.
A more elegant and versatile application for evaluating and recording various events – even over many measurement cycles – is the automatic parameter measurement method. For many measurement tasks, parameters are already available. The measurement of a frequency or the pulse width is almost automatic and can also be statistically evaluated and graphically visualized.
The whole thing does not happen over a measuring cycle but is done for a very long time. A distinction is made here between vertical and horizontal parameters. Typical vertical parameters are amplitude, minimum or maximum, the area between curve and time axis, peak to peak values. Representatives of the horizontal parameters are frequency, time offset, the distance between two edges or pulse widths.
Especially with serial buses, the user not only expects support from his oscilloscope in the trigger area. Also, various parameters for serial buses are offered. Optimal evaluation of the physical layer of a serial bus also includes the ability to decode real-time data directly in the oscilloscope. From simple buses like Lin, SPI or I²C to complex ones like FlexRay, USB and SATA, every protocol must be available.
Capture properties and errors of a bus
The decoding can be offered in various forms. Ideally, the decoding of the messages is both color-coded and output in text form directly above the measured signal on the screen. It is also possible to display the decoding of the data packets in a table. These tables can be easily exported and evaluated in other programs. By linking the table with the recorded data, the desired data package can be zoomed in quickly and easily.
The combination of triggers, parameters and decoding makes it possible to capture errors and characteristics of a bus. On some buses, such as USB 2.0, it is possible to present a complete log analyzer overview on the oscilloscope. This allows the user to easily check if the error is in the physical or protocol level.