by Barrett Poe, Keysight Technologies, Inc.
User interfaces have become more advanced and measurements more automated – and less tolerant of improper measurement setups.
Long before every oscilloscope carried an autoscale button, and before touch-screen triggering was a gleam in someone’s eye, aspiring engineers would need to manually adjust settings for the time base and voltage sensitivity. And they would have to do so before even considering the way they would hook the probes to their projects. After all, how would other students react if they found out who fried the only oscilloscope in the department?
Now, more affordable equipment is becoming available to universities. There’s a trend toward multiple well-stocked labs fitted with the newest mixed-signal digital oscilloscopes. Measurement automation is gaining traction, but at a cost: Some students may miss out on learning how their lab equipment can bring faster, more efficient lab hours and streamlined report writing. Here is a list of the basics that can help newly minted engineers excel, and help engineering students stand out among their classmates.
Take Advantage of Open Lab Hours
This isn’t technically a function of an oscilloscope, but it will enable you to try out features without being pressed for time or fighting over bench space. With the advent of cheap and accessible microcontrollers, the maker revolution, and the ever increasing popularity of the Internet of Things (IoT), there are many reasons to make use of open lab time. Lab equipment is a resource that can promote learning, not sit on a shelf to collect dust. In school, most of the time all it takes to get an after-hours lab key is a desire to learn and a professor’s signature. At work, after-hours access to equipment is often even easier.
Avoid the “Auto” Temptation
While the autoscale button has useful function in some situations, you will gain more insight into your measurement by avoiding it. Additionally, the autoscale button will not always function as you intend. For example, trying to probe the envelope of an amplitude-modulated waveform can lead to misleading measurements and confusion. If you refrain from using this feature, you will have a better understanding of your measurement and how the scope is being configured. This also leads to the need for knowledge about the basics of triggering…
To master your measurement setup, you’ll need to know how to set up the triggering basics — the channel on which you are triggering, the voltage at which you are triggering, and settings for rising/falling edges. These are mostly straightforward. They’re covered in the first or second term of introductory EE classes. Besides these basics, a few more advanced features can help out when triggering on digital signals, especially when they have periodic packets of information.
If you see your signal overlap with itself, as in the accompanying figure, you may need to adjust the Trigger Holdoff Time. Trigger Holdoff Time, or just Holdoff, is the minimum amount of time between a trigger event and when the trigger resets for the next trigger event. In general, you want to set this period to a value just shorter than the period of the incoming packets. In this way, you can trigger on the first transition of one packet and then “hold-off” on the next trigger until just before the next packet arrives.
Roll & XY Deflection
Two more useful modes on most modern scopes are Roll and XY Deflection. Roll mode removes the triggering and displays the channel outputs as a steady stream of data. Similar to how you would read a stock ticker, the newest data comes in on the right and the old data leaves on the left. In this way, one can get a real-time glance at what is on the probe, without having to wait for the trigger to activate.
This feature is especially useful for measuring slow-moving signals where you would like to know the data you capture before the trigger threshold voltage is met. Two application examples include voltage discharging from a large capacitor, or a pulse monitor that senses a heartbeat every second. The timebase can still be modified to set the time for data to display before leaving the screen.
XY Deflection mode removes the timebase and allows the user to control the x and y axes with two channels of the scope. Back in the days of cathode-ray oscilloscopes, the scope could be configured to deflect the electron beam based on the voltage of the two channels — one for the x-axis, one for the y-axis. This feature is useful for displaying hysteresis on Schmitt triggers. In more advanced signal processing, this mode has practical applications in audio devices for measuring the phase between left and right channels, where phase information will produce a Lissajous curve.
Report writing is a tedious aspect of engineering that will dominate your entire college and professional career. Getting in good habits early will save you time and stress in the long run. You may be tempted to save under the equipment’s default naming/numbering scheme and sort out the data later. This can be a dangerous game to play. Instead, save acquired data with descriptive names. Don’t forget to add a name label on each channel to provide further documentation for the test setup.
Another feature of modern lab equipment is the ability to save and recall test setups. Taking a few minutes to store the timebase, channel sensitivity settings, channel names, and trigger settings into a test setup file will help you pick up where you left off at the next lab session. Adding a setup file for power supplies and signal generators will also cut setup time and prevent missteps the next time you turn on the instrument.
Math functions on an oscilloscope are the powerhouse of report writing. Math measurements not only help calculate data, they can also display these results on the screen, a feature helpful for documenting results. With a little creativity, it’s easy to acquire more than just your system’s time data. To capture the bandwidth of a filter, for example, use a frequency sweeping sine wave and the FFT (Fast Fourier Transform) function, then turn on the scope’s persistence display feature. After sweeping a few times you can measure the corner frequencies easily. In addition to the internal math functions supplied with the scope, you can define your own math functions to really push the limits of your measurement equipment. Just be careful not to rely on automated measurements too much. When in doubt, the cursors are handy as well.
Even after mastering the above techniques, there is always more to learn. Active probing, segmented memory, Non Return to Zero (NRZ) protocols, and Mixed Signal Oscilloscopes (MSOs) are just a few of the additional features and capabilities found in modern oscilloscopes. As data rates skyrocket in the fifth-generation (5G) wireless space and oscilloscope bandwidths rise beyond 100 GHz, knowledge of how to use your equipment will grant insight that will give your measurements the competitive edge.
Keysight Technologies Inc.