If your sample rate is not fast enough, you won’t be able to see your signal accurately on the oscilloscope screen. Sample rate is the number of samples an oscilloscope can acquire per second. This determines the resolution of your waveform. Read on to learn why.
A sample is a single value at a point in time. You could think of a sample like one piece in a puzzle. The more pieces you assemble over time, the more apparent and complete the picture becomes.
But unlike a puzzle, reconstructing a waveform on an oscilloscope is not solely dependent on the number of samples that are strung together. The speed at which you sample matters too. A puzzle is a static picture. Therefore, it doesn’t matter how long it takes you to assemble a puzzle – the result will still be a complete picture in the end. However, electric waveforms change with time. So, to get a complete picture of the waveform, we need to sample fast enough to capture it. That is why we talk about the specification in terms of a rate. We need a fast sample rate to properly display our device’s signals on our test equipment.
We know from Harry Nyquist that we need to take equally spaced samples of a signal at at least twice the rate of the signal’s highest frequency component to represent that signal without errors.
Fsampling ≥ 2Fsignal
This definition is given as a minimum requirement for proper sampling. You want your oscilloscope to provide more than just a minimum requirement.
There are two key oscilloscope specs that determine if your signal will be displayed properly on screen: bandwidth and sample rate. In my previous blog “What is Bandwidth and How Much Do You Need,” we discussed the importance of bandwidth. From that blog you’ll know that without enough bandwidth, you’ll have an attenuated and distorted signal. But, it’s also important to know that without enough sample rate, you will be without all the waveform information that is necessary to display the frequency of your signal, exact rise and fall times, the height and shape of your signal, and any glitch or anomaly that may be occurring.
When you probe your device and connect it to an oscilloscope, you are sending an analog signal into the oscilloscope. Then, the scope samples and digitizes the signal, saves it in memory, and displays it on screen.
Figure 1. Simplified block diagram of signal flow from a DUT through an oscilloscope.
The default sampling setting on your Keysight oscilloscope is automatic in real-time sampling mode. Automatic sampling will select the sample rate for you. The scope will choose the highest sampling rate possible, using as much memory as necessary to fill the display with your waveform information. In real-time sampling mode, all the samples of the waveform are taken from one trigger event and are evenly spaced in time. (If you aren’t familiar with the term trigger, that is basically the event that time-correlates your device’s waveform within the oscilloscope, allowing the waveform to be steadily displayed on screen.) The scope may also apply interpolation to fill in gaps between samples.
If you don’t want the oscilloscope to select the sample rate for you, most oscilloscopes allow you to set the sample rate yourself. If you set the sample rate yourself, remember: two times the frequency is the absolute minimum rate you should use. When it comes to oscilloscopes, I recommend choosing a sample rate faster than this. Usually choosing a sample rate that is 3 to 5 times the bandwidth of the oscilloscope will give you a high-enough sampling rate to capture the details of your signal, including its frequency of oscillation and the rise times of your waveforms. You need a sample rate that will provide enough detail to see any unexpected glitches or anomalies.
The more samples you have in each period, the more signal detail you'll capture.
One last thing to double check is the sample rate of the oscilloscope when all channels are turned on. Typically, when multiple channels are in use, the sample rate is split up among the channels. If you are using more than one channel, you’ll want to make sure the sampling rate is still sufficient.
The Specs You Need to Know
While bandwidth is the number one oscilloscope specification, sample rate is a close second. The oscilloscope sample rate determines the amount of waveform information captured and displayed on screen. You need a sample rate that will accurately show all aspects of your signal including its standard shape, accurate rise times, and glitches. You could be missing vital design flaws without being able to view a glitch, or you could waste hours trying to determine why your signal looks differently than you expected just because your scope was under sampling.
To learn about the other need-to-know oscilloscope specs to set you up for successful measurements, check out the Basic Oscilloscope Fundamentals application note.