A brief analysis of the thickness properties of oscilloscope waveforms and the impact of noise on waveform thickness

Oscilloscope waveforms represent real electronic signals. When evaluating an oscilloscope's performance, you can look at its ability to display a waveform with the same shape as the target signal. Assuming the oscilloscope has adequate basic specifications - such as bandwidth, sampling rate and isofrequency response, should the oscilloscope display thick or thin waveforms better? The answer to this question is the same as most engineering questions: "It depends on the specific situation."

Now let's look at the properties of oscilloscopes and signals that help users determine if a waveform is thick or thin. Two key properties that will give users an idea of ​​their oscilloscope's ability to display the signal of interest are update rate and noise.

Effect of update rate on waveform thickness

Update rate refers to the number of waveforms that an oscilloscope acquires, processes, and displays in one second. The higher the update rate, the quicker the oscilloscope can display the signal being measured. The lower the update rate, the longer it takes the oscilloscope to display details associated with a particular waveform. Today, oscilloscope update rates range from 1 million waveforms per second to 1 waveform every few seconds. The same oscilloscope can display a different range of rates simply by changing the oscilloscope's settings. The update rate can be affected by several oscilloscope settings, including changing the acquisition memory depth, which can have a significant impact on memory depth.

Let's look at a simple example. The top half of Figure 1 shows two oscilloscopes of equal bandwidth from a well-known manufacturer, running continuously and connected to the exact same 10 MHz sine wave. One of the oscilloscopes shows a thicker waveform, and the other shows a thinner waveform. This results in different measurements. Which one is more accurate? One of the biggest differences between the two oscilloscopes is the update rate. Using the same settings, one of the oscilloscopes updates at 1 million waveforms per second, which is 16,000 times faster than the other oscilloscope.

How does this affect the waveform? The bottom half of Figure 1 shows how two oscilloscopes connected to the same signal would look when infinite persistence is turned on. Both oscilloscopes build images for a longer duration. After 10 seconds, the oscilloscopes show the same waveform shape and waveform thickness. In this case, the original oscilloscope with the higher data rate is able to show the thicker waveform, giving a clearer representation of what each oscilloscope is displaying. By turning on infinite persistence, we can make a quick assessment.

Figure 1. Two oscilloscopes with equal bandwidth and similar noise are connected to the same signal.

Both oscilloscopes have similar noise. The screenshot above shows the Tek DPO5104A, which has a much narrower waveform, providing more detail. The Agilent DSOX4104A displays a wider waveform. Why is there such a difference? It's all about the update rate.

Turn on infinite persistence and wait 10 seconds. Both oscilloscopes show the same waveform thickness. The Agilent scope updates at 1 million waveforms/second, while the Tek scope updates at only 60 waveforms/second in normal mode. The waveform thickness is related to the amount of noise the oscilloscope adds to the initial signal.

The text in the picture is in Chinese and English

Both scopes are connected to the same signal, with the same setTIngs

Now with infinite persistence turned on. Both oscilloscopes connected to the same signal, with the same settings

Now enable infinite afterglow.