Understanding oscilloscope waveform thickness properties

The oscilloscope waveform represents the real electrical signal. When evaluating an oscilloscope's performance, one key factor is its ability to accurately display the shape of the target signal. Assuming the oscilloscope has sufficient technical specifications—such as bandwidth, sample rate, and flat frequency response—the question arises: does a coarse or fine waveform provide better results? The answer, like most engineering questions, is not straightforward—it depends on the situation. To determine whether an oscilloscope displays a coarse or fine waveform, it’s important to understand two key factors: update rate and noise. These properties help users assess how well their oscilloscope captures and displays the actual signal. The update rate refers to the number of waveforms the oscilloscope can acquire, process, and display in less than one second. A higher update rate means the oscilloscope can capture more details of the signal faster. Lower update rates may result in longer intervals between waveform updates, which can make the displayed waveform appear coarser. Oscilloscopes typically have update rates ranging from 1 million waveforms per second down to just one per second. Settings such as memory depth can significantly impact this rate. Let’s look at an example. In Figure 1, two oscilloscopes with similar bandwidth and noise levels are connected to the same 10 MHz sine wave. One shows a thicker waveform, while the other appears thinner. This difference is due to their update rates. One has a much higher update rate (1 million waveforms per second), while the other only captures 60 waveforms per second in normal mode. When infinite persistence is enabled, both eventually show the same waveform thickness after 10 seconds. However, the oscilloscope with the higher update rate displays a thicker waveform initially, indicating more detail. Infinite persistence helps evaluate waveform thickness by accumulating multiple acquisitions over time. It allows you to see how the oscilloscope handles noise and how quickly it updates the display. Now, let’s consider the effect of noise on waveform thickness. Oscilloscope accuracy is generally high in the horizontal domain but can be reduced in the vertical domain due to internal noise. This noise gets added to the signal being measured, making the waveform appear wider. To test for this, disconnect all inputs and set the oscilloscope to a 50 Ω or 1 MΩ input path. Enable infinite persistence and measure the waveform height. A thicker waveform indicates higher internal noise. Figure 2 shows two oscilloscopes with the same update rate but different noise levels. One displays a thicker waveform, clearly due to higher noise. This highlights that waveform thickness isn’t always about speed—it can also reflect the quality of the oscilloscope’s design. To quickly characterize your oscilloscope’s noise, use the built-in tools or perform a simple test. Set the oscilloscope to a low vertical scale (e.g., 100 mV/div to 10 mV/div) and observe the waveform. Higher sensitivity will reveal more noise, helping you quantify it using AC RMS measurements. Oscilloscope vendors often provide noise specifications, but if not available, you can measure it yourself. Reducing the bandwidth can also help minimize noise, especially when dealing with narrow signals. When analyzing a target signal, it can be hard to tell if the noise comes from the signal itself or the oscilloscope. Using infinite persistence and comparing normal and persistent modes can help identify the source. High-noise, low-update-rate oscilloscopes may start with a fine waveform but become coarse under persistence. Conversely, low-noise, high-update-rate models maintain clarity. Modes like averaging and high-resolution can reduce noise. Averaging works best for repetitive signals, while high-resolution mode averages adjacent samples to improve clarity, though it may reduce effective sampling rate and bandwidth. Understanding these factors helps you choose the right oscilloscope for your application. Whether you're looking for accurate signal representation or detailed noise analysis, knowing how your oscilloscope performs under different conditions ensures better results.

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