The oscilloscope waveform reflects the actual 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 meets basic specifications like bandwidth, sample rate, and flat frequency response, should a coarse or fine waveform be better? The answer is not straightforward—it depends on the situation.
Let’s explore the characteristics of both the oscilloscope and the signals it measures. These properties help users determine whether a waveform appears coarse or fine. Two critical factors that influence the oscilloscope’s ability to display the target signal are the update rate and noise.
**The Impact of Update Rate on Waveform Thickness**
The update rate refers to how many waveforms the oscilloscope can acquire, process, and display in less than one second. A higher update rate means the oscilloscope can show the signal under test more quickly. Conversely, a lower update rate results in longer display times for specific waveform details. Oscilloscopes today have update rates ranging from 1 million waveforms per second down to just one waveform per second. Changing settings, such as memory depth, can significantly affect the update rate.
Consider a simple example: two well-known oscilloscopes with equal bandwidths are connected to the same 10 MHz sine wave. One displays a thicker waveform, while the other shows a thinner one. This difference affects measurement accuracy. The main distinction between the two is their update rate. One has an update rate of 1 million waveforms per second, while the other is only 60 waveforms per second.
When infinite persistence is enabled, both oscilloscopes eventually display the same waveform thickness after 10 seconds. However, the one with a higher update rate initially shows a thicker waveform, making it easier to interpret the signal. Using infinite persistence is a quick way to evaluate this.
**The Effect of Oscilloscope Noise on Waveform Thickness**
How accurate is an oscilloscope’s measurement? While horizontal time base accuracy is typically very high, vertical measurement accuracy can be significantly affected by noise. Internal noise from the oscilloscope can mix with the signal under test, leading to convolution and distortion during digitization, storage, processing, and display. The ADC cannot distinguish between internal noise and actual signal noise, so both contribute to the final waveform.
To assess your oscilloscope’s noise, you can perform a quick test. Disconnect all inputs, set the oscilloscope to 50 Ω input, and enable infinite persistence. The thicker the waveform, the more internal noise the oscilloscope produces. You can also measure the AC true RMS voltage to quantify the noise. Adjusting the vertical scale to a more sensitive setting (e.g., 100 mV/div to 10 mV/div) will make the noise more visible, helping you understand the oscilloscope’s noise level.
Oscilloscope vendors often provide noise specifications in product literature. If not available, you can calculate it yourself. Reducing the oscilloscope’s bandwidth can also help minimize noise, especially if the signal is narrow.
**Understanding the Target Signal**
The target signal itself may have varying levels of noise. It can be challenging to determine whether the noise on the oscilloscope comes from the signal or the device itself. When the ADC digitizes the signal, it treats both types of noise equally. To distinguish between them, use the methods above to evaluate the oscilloscope’s internal noise. Enable infinite persistence and observe if the waveform thickens or remains consistent.
Infinite persistence also reveals how the oscilloscope’s noise affects the signal. Testing known waveforms and comparing normal and infinite persistence modes helps identify the oscilloscope’s noise and update rate characteristics. For instance, a high-noise, low-update-rate oscilloscope may start with a fine waveform but become coarse with infinite persistence, while a low-noise, high-update-rate model will maintain a clean display regardless of mode.
**Noise Reduction Techniques**
Average mode reduces noise by capturing multiple waveforms and averaging them, which is ideal for repetitive signals. High-resolution mode also reduces noise by averaging adjacent samples, improving clarity for both repetitive and single-shot signals. However, this method reduces the effective sampling rate and bandwidth.
Choosing the right oscilloscope involves understanding how it handles noise and update rate. With the right tools and techniques, you can ensure your oscilloscope faithfully reproduces the signal you’re testing. Whether you're looking for a device that displays thin or thick waveforms, the right configuration will give you accurate and reliable results.
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