The oscilloscope waveform reveals the true 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, does it display a coarse or fine waveform better? The answer, as with most engineering questions, is: “It depends on the situation.â€
Let’s dive into the properties of the oscilloscope and the signals being measured. These characteristics help users determine whether a waveform appears coarse or fine. Two critical factors that influence how well the oscilloscope displays 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 more details of the signal faster. A lower update rate results in longer intervals between waveform updates, which can make the displayed waveform appear coarser.
Modern oscilloscopes have update rates ranging from 1 million waveforms per second down to just one waveform per second. Adjusting settings such as acquisition memory depth can significantly affect this rate. For example, increasing memory depth may reduce the update rate, depending on the scope's architecture.
Consider a simple example: two oscilloscopes with the same bandwidth, connected to the same 10 MHz sine wave. One shows a thicker waveform, while the other shows a thinner one. This difference can be attributed to their update rates. One might have an update rate of 1 million waveforms per second, while the other only captures 60 waveforms per second. The higher update rate allows for a denser and more detailed display.
When using infinite persistence mode, both scopes will eventually display the same waveform thickness after 10 seconds. However, the scope with a higher update rate initially shows a thicker waveform, capturing more detail over time.
**Effect of Oscilloscope Noise on Waveform Thickness**
How accurate is the oscilloscope’s measurement? While horizontal timing accuracy is usually very high, vertical accuracy can be significantly affected by noise. Internal noise from the oscilloscope can mix with the actual signal, making it harder to distinguish between real signal variations and noise.
To assess this, you can 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 more internal noise. You can also use AC RMS measurements to quantify the noise at different vertical scales.
Oscilloscopes with higher noise levels tend to display wider waveforms, even when update rates are similar. Active probes, which offer lower noise compared to passive ones, can also impact the overall noise level.
**Understanding the Target Signal**
The target signal itself may have low or high noise. It can be challenging to tell if the noise seen on the screen comes from the signal or the oscilloscope. The ADC cannot differentiate between them, so it records and displays both.
To evaluate this, use the method above to check the oscilloscope’s internal noise. Turn on infinite persistence and observe the waveform. If the waveform thickens significantly, it likely has more internal noise.
Infinite persistence also helps visualize how noise affects the signal. Testing known waveforms in normal and infinite persistence modes can reveal the oscilloscope’s noise and update rate characteristics.
**Noise Reduction Techniques**
Average mode reduces noise by averaging multiple acquisitions, which works best for repetitive signals. High-resolution mode averages adjacent samples to reduce noise but lowers the effective sampling rate and bandwidth.
These techniques allow for clearer waveform displays, though they come with trade-offs. Choosing the right mode depends on the signal type and the level of detail needed.
By understanding these factors, you can now select an oscilloscope that faithfully reproduces your signal or determine how your current device displays fine or coarse waveforms. With this knowledge, you're better equipped to make informed decisions about your test setup.
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