What are the important performance indicators of oscilloscopes, sample rate, memory depth, bandwidth, etc.
Important Oscilloscope Performance Metrics
Many oscilloscope performance metrics can significantly affect the performance of the instrument and, in turn, the ability to accurately test devices. This section will cover the basics of these metrics. They will also help you become familiar with oscilloscope terminology and allow you to make an informed decision when selecting the most appropriate oscilloscope.
Bandwidth
Bandwidth is one of the most important characteristics of an oscilloscope because it indicates the frequency domain range of the oscilloscope. In other words, it determines the range of signals (in terms of frequency) that can be accurately displayed and tested. Bandwidth is measured in Hz. If the bandwidth is insufficient, the oscilloscope will not be able to accurately display the actual signal. For example, the amplitude of the signal may be incorrect, edges may be unclear, and waveform detail may be lost. The bandwidth of an oscilloscope is the lowest frequency at which the input signal is attenuated by 3 dB. There is another way to view the bandwidth: If you enter a pure sine wave into the oscilloscope, the bandwidth is the minimum frequency at which the displayed amplitude is 70.7% of the actual signal amplitude.
Number of Channels
Channels refer to the independent inputs of the oscilloscope. The number of oscilloscope channels usually ranges from 2 to 20. Typically, there are 2 or 4 channels. The types of signals carried by the channels also vary. Some oscilloscopes have only digital channels (these instruments are called DSOs, or Digital Signal Oscilloscopes). Others are called mixed-signal oscilloscopes (MSOs), and they have both analog and digital channels. For example, the Keysight InfiniiVision Series MSO has 20 channels, 16 of which are digital and 4 of which are analog. It is critical to ensure that you have enough channels for your application. If you have two channels but need to display four signals at the same time, you will obviously have a problem.
Sampling rate
The sample rate of an oscilloscope is the number of samples per second that can be acquired by the oscilloscope. It is recommended that an oscilloscope's sample rate should be at least 2.5 times its bandwidth. Ideally, however, the sampling rate should be 3 times the bandwidth or higher. Extreme care is needed when evaluating the oscilloscope's claimed sample rate specifications. Manufacturers often refer to the sample rate that an oscilloscope can achieve as the maximum sample rate, but sometimes this maximum sample rate is only possible when using one or two channels.
If more channels are used at the same time, the sample rate may be reduced. Therefore, it is best to check how many channels you can use while maintaining the specified maximum sample rate. If the oscilloscope's sample rate is too low, the signal you see on the oscilloscope will be less accurate. For example, if you want to view a waveform with a low sample rate, the oscilloscope will only generate two points per cycle (Figure 26).
For the same waveform, the sample rate increases so that you can sample seven times per cycle (Figure 27). Obviously, the higher the number of samples per second, the clearer and more accurate the waveform display. If we keep increasing the sampling rate of the waveform in the above example, the sampling points will eventually appear to be almost continuous. In fact, the oscilloscope usually uses sin(x)/x interpolation between sample points.
Memory Depth
As mentioned earlier, a digital oscilloscope uses an A/D (analog-to-digital) converter to digitize the input waveform. The digitized data is then stored in the oscilloscope's high-speed memory. Memory depth refers to exactly how many samples or points can be stored, and for how long. Memory depth plays an important role in the oscilloscope's sampling rate. Ideally, the sample rate will remain constant regardless of the oscilloscope settings.
However, such an oscilloscope requires a huge amount of storage space to achieve large time/grid settings, so it is expensive and can seriously affect and limit the number of customers who can afford it. Conversely, the sample rate decreases as the time range increases. Memory depth is important because the greater the memory depth of the oscilloscope, the longer the waveform can be captured at full sampling speed.
In a mathematical sense, this can be expressed by the following formula: Memory depth = (sample rate) * (display time) Therefore, if you want to observe long periods between two points at high resolution, you need deep memory. It is also important to check the response performance of the oscilloscope when the deepest memory depth setting is used. In this mode, the oscilloscope's waveform capture rate performance is usually severely degraded, so many engineers use deep memory only when necessary.
Waveform Capture Rate
Waveform capture rate is the rate at which an oscilloscope can acquire and update a waveform display. Although the oscilloscope appears to the human eye to display a "real-time" waveform, this is because the update rate is too fast for the human eye to detect changes. In fact, there is some dead time between waveform acquisitions (Figure 28). During this silent time, a portion of the waveform is not displayed on the oscilloscope.
Therefore, if some rare event or burr occurs during one of these silent moments, you will not see it. This shows why a fast waveform capture rate is important. The faster the waveform capture rate, the shorter the dead time, which means the higher the probability of catching a rare event or burr. For example, let's say you are displaying a signal that has a burr only once every 50,000 cycles. If the oscilloscope's waveform capture rate is 100,000 waveforms per second, then you will capture this burr on average twice per second.
Conversely, if the oscilloscope has a waveform capture rate of 800 waveforms per second, it will take an average of one minute to capture one burr. This is too long of an observation time. The waveform capture rate specifications need to be read carefully. Some manufacturers require specific acquisition modes to meet their advertised waveform capture rate specifications. These acquisition modes can severely limit the oscilloscope's performance in terms of memory depth, sample rate, and waveform reconstruction. Therefore, it is a good idea to check the performance of the oscilloscope at this point when it is displaying waveforms at the maximum waveform capture rate.
Important Oscilloscope Performance Metrics
Many oscilloscope performance metrics can significantly affect the performance of the instrument and, in turn, the ability to accurately test devices. This section will cover the basics of these metrics. They will also help you become familiar with oscilloscope terminology and allow you to make an informed decision when selecting the most appropriate oscilloscope.
Bandwidth
Bandwidth is one of the most important characteristics of an oscilloscope because it indicates the frequency domain range of the oscilloscope. In other words, it determines the range of signals (in terms of frequency) that can be accurately displayed and tested. Bandwidth is measured in Hz. If the bandwidth is insufficient, the oscilloscope will not be able to accurately display the actual signal. For example, the amplitude of the signal may be incorrect, edges may be unclear, and waveform detail may be lost. The bandwidth of an oscilloscope is the lowest frequency at which the input signal is attenuated by 3 dB. There is another way to view the bandwidth: If you enter a pure sine wave into the oscilloscope, the bandwidth is the minimum frequency at which the displayed amplitude is 70.7% of the actual signal amplitude.
Number of Channels
Channels refer to the independent inputs of the oscilloscope. The number of oscilloscope channels usually ranges from 2 to 20. Typically, there are 2 or 4 channels. The types of signals carried by the channels also vary. Some oscilloscopes have only digital channels (these instruments are called DSOs, or Digital Signal Oscilloscopes). Others are called mixed-signal oscilloscopes (MSOs), and they have both analog and digital channels. For example, the Keysight InfiniiVision Series MSO has 20 channels, 16 of which are digital and 4 of which are analog. It is critical to ensure that you have enough channels for your application. If you have two channels but need to display four signals at the same time, you will obviously have a problem.
Sampling rate
The sample rate of an oscilloscope is the number of samples per second that can be acquired by the oscilloscope. It is recommended that an oscilloscope's sample rate should be at least 2.5 times its bandwidth. Ideally, however, the sampling rate should be 3 times the bandwidth or higher. Extreme care is needed when evaluating the oscilloscope's claimed sample rate specifications. Manufacturers often refer to the sample rate that an oscilloscope can achieve as the maximum sample rate, but sometimes this maximum sample rate is only possible when using one or two channels.
If more channels are used at the same time, the sample rate may be reduced. Therefore, it is best to check how many channels you can use while maintaining the specified maximum sample rate. If the oscilloscope's sample rate is too low, the signal you see on the oscilloscope will be less accurate. For example, if you want to view a waveform with a low sample rate, the oscilloscope will only generate two points per cycle (Figure 26).
For the same waveform, the sample rate increases so that you can sample seven times per cycle (Figure 27). Obviously, the higher the number of samples per second, the clearer and more accurate the waveform display. If we keep increasing the sampling rate of the waveform in the above example, the sampling points will eventually appear to be almost continuous. In fact, the oscilloscope usually uses sin(x)/x interpolation between sample points.
Memory Depth
As mentioned earlier, a digital oscilloscope uses an A/D (analog-to-digital) converter to digitize the input waveform. The digitized data is then stored in the oscilloscope's high-speed memory. Memory depth refers to exactly how many samples or points can be stored, and for how long. Memory depth plays an important role in the oscilloscope's sampling rate. Ideally, the sample rate will remain constant regardless of the oscilloscope settings.
However, such an oscilloscope requires a huge amount of storage space to achieve large time/grid settings, so it is expensive and can seriously affect and limit the number of customers who can afford it. Conversely, the sample rate decreases as the time range increases. Memory depth is important because the greater the memory depth of the oscilloscope, the longer the waveform can be captured at full sampling speed.
In a mathematical sense, this can be expressed by the following formula: Memory depth = (sample rate) * (display time) Therefore, if you want to observe long periods between two points at high resolution, you need deep memory. It is also important to check the response performance of the oscilloscope when the deepest memory depth setting is used. In this mode, the oscilloscope's waveform capture rate performance is usually severely degraded, so many engineers use deep memory only when necessary.
Waveform Capture Rate
Waveform capture rate is the rate at which an oscilloscope can acquire and update a waveform display. Although the oscilloscope appears to the human eye to display a "real-time" waveform, this is because the update rate is too fast for the human eye to detect changes. In fact, there is some dead time between waveform acquisitions (Figure 28). During this silent time, a portion of the waveform is not displayed on the oscilloscope.
Therefore, if some rare event or burr occurs during one of these silent moments, you will not see it. This shows why a fast waveform capture rate is important. The faster the waveform capture rate, the shorter the dead time, which means the higher the probability of catching a rare event or burr. For example, let's say you are displaying a signal that has a burr only once every 50,000 cycles. If the oscilloscope's waveform capture rate is 100,000 waveforms per second, then you will capture this burr on average twice per second.
Conversely, if the oscilloscope has a waveform capture rate of 800 waveforms per second, it will take an average of one minute to capture one burr. This is too long of an observation time. The waveform capture rate specifications need to be read carefully. Some manufacturers require specific acquisition modes to meet their advertised waveform capture rate specifications. These acquisition modes can severely limit the oscilloscope's performance in terms of memory depth, sample rate, and waveform reconstruction. Therefore, it is a good idea to check the performance of the oscilloscope at this point when it is displaying waveforms at the maximum waveform capture rate.