How to accurately measure power integrity with an oscilloscope?
“In the test and measurement work, we will encounter such problems. The rail voltage and tolerance are getting smaller and smaller, and it becomes more and more difficult to accurately measure the power integrity. In the past, any oscilloscope was able to measure ripples with 10% tolerance on the 5V power rail, because the 500mV requirement is much higher than the noise level of the oscilloscope; but now, it is difficult to measure 1V regardless of the oscilloscope used Ripple voltage with 2% tolerance on the power rail. How should we measure in this case? Antai tells you some tips for you to use an oscilloscope to accurately measure power integrity.
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In the test and measurement work, we will encounter such problems. The rail voltage and tolerance are getting smaller and smaller, and it becomes more and more difficult to accurately measure the power integrity. In the past, any oscilloscope was able to measure ripples with 10% tolerance on the 5V power rail, because the 500mV requirement is much higher than the noise level of the oscilloscope; but now, it is difficult to measure 1V regardless of the oscilloscope used Ripple voltage with 2% tolerance on the power rail. How should we measure in this case? Antai tells you some tips for you to use an oscilloscope to accurately measure power integrity.
Figure 1: The DC voltage of the power rail and its tolerance.
Tip 1: Adjust the display characteristic waveform intensity (waveform intensity)
To measure the DC voltage tolerance of the power rail, it is necessary to measure the worst-case voltage peak-to-peak value (Vpp). This can be perfectly achieved through automated measurement; sometimes visual judgment is also useful. All oscilloscopes have display settings, and users can see through Use this setting to change the waveform intensity. The intensity value is usually set to about 50%. Setting the intensity to a higher value allows users to more easily view the oscilloscope pixels corresponding to waveforms that appear less frequently. But the disadvantage of increasing the waveform intensity is that it is more difficult to determine the frequency of the waveform displayed on any particular pixel; although this is important for observing the modulated signal, this resolution is usually not important for power integrity measurements.
Infinite persistence: Turning on the infinite duration mode allows the continuously acquired waveforms to be displayed cumulatively; infinite duration is also very useful for file building. The oscilloscope can display the DC voltage tolerance range for a longer period of time.
Color grading: Turn on the color grading mode to generate a 3D image of the power rail waveform; color grading combined with an infinite duration display helps to understand the power rail signal more deeply.
Tip 2: Reduce noise and choose a low-noise oscilloscope
If the signal strength is less than the noise of the oscilloscope and probe/cable system, you will never be able to measure the signal. After the signal enters the oscilloscope and before entering the analog-to-digital converter (ADC), the front-end noise will be added; then each stored sample contains the original signal value, there will also be some offset (offset), the size of the offset It depends on the amount of noise that exists when the sample is taken. The user will see a thicker waveform on the oscilloscope’s display, not to confuse it with the fast update rate. Peak-to-peak values larger than the real signal will be displayed and measured.
The best way is to use a less noisy oscilloscope. How to determine the noise level of an oscilloscope? Most oscilloscope manufacturers provide product specification sheets that list the typical root mean square (RMS) noise values of that particular oscilloscope; these noise values are characterized based on a large number of oscilloscope samples. Noise is a characteristic rather than a specification. Manufacturers only provide typical values of RMS noise, but the peak-to-peak value of noise is actually an important factor that affects accurate measurement of ripple.
Figure 2: Noise is the main cause of inaccurate measurement of power rail DC ripple
An easy way is to measure it yourself. Fast characterization takes only a few minutes, and no external equipment is required. Disconnect all inputs of the oscilloscope, turn on the Vpp measurement, set the vertical scale and sampling rate of the noise measurement, and let the oscilloscope run until a stable and consistent Vpp noise value is obtained. The noise level depends on the vertical sensitivity setting, bandwidth setting and impedance selection (50Ω or 1MΩ), and there will be slight differences on different channels on the same oscilloscope.
Choose the signal path impedance with the lowest noise: The oscilloscope used to measure power integrity usually has two signal path impedances: 50Ω and 1MΩ. Generally, 50Ω impedance is usually less noisy and supports the full bandwidth of the oscilloscope. The noise on the 1MΩ path may be two to three times the noise on the 50Ω path, and the bandwidth on the 1MΩ path is usually limited to 500MHz, so the 50Ω path is the best choice for measuring power integrity. The output impedance of the power rail is usually in the mΩ level. For cable measurement equipment without any probes, the 50Ω path has a DC input impedance of 50Ω, which will produce some load effects, which will reduce the DC amplitude of the power rail. Using a dedicated power integrity probe can minimize the impact of this problem.
Use the most sensitive vertical scale: The noise level of the oscilloscope is related to the vertical scale value of the full Screen of the oscilloscope. Therefore, using a more sensitive vertical resolution will reduce the total amount of noise in the measurement. In addition, when amplifying the signal to cover most of the vertical range, the oscilloscope will make fuller use of the ADC resolution, and the Vpp measurement value will be more accurate at this time.
Bandwidth-limited noise has broadband characteristics. Turn on the FFT function when the oscilloscope is not connected to the input, and you can see the noise that exists on the entire bandwidth of the oscilloscope. Turning on the bandwidth limit filter can reduce broadband noise and help measure the power rail more accurately, but the disadvantage is that if the bandwidth limit setting is too low, higher frequency anomalies will not be displayed. How much bandwidth should be used? The answer is that it depends on the specific signal. Although the switching speed may be in the kHz range, fast edges can produce harmonics in the MHz range. For higher frequency coupled signals, including frequency harmonics, a larger bandwidth is required to capture these signals.
Using a probe with a 1:1 attenuation ratio can significantly improve the accuracy of measuring power integrity; a probe with a higher attenuation ratio will amplify noise, and a higher attenuation ratio will limit the vertical sensitivity that can be used. For example, using a probe with an attenuation ratio of 1:1 on an oscilloscope with an input as low as 1 mV/div can reduce the sensitivity to 1mV/div, while a probe with an attenuation ratio of 10:1 can only be set to 10 mV/div. div.
How to detect the power rail signal is as important as other techniques. Some users link the power rail to an SMA connector with high signal quality and easy connection; some users choose to weld the connection, and some users choose to use a clamp as a simple contact in the bypass capacitor; others use a handheld probe . Each technique has its own pros and cons in terms of ease of use, required pre-planning, and signal quality.
Figure 3: For small signals, using a probe with an attenuation ratio of 1:1 can obtain more accurate measurement results
Tip 3: Achieve sufficient offset AC coupling and blocking capacitance
The built-in offset of the oscilloscope is usually not enough for the user to place the waveform in the center of the display and magnify the display. This leads to two negative factors: the oscilloscope uses only a small part of the ADC vertical resolution and uses a larger vertical scale, thus Generate additional noise; this will reduce the quality of the measurement.
If the blocking capacitor or the AC coupling mode of the oscilloscope is used on the selected path and probe, the DC component in the signal will be removed; this can solve some problems, but the actual DC value and drift will not be seen.
Probes with built-in offsets Some probes have additional built-in offsets. The advantage is that they allow users to obtain enough offsets to see the true DC value and low-frequency characteristics, such as drift and dips ( sag).
Tip 4: Evaluate the switch and EMI frequency domain diagram
Characterizing power rails usually requires ensuring that no interfering signals are coupled to the power rails. In addition, you sometimes need to consider switching harmonics. These interference factors cannot be determined by viewing the time-domain waveform, but these interferences can be seen in the frequency domain through the FFT function of the oscilloscope.
How much bandwidth do I need to view frequency domain waveforms? This depends on the potential signals that may be coupled on the power rail, including frequency signals and fast edge harmonics.
Figure 4: View the waveform of the power rail in the time domain to get Vpp
But to find and isolate the coupled signal on the power rail (such as the 2.4 GHz Wi-Fi signal in this example), you need to use a frequency domain map.
Tip 5: Accelerate the measurement speed and the effect of the update rate on the power integrity measurement speed
Power rail measurement needs to find the worst-case voltage value, and establishing high reliability means making hundreds or thousands of measurements over a longer period of time; this will take a long time and the process will be boring. The uniqueness of power integrity measurements is that they usually require a long time span. In order to maintain a higher bandwidth, the oscilloscope requires a higher sampling rate, which will take up a lot of memory.
The waveform update rate is used to describe the speed at which the oscilloscope processes the memory, displays the results on the display, and starts to acquire new data; the update rate of the digital oscilloscope is as high as 1 million waveforms per second. The fast update rate means that measurements such as Vpp and FFT can be completed faster. The maximum update rate of many oscilloscopes is in the range of tens or hundreds of samples per second, which means that this oscilloscope requires more time to accurately obtain the worst-case tolerance test than an oscilloscope with a higher update rate. Several levels out. An oscilloscope with a high update rate allows users to complete accurate measurements faster.
Having said so much, Aetna will summarize for everyone:
1. Choosing a low-noise oscilloscope is essential to accurately measure power integrity;
2. The oscilloscope is used with probes with attenuation ratio of 1:1, built-in offset, high bandwidth, high DC resistance and integrated voltmeter to improve measurement performance;
3. Understanding and correctly setting a series of oscilloscope properties, such as vertical scale and bandwidth limit filter, can improve the accuracy of measurement results;
4. Adding a frequency domain diagram allows users to quickly isolate coupled signals;
5. Fast update rate allows users to test power rails more quickly
In the test and measurement work, we will encounter such problems. The rail voltage and tolerance are getting smaller and smaller, and it becomes more and more difficult to accurately measure the power integrity. In the past, any oscilloscope was able to measure ripples with 10% tolerance on the 5V power rail, because the 500mV requirement is much higher than the noise level of the oscilloscope; but now, it is difficult to measure 1V regardless of the oscilloscope used Ripple voltage with 2% tolerance on the power rail. How should we measure in this case? Antai tells you some tips for you to use an oscilloscope to accurately measure power integrity.
Figure 1: The DC voltage of the power rail and its tolerance.
Tip 1: Adjust the display characteristic waveform intensity (waveform intensity)
To measure the DC voltage tolerance of the power rail, it is necessary to measure the worst-case voltage peak-to-peak value (Vpp). This can be perfectly achieved through automated measurement; sometimes visual judgment is also useful. All oscilloscopes have display settings, and users can see through Use this setting to change the waveform intensity. The intensity value is usually set to about 50%. Setting the intensity to a higher value allows users to more easily view the oscilloscope pixels corresponding to waveforms that appear less frequently. But the disadvantage of increasing the waveform intensity is that it is more difficult to determine the frequency of the waveform displayed on any particular pixel; although this is important for observing the modulated signal, this resolution is usually not important for power integrity measurements.
Infinite persistence: Turning on the infinite duration mode allows the continuously acquired waveforms to be displayed cumulatively; infinite duration is also very useful for file building. The oscilloscope can display the DC voltage tolerance range for a longer period of time.
Color grading: Turn on the color grading mode to generate a 3D image of the power rail waveform; color grading combined with an infinite duration display helps to understand the power rail signal more deeply.
Tip 2: Reduce noise and choose a low-noise oscilloscope
If the signal strength is less than the noise of the oscilloscope and probe/cable system, you will never be able to measure the signal. After the signal enters the oscilloscope and before entering the analog-to-digital converter (ADC), the front-end noise will be added; then each stored sample contains the original signal value, there will also be some offset (offset), the size of the offset It depends on the amount of noise that exists when the sample is taken. The user will see a thicker waveform on the oscilloscope’s display, not to confuse it with the fast update rate. Peak-to-peak values larger than the real signal will be displayed and measured.
The best way is to use a less noisy oscilloscope. How to determine the noise level of an oscilloscope? Most oscilloscope manufacturers provide product specification sheets that list the typical root mean square (RMS) noise values of that particular oscilloscope; these noise values are characterized based on a large number of oscilloscope samples. Noise is a characteristic rather than a specification. Manufacturers will only provide a typical value of RMS noise, but the peak-to-peak value of noise is actually an important factor that affects accurate measurement of ripple.
Figure 2: Noise is the main cause of inaccurate measurement of power rail DC ripple
An easy way is to measure it yourself. Fast characterization takes only a few minutes, and no external equipment is required. Disconnect all inputs of the oscilloscope, turn on the Vpp measurement, set the vertical scale and sampling rate of the noise measurement, and let the oscilloscope run until a stable and consistent Vpp noise value is obtained. The noise level depends on the vertical sensitivity setting, bandwidth setting and impedance selection (50Ω or 1MΩ), and there will be slight differences on different channels on the same oscilloscope.
Choose the signal path impedance with the lowest noise: The oscilloscope used to measure power integrity usually has two signal path impedances: 50Ω and 1MΩ. Generally, 50Ω impedance is usually less noisy and supports the full bandwidth of the oscilloscope. The noise on the 1MΩ path may be two to three times the noise on the 50Ω path, and the bandwidth on the 1MΩ path is usually limited to 500MHz, so the 50Ω path is the best choice for measuring power integrity. The output impedance of the power rail is usually in the mΩ level. For cable measurement equipment without any probes, the 50Ω path has a DC input impedance of 50Ω, which will produce some load effects, which will reduce the DC amplitude of the power rail. Using a dedicated power integrity probe can minimize the impact of this problem.
Use the most sensitive vertical scale: The noise level of the oscilloscope is related to the vertical scale value of the full screen of the oscilloscope. Therefore, using a more sensitive vertical resolution will reduce the total amount of noise in the measurement. In addition, when amplifying the signal to cover most of the vertical range, the oscilloscope will make fuller use of the ADC resolution, and the Vpp measurement value will be more accurate at this time.
Bandwidth-limited noise has broadband characteristics. Turn on the FFT function when the oscilloscope is not connected to the input, and you can see the noise that exists on the entire bandwidth of the oscilloscope. Turning on the bandwidth limit filter can reduce broadband noise and help measure the power rail more accurately, but the disadvantage is that if the bandwidth limit setting is too low, higher frequency anomalies will not be displayed. How much bandwidth should be used? The answer is that it depends on the specific signal. Although the switching speed may be in the kHz range, fast edges can produce harmonics in the MHz range. For higher frequency coupled signals, including frequency harmonics, a larger bandwidth is required to capture these signals.
Using a probe with a 1:1 attenuation ratio can significantly improve the accuracy of measuring power integrity; a probe with a higher attenuation ratio will amplify noise, and a higher attenuation ratio will limit the vertical sensitivity that can be used. For example, using a probe with an attenuation ratio of 1:1 on an oscilloscope with an input as low as 1 mV/div can reduce the sensitivity to 1mV/div, while a probe with an attenuation ratio of 10:1 can only be set to 10 mV/div. div.
How to detect the power rail signal is as important as other techniques. Some users connect the power rail to an SMA connector with high signal quality and easy connection; some users choose to weld the connection, and some users choose to use a clamp as a simple contact in the bypass capacitor; others use a handheld probe . Each technique has its own pros and cons in terms of ease of use, required pre-planning, and signal quality.
Figure 3: For small signals, using a probe with an attenuation ratio of 1:1 can obtain more accurate measurement results
Tip 3: Achieve sufficient offset AC coupling and blocking capacitance
The built-in offset of the oscilloscope is usually not enough for the user to place the waveform in the center of the display and magnify the display. This leads to two negative factors: the oscilloscope uses only a small part of the ADC vertical resolution and uses a larger vertical scale, thus Generate additional noise; this will reduce the quality of the measurement.
If the blocking capacitor or the AC coupling mode of the oscilloscope is used on the selected path and probe, the DC component in the signal will be removed; this can solve some problems, but the actual DC value and drift will not be seen.
Probes with built-in offset. Some probes have additional built-in offsets. The advantage of these probes is that they allow users to obtain enough offset to see the true DC value and low-frequency characteristics, such as drift and dips ( sag).
Tip 4: Evaluate the switch and EMI frequency domain diagram
Characterizing power rails usually requires ensuring that no interfering signals are coupled to the power rails. In addition, you sometimes need to consider switching harmonics. These interference factors cannot be determined by viewing the time-domain waveform, but these interferences can be seen in the frequency domain through the FFT function of the oscilloscope.
How much bandwidth do I need to view frequency domain waveforms? This depends on the potential signals that may be coupled on the power rail, including frequency signals and fast edge harmonics.
Figure 4: View the waveform of the power rail in the time domain to get Vpp
But to find and isolate the coupled signal on the power rail (such as the 2.4 GHz Wi-Fi signal in this example), you need to use a frequency domain map.
Tip 5: Accelerate the measurement speed and the effect of the update rate on the power integrity measurement speed
Power rail measurement needs to find the worst-case voltage value, and establishing high reliability means making hundreds or thousands of measurements over a longer period of time; this will take a long time and the process will be boring. The uniqueness of power integrity measurements is that they usually require a long time span. In order to maintain a higher bandwidth, the oscilloscope requires a higher sampling rate, which will take up a lot of memory.
The waveform update rate is used to describe the speed at which the oscilloscope processes the memory, displays the results on the display, and starts to acquire new data; the update rate of the digital oscilloscope is as high as 1 million waveforms per second. The fast update rate means that measurements such as Vpp and FFT can be completed faster. The maximum update rate of many oscilloscopes is in the range of tens or hundreds of samples per second, which means that this oscilloscope requires more time to accurately obtain the worst-case tolerance test than an oscilloscope with a higher update rate. Several levels out. An oscilloscope with a high update rate allows users to complete accurate measurements faster.
Having said so much, Aetna will summarize for everyone:
1. Choosing a low-noise oscilloscope is essential to accurately measure power integrity;
2. The oscilloscope is used with probes with attenuation ratio of 1:1, built-in offset, high bandwidth, high DC resistance and integrated voltmeter to improve measurement performance;
3. Understanding and correctly setting a series of oscilloscope properties, such as vertical scale and bandwidth limit filter, can improve the accuracy of measurement results;
4. Adding a frequency domain diagram allows users to quickly isolate coupled signals;
5. Fast update rate allows users to test power rails more quickly
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