Noise is a common problem everywhere. Almost everyone who deals with a circuit spends a certain amount of time dealing with noise, either finding a source of noise to fix it, or reducing the effect of noise on the measurement.
Noise can come from endless sources, including internal or external sources of the design, and noise can block the signal of interest. You may be having trouble measuring low-voltage (mV) signals, such as in radar transmissions or heart monitors. Noise can make it difficult to find the actual voltage of the signal, it can increase jitter, and make timing measurements difficult. You may want a clean, noise-free trace, focusing on the signal expected in your design. Clean trajectories can be used for reports and documentation to clearly show how the design is performing.
Your oscilloscope provides features and tools to help you deal with noise.
This application note will review common oscilloscope functions to reduce noise during measurements, including innovative tools available only on Tektronix MSO2000 and DPO2000 Series oscilloscopes. With FilterVuTM variable low-pass filter, you can filter out unwanted noise from the signal while still reaching the full bandwidth of the oscilloscope, capturing unexpected glitches, allowing you to focus on the signal of interest without Missing key high-frequency events.
Measuring Noisy Signals with an Oscilloscope
1.Requires stable trigger
Before analyzing a signal, you need a stable display. If the signal is noisy, a stable display can be a problem, making it difficult to create a stable trigger. Most oscilloscopes have multiple features to help you solve this problem. Zh
In general, the first step in creating a stable trigger is to test which trigger coupling mode works best. Many Tektronix oscilloscopes offer high frequency (HF) rejection, low frequency (LF) rejection, and noise rejection trigger coupling options, each of which can be used to create a stable trigger for the signal. Zh
HF Reject performs low-pass filtering on the trigger path, trying to ignore any high-frequency instability or noise. LF Reject performs high-pass filtering on the trigger path, trying to prevent low-frequency signals from generating noise. Noise Reject increases the required trigger stickiness to prevent random noise from triggering. It can be difficult to predict how these patterns will affect your particular signal, and try them one by one if necessary to maintain a stable trigger. The trigger system in most oscilloscopes also provides trigger holdoff control. This control can only be triggered after a user-specified delay timer. If the signal is repetitive, try adjusting trigger holdoff and ignore some false triggers. Zh
If the trigger is still unstable, most oscilloscopes provide a bandwidth-limiting filter to pass the signal through a low-pass filter. Low-pass filters generally provide only a few frequency settings, usually not lower than 20MHz. For many applications, such as debugging power issues, this setting may not be low enough. Try different bandwidth settings until a stable trigger is achieved.
2.Reduce noise on the displayed signal
Once a stable trigger is obtained, you can further adjust the noise display on the oscilloscope. There are several tools to do this: Bandwidth Limiting Filter (as mentioned earlier), Average Acquisition Mode, HiRes Acquisition Mode, and FilterVu Low Pass Filter. FilterVu Low Pass Filter is one of the Tektronix MSO / DPO2000 Series Oscilloscope New features.
3. Bandwidth limited filtering
The bandwidth limit filter reduces the oscilloscope's bandwidth to the selected frequency. That is, frequencies higher than the selected frequency will be attenuated or completely removed from the trigger path and the acquisition and display path. Bandwidth limiting filters can be used not only to maintain stable triggers, but also to reduce the amount of noise displayed on the oscilloscope.
Using a band-limiting filter is one of the easiest ways to reduce noise in an oscilloscope. It is especially suitable if all unwanted noise frequencies are above a fixed cutoff frequency. However, it also removes any high-speed glitches that may occur. The bandwidth limit settings provided by the oscilloscope are generally very limited. Standard options include 250MHz and 20MHz.
4.Average acquisition mode
The average acquisition mode performs several complete acquisitions and averages point by point to obtain the average voltage of each time sample in the acquisition. The user can adjust the number of acquisitions included in the average. Noise is generally random between acquisitions, sometimes rising and sometimes falling. When these random changes are averaged over a sufficient number of acquisitions, they will cancel out and produce a stable signal on the screen. To take advantage of the average acquisition mode, your waveform must be repetitive. Non-repetitive waveforms or single events cannot be averaged. Zh
The average acquisition mode reduces all kinds of uncorrelated signals and random noise, even at very low frequencies. In addition, it applies to all oscilloscope time / division settings.
Because multiple waveforms must be acquired to create an averaged waveform, the display screen update rate may be slow when the input signal changes or the front panel knob changes. This means that occasional glitches may be missed. Zh
In some applications, the average acquisition mode is better than a bandwidth-limited filter because the full bandwidth of the oscilloscope can be used to capture high-frequency repetitive events.
Noise Reduction: Bandwidth Limiting Filter and Average Acquisition Mode
The default acquisition and small voltage sine wave display screen. Note that the noise of the signal is 30mV.
The bandwidth limit filter is set to 20 MHz. Note that the amount of noise has been greatly reduced. This indicates that part of the noise is greater than 20 MHz, but there is still some lower frequency noise.
Average acquisition mode with an average of 32 times. Note that the sine wave is very clean and has almost no noise. Can remove random noise at all frequencies on average.
5.HiRes acquisition mode
Some oscilloscopes include the HiRes acquisition mode, which is similar to the average acquisition mode because it uses averaging to remove noise. HiRes performs rectangular wave string averaging on each acquisition, averaging multiple adjacent samples within a acquired waveform to produce an averaged sample. This reduces high-frequency noise, because high-speed variations in voltage caused by noise can be offset. It also reduces the sampling rate because it converts multiple samples into one sample. Therefore, HiRes acquisition mode is only suitable for slower time / division settings. In this case, the oscilloscope still has a sufficient sample rate to represent the signal under test.
Unlike the average acquisition mode, the HiRes acquisition mode can be used on non-repetitive and single-shot waveforms. In addition, since only one waveform needs to be acquired, the HiRes acquisition mode displays the display update speed much faster after the input or front panel settings change. Combining multiple adjacent samples in time also reduces the chance of generating false signals at lower time / div settings.
Because HiRes acquisition mode is a type of low-pass filtering, you may miss high-speed glitches on the signal. HiRes acquisition mode will pass some high-frequency noise, which may obscure the signal shape and edge position. It is generally not indicated which frequencies are removed from the HiRes acquisition mode.
The HiRes acquisition mode may reduce the frequency of some artifacts from the display; due to the poor frequency selection of the HiRes low-pass filter, some frequencies of artifacts may still exist.
Noise reduction: average acquisition mode and HiRes acquisition mode
6.DSP FilterSome oscilloscopes provide post-processing DSP filters
Remove some frequency noise from the signal. Can fully control the filtering frequency. Although these filters may be flexible, they are usually slow and only suitable for single shot or slower update displays. They may filter out important glitches or anomalies of interest without your knowledge.
FilterVuTM variable low-pass filtering
Tektronix MSO / DPO2000 Series oscilloscopes provide a powerful feature—FilterVuTM Variable Low Pass Filtering, which helps filter unwanted noise from the signal. FilterVu allows selection of the low-pass filtering frequency to be applied to the displayed acquisition. In addition to the low-pass filtered trajectory, you can use an inconspicuous background trajectory to display the original acquisition of peak detection (min / max sampling) in a clean filtered waveform to prevent any unexpected high-frequency glitches or high Amplitude noise. Zh
With FilterVu variable low-pass filter, the background trace is displayed under the clean filtered waveform, and the background trace shows the original acquisition of peak detection.
The low-pass filter cutoff frequency can be adjusted from the front panel to control the amount of noise you want to reduce. Filtering frequency readings allows you to characterize what frequency noise is on the signal without having to set up troublesome FFTs (fast Fourier transforms). This adjustment can even be used for the acquired single-shot waveform, and then the signal can be carefully detected. Zh
As part of the acquisition process, FilterVu can quickly update the HiRes acquisition mode display, with the flexibility and control of a post-processing DSP filter, while maintaining a background image, displaying high-frequency glitches and noise amplitude. Zh
Peak detection background traces until the peak drift of the oscilloscope's bandwidth capture signal, even for single-shot waveforms. This means that when detecting a signal at the lowest time / division setting, any glitches that can be captured at the fastest time / division setting are still displayed. Zh
FilterVu captures the power-on power of a switch-mode power supply. Notice the small negative spikes on the left side of the screen. FilterVu's glitch capture feature shows this spike (circled in red). Other oscilloscopes may miss this glitch.
Noise can come from endless sources, including internal or external sources of the design, and noise can block the signal of interest. You may be having trouble measuring low-voltage (mV) signals, such as in radar transmissions or heart monitors. Noise can make it difficult to find the actual voltage of the signal, it can increase jitter, and make timing measurements difficult. You may want a clean, noise-free trace, focusing on the signal expected in your design. Clean trajectories can be used for reports and documentation to clearly show how the design is performing.
Your oscilloscope provides features and tools to help you deal with noise.
This application note will review common oscilloscope functions to reduce noise during measurements, including innovative tools available only on Tektronix MSO2000 and DPO2000 Series oscilloscopes. With FilterVuTM variable low-pass filter, you can filter out unwanted noise from the signal while still reaching the full bandwidth of the oscilloscope, capturing unexpected glitches, allowing you to focus on the signal of interest without Missing key high-frequency events.
Measuring Noisy Signals with an Oscilloscope
1.Requires stable trigger
Before analyzing a signal, you need a stable display. If the signal is noisy, a stable display can be a problem, making it difficult to create a stable trigger. Most oscilloscopes have multiple features to help you solve this problem. Zh
In general, the first step in creating a stable trigger is to test which trigger coupling mode works best. Many Tektronix oscilloscopes offer high frequency (HF) rejection, low frequency (LF) rejection, and noise rejection trigger coupling options, each of which can be used to create a stable trigger for the signal. Zh
HF Reject performs low-pass filtering on the trigger path, trying to ignore any high-frequency instability or noise. LF Reject performs high-pass filtering on the trigger path, trying to prevent low-frequency signals from generating noise. Noise Reject increases the required trigger stickiness to prevent random noise from triggering. It can be difficult to predict how these patterns will affect your particular signal, and try them one by one if necessary to maintain a stable trigger. The trigger system in most oscilloscopes also provides trigger holdoff control. This control can only be triggered after a user-specified delay timer. If the signal is repetitive, try adjusting trigger holdoff and ignore some false triggers. Zh
If the trigger is still unstable, most oscilloscopes provide a bandwidth-limiting filter to pass the signal through a low-pass filter. Low-pass filters generally provide only a few frequency settings, usually not lower than 20MHz. For many applications, such as debugging power issues, this setting may not be low enough. Try different bandwidth settings until a stable trigger is achieved.
2.Reduce noise on the displayed signal
Once a stable trigger is obtained, you can further adjust the noise display on the oscilloscope. There are several tools to do this: Bandwidth Limiting Filter (as mentioned earlier), Average Acquisition Mode, HiRes Acquisition Mode, and FilterVu Low Pass Filter. FilterVu Low Pass Filter is one of the Tektronix MSO / DPO2000 Series Oscilloscope New features.
3. Bandwidth limited filtering
The bandwidth limit filter reduces the oscilloscope's bandwidth to the selected frequency. That is, frequencies higher than the selected frequency will be attenuated or completely removed from the trigger path and the acquisition and display path. Bandwidth limiting filters can be used not only to maintain stable triggers, but also to reduce the amount of noise displayed on the oscilloscope.
Using a band-limiting filter is one of the easiest ways to reduce noise in an oscilloscope. It is especially suitable if all unwanted noise frequencies are above a fixed cutoff frequency. However, it also removes any high-speed glitches that may occur. The bandwidth limit settings provided by the oscilloscope are generally very limited. Standard options include 250MHz and 20MHz.
4.Average acquisition mode
The average acquisition mode performs several complete acquisitions and averages point by point to obtain the average voltage of each time sample in the acquisition. The user can adjust the number of acquisitions included in the average. Noise is generally random between acquisitions, sometimes rising and sometimes falling. When these random changes are averaged over a sufficient number of acquisitions, they will cancel out and produce a stable signal on the screen. To take advantage of the average acquisition mode, your waveform must be repetitive. Non-repetitive waveforms or single events cannot be averaged. Zh
The average acquisition mode reduces all kinds of uncorrelated signals and random noise, even at very low frequencies. In addition, it applies to all oscilloscope time / division settings.
Because multiple waveforms must be acquired to create an averaged waveform, the display screen update rate may be slow when the input signal changes or the front panel knob changes. This means that occasional glitches may be missed. Zh
In some applications, the average acquisition mode is better than a bandwidth-limited filter because the full bandwidth of the oscilloscope can be used to capture high-frequency repetitive events.
Noise Reduction: Bandwidth Limiting Filter and Average Acquisition Mode
The default acquisition and small voltage sine wave display screen. Note that the noise of the signal is 30mV.
The bandwidth limit filter is set to 20 MHz. Note that the amount of noise has been greatly reduced. This indicates that part of the noise is greater than 20 MHz, but there is still some lower frequency noise.
Average acquisition mode with an average of 32 times. Note that the sine wave is very clean and has almost no noise. Can remove random noise at all frequencies on average.
5.HiRes acquisition mode
Some oscilloscopes include the HiRes acquisition mode, which is similar to the average acquisition mode because it uses averaging to remove noise. HiRes performs rectangular wave string averaging on each acquisition, averaging multiple adjacent samples within a acquired waveform to produce an averaged sample. This reduces high-frequency noise, because high-speed variations in voltage caused by noise can be offset. It also reduces the sampling rate because it converts multiple samples into one sample. Therefore, HiRes acquisition mode is only suitable for slower time / division settings. In this case, the oscilloscope still has a sufficient sample rate to represent the signal under test.
Unlike the average acquisition mode, the HiRes acquisition mode can be used on non-repetitive and single-shot waveforms. In addition, since only one waveform needs to be acquired, the HiRes acquisition mode displays the display update speed much faster after the input or front panel settings change. Combining multiple adjacent samples in time also reduces the chance of generating false signals at lower time / div settings.
Because HiRes acquisition mode is a type of low-pass filtering, you may miss high-speed glitches on the signal. HiRes acquisition mode will pass some high-frequency noise, which may obscure the signal shape and edge position. It is generally not indicated which frequencies are removed from the HiRes acquisition mode.
The HiRes acquisition mode may reduce the frequency of some artifacts from the display; due to the poor frequency selection of the HiRes low-pass filter, some frequencies of artifacts may still exist.
Noise reduction: average acquisition mode and HiRes acquisition mode
6.DSP FilterSome oscilloscopes provide post-processing DSP filters
Remove some frequency noise from the signal. Can fully control the filtering frequency. Although these filters may be flexible, they are usually slow and only suitable for single shot or slower update displays. They may filter out important glitches or anomalies of interest without your knowledge.
FilterVuTM variable low-pass filtering
Tektronix MSO / DPO2000 Series oscilloscopes provide a powerful feature—FilterVuTM Variable Low Pass Filtering, which helps filter unwanted noise from the signal. FilterVu allows selection of the low-pass filtering frequency to be applied to the displayed acquisition. In addition to the low-pass filtered trajectory, you can use an inconspicuous background trajectory to display the original acquisition of peak detection (min / max sampling) in a clean filtered waveform to prevent any unexpected high-frequency glitches or high Amplitude noise. Zh
With FilterVu variable low-pass filter, the background trace is displayed under the clean filtered waveform, and the background trace shows the original acquisition of peak detection.
The low-pass filter cutoff frequency can be adjusted from the front panel to control the amount of noise you want to reduce. Filtering frequency readings allows you to characterize what frequency noise is on the signal without having to set up troublesome FFTs (fast Fourier transforms). This adjustment can even be used for the acquired single-shot waveform, and then the signal can be carefully detected. Zh
As part of the acquisition process, FilterVu can quickly update the HiRes acquisition mode display, with the flexibility and control of a post-processing DSP filter, while maintaining a background image, displaying high-frequency glitches and noise amplitude. Zh
Peak detection background traces until the peak drift of the oscilloscope's bandwidth capture signal, even for single-shot waveforms. This means that when detecting a signal at the lowest time / division setting, any glitches that can be captured at the fastest time / division setting are still displayed. Zh
FilterVu captures the power-on power of a switch-mode power supply. Notice the small negative spikes on the left side of the screen. FilterVu's glitch capture feature shows this spike (circled in red). Other oscilloscopes may miss this glitch.