How Oscilloscopes Handle Noisy Signals

How does an oscilloscope handle noisy signals?

Noise is a common problem that is ubiquitous. Almost everyone who works with circuits has to deal with noise at some point, either by finding its source and fixing it or reducing its effect on measurements.
Noise can come from an endless number of sources, both internal to the design and external to it, and can obscure signals of interest. You may be having trouble measuring low voltage (mV) signals, such as in radar transmissions or heart monitors. Noise can make it hard to find the actual voltage of a signal, can increase jitter, and make timing measurements difficult. You may want a clean trace free of noise to focus on the intended signal in your design. A clean trace can be used for reports and documentation to clearly show how your design is operating.
Your oscilloscope provides features and tools to help you deal with noise. This application note will review common oscilloscope features to reduce noise in your measurements, including innovative tools available only on the Tektronix MSO2000 and DPO2000 Series oscilloscopes. With the FilterVuTM variable low-pass filter, you can filter unwanted noise from your signal while still using the full bandwidth of the oscilloscope to capture unexpected glitches, allowing you to focus on your signal of interest without missing critical high-frequency events.
Measuring Noisy Signals with an Oscilloscope
Require a Stable Trigger
Before analyzing a signal, you need a stable display, which can be a problem if the signal is noisy, making it difficult to create a stable trigger. Most oscilloscopes have a variety of features to help you solve this problem.
Often, the first step in creating a stable trigger is to test which trigger coupling mode works best. Many Tektronix oscilloscopes offer high-frequency (HF) reject, low-frequency (LF) reject, and noise reject trigger coupling options, each of which can be used to create a stable trigger for your signal.
HF Reject performs low-pass filtering on the trigger path, attempting to ignore any high-frequency instabilities or noise. LF Reject performs high-pass filtering on the trigger path, attempting to prevent low-frequency signals from generating noise. Noise Reject increases the required trigger hysteresis value to prevent random noise from causing a trigger. It can be difficult to predict how these modes will affect your specific signal, so try each one if necessary to maintain a stable trigger.
The trigger system in most oscilloscopes also provides a trigger holdoff control. This control allows triggering only after a user-specified delay timer. If the signal is repetitive, try adjusting the trigger holdoff to ignore some of the false triggers.
If the trigger is still unstable, most oscilloscopes provide a bandwidth limit filter that passes the signal through a low-pass filter. The low-pass filter generally only offers a few frequency settings, usually no lower than 20MHz. For many applications, such as debugging power supply problems, this setting may not be low enough. Try different bandwidth settings until a stable trigger is achieved.
Reduce the noise on the displayed signal
Once a stable trigger is obtained, the noise display can be further adjusted on the oscilloscope. There are several tools to accomplish this: bandwidth limit filters (described previously), average acquisition mode, HiRes acquisition mode, and FilterVu low-pass filter, which is a new feature available on Tektronix MSO/DPO2000 series oscilloscopes.

Bandwidth Limit Filters
Bandwidth limit filters reduce the bandwidth of the oscilloscope to a selected frequency. That is, frequencies above the selected frequency are attenuated or completely removed from the trigger path and the acquisition and display paths. Bandwidth limit filters can be used not only to maintain a stable trigger, but also to reduce the amount of noise displayed on the oscilloscope.
Using a bandwidth limit filter is one of the simplest ways to reduce noise in an oscilloscope, and is particularly suitable if all unwanted noise frequencies are above a fixed cutoff frequency. However, it will also remove any high-speed glitches that may occur.
The bandwidth limit settings provided by oscilloscopes are generally very limited, with standard options including 250MHz and 20MHz.
Average Acquisition Mode
Average acquisition mode takes a number of complete acquisitions, averages them point by point, and obtains the average voltage at each time sample in the acquisition. The number of acquisitions included in the average is user adjustable. Noise is generally random between acquisitions, sometimes rising and sometimes falling. When these random variations are averaged over a sufficient number of acquisitions, they will cancel out, producing a stable signal on the screen. To take advantage of average acquisition mode, your waveform must be repetitive. Non-repetitive waveforms or single-shot events cannot be averaged.
Average acquisition mode reduces all types of uncorrelated signals and random noise, even at very low frequencies. In addition, it works at all oscilloscope time/div settings.
Because multiple waveforms must be acquired to create one averaged waveform, the display may update slowly when the input signal changes or the front panel knob is changed. This means that infrequent glitches may be missed.
In some applications, average acquisition mode is better than bandwidth-limiting filters because the full bandwidth of the oscilloscope can be used to capture high-frequency repetitive events.

Noise Reduction: Bandwidth Limiting Filters and Averaging Acquisition Mode

Default acquisition and display of a small voltage sine wave. Note that the noise on 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 some amount of noise is above 20 MHz, but there is still some noise at lower frequencies.

Average acquisition mode with 32 averages. Note how clean the sine wave is, with almost no noise. Averaging removes random noise at all frequencies.

HiRes Acquisition Mode
Some oscilloscopes include a HiRes acquisition mode, which is similar to the Average acquisition mode in that it uses averaging to eliminate noise. HiRes performs a boxcar average on each acquisition, averaging multiple adjacent samples within one acquired waveform to produce one averaged sample. This reduces high frequency noise because averaging cancels out the high speed changes in voltage caused by noise. It also reduces the sample rate because it converts multiple samples into one sample. Therefore, the HiRes acquisition mode is only useful at slower time/div settings where the oscilloscope still has enough sample rate to represent the signal being measured.
Unlike the Average acquisition mode, the HiRes acquisition mode can be used on non-repeating waveforms and single-shot waveforms. In addition, since only one waveform is acquired, the HiRes acquisition mode updates the display much faster after changes to inputs or front panel settings. Combining multiple samples that are adjacent in time also reduces the chance of aliasing at slower time/div settings.
Because the HiRes acquisition mode is a form of low pass filtering, you may miss high speed glitches on your signal. HiRes acquisition mode will pass some high frequency noise, which may obscure signal shape and edge location. It is generally not specified which frequencies are removed in HiRes acquisition mode.
HiRes acquisition mode may reduce some aliased frequencies from the display; due to the poor frequency selectivity of the HiRes low pass filter, some aliased frequencies may still be present.

Noise reduction: Average acquisition mode and HiRes acquisition mode

DSP Filters
Some oscilloscopes offer post-processing DSP filters to remove certain frequencies of noise from a signal. The filter frequency can be fully controlled. Although these filters can be flexible, they are often slow and only suitable for single-shot or slow update rate displays. They can filter out important glitches or anomalies of interest without you knowing it.
FilterVuTM Variable Low-Pass Filtering
Tektronix MSO/DPO2000 Series oscilloscopes offer a powerful feature, FilterVuTM variable low-pass filtering, to help filter out unwanted noise from a signal. FilterVu allows you to select the low-pass filter frequency that is applied to the displayed acquisition. In addition to the low-pass filtered trace, the peak detected (min/max sampling) raw acquisition can be displayed under the clean filtered waveform with an inconspicuous background trace, preventing you from missing any unexpected high-frequency glitches or high-amplitude noise (see Figure 1).

Figure 1. The clean filtered waveform is shown below the background trace after passing it through the FilterVu variable low-pass filter, which shows the peak-detected raw acquisition.

The low pass filter cutoff frequency can be adjusted from the front panel to control the amount of noise you want to reduce. The filter frequency readout allows characterization of what frequencies of noise are present on the signal without having to set up a cumbersome FFT (Fast Fourier Transform). This adjustment can even be applied to single-shot waveforms after acquisition, allowing careful inspection of the signal.
As part of the acquisition process, FilterVu quickly updates the HiRes acquisition mode display, giving you the flexibility and control of post-processing DSP filters while maintaining a background image showing high frequency glitches and noise amplitudes.
The peak detect background trace captures peak excursions of the signal up to the bandwidth of the oscilloscope, even on single-shot waveforms. This means any glitches that could be captured at the fastest time/div setting will still be displayed when inspecting the signal at the slowest time/div setting.

Reducing Noise with FilterVuTM Variable Low-Pass Filter

FilterVu captures the power on of a switch mode power supply. Note the small negative spike on the left side of the screen. FilterVu's glitch capture feature shows this spike (circled in red). Other oscilloscopes may miss this glitch.

Summary
Noise is a challenging issue that is ubiquitous in all electrical design and debug work. In this application note, we discussed some of the oscilloscope tools that can be used to reduce, understand, and characterize measurement noise. The FilterVu low-pass filter in the Tektronix MSO2000 and DPO2000 Series oscilloscopes adds a powerful and flexible tool at a very low cost, allowing you to better handle noise issues in your designs.