HIGH-RESOLUTION ACQUISITION FILTERING
Similar to average acquisition filtering because it uses averaging to eliminate noise, the high-resolution acquisition filter performs a box-car average on each acquisition (Fig. 3). In this manner, it averages several adjacent samples within a single acquired waveform to create a single averaged sample.
High-resolution acquisition filtering has the effect of reducing high-frequency noise, because the average will cancel out the high-speed variance in voltages caused by the noise. It also reduces the sample rate because it converts many samples into one. As a result, high-resolution acquisition filtering is limited to slower time/division settings where the oscilloscope still has sufficient sample rate to represent the measured signal.
Unlike average acquisition filtering, high-resolution acquisition filtering can be used on non-repetitive and single-shot waveforms. Because only one waveform needs to be acquired, a scope’s highresolution acquisition mode provides a much faster update to the display after an input or front-panel setting change. Combining neighboring samples in time also reduces the chance of aliasing at slower time/division settings.
Because high-resolution acquisition filtering is a type of lowpass filtering, it may cause the user to miss high-speed glitches on the signal. Typically, no indication is given of what frequencies, if any, are being removed in high-resolution acquisition filtering. It may reduce some aliased frequencies from the display. Other aliased frequencies may still be present due to the poor frequency- selectivity nature of the high-resolution low-pass filter.
DSP FILTERS
Some oscilloscopes offer post-processing DSP filters that remove certain frequencies of noise from the signal, giving the user complete control over the filter frequency. While these filters may be flexible, they often are slow and suitable only for singleshot or slow update-rate displays. There’s also a risk that the DSP will filter out interesting and important glitches or anomalies without the user’s knowledge.
VARIABLE LOW-PASS FILTERING
Variable low-pass filters, which are coming on the market, are another method of removing unwanted noise. They allow the user to select a low-pass filter frequency to apply to the displayed acquisition. In addition to the low-pass filtered trace, the filters can ensure that the user doesn’t miss any unexpected high-frequency glitches or large-magnitude noise. It does so by providing a background trace showing the peak-detected (min/max sampled) raw acquisition underneath the clean filtered waveform (Fig. 4).
The low-pass filter cutoff frequency can be adjusted to control the amount of noise reduction. Filter-frequency readouts let the user characterize what frequencies of noise are on a given signal without the need to set up a more cumbersome fast Fourier transform (FFT).
A glitch that can be captured at the fastest time/division setting ideally would still be shown when inspecting the signal at the slowest time/division setting. This is where a peak-detect background trace capability is useful to capture peak excursions of the signal up to the oscilloscope’s bandwidth—even for singleshot waveforms.
In an example showing the use of a variable low-pass filter to capture the power-on of a switched-mode power supply, one can observe a small negative spike to the left of the display and oscillation to the right (Fig. 5a). By changing the filter-frequency cutoff to 550 kHz, the display shows that the oscillation has been removed from the main signal (Fig. 5b).
Thus, it’s apparent that the oscillation is between 550 kHz and 1.1 MHz. This analysis can be performed while stopped on the same single-shot capture. Note also that the spike is still shown in the glitch capture background, even though the foreground trace was filtered.
As with high-resolution acquisition filtering, variable low-pass filtering isn’t available at all time/division settings. At faster settings, the range of the filter is reduced. At the fastest time/division settings, no filtering is available because the low-pass filter works by reducing the number of sample points in the waveform.
At many time/division settings, the oscilloscope runs at a reduced sample rate and there are many extra points. When the oscilloscope runs at or near its full sample rate, fewer extra points exist, reducing the variable low-pass functionality. As such, average acquisition is a better choice for reducing noise at the fastest time/division settings.
Variable low-pass filtering can be used on repetitive, nonrepetitive, and single-shot waveforms. The filter-frequency adjustability allows the user to remove noise without rolling off the signal. Compared to the bandwidth-limit filter, variable low-pass filtering can handle lower frequencies (less than 1 MHz).
Another problem can be the effects of aliasing. Variable low-pass filtering deals with aliasing by passing no more than 1% of the high-frequency content that causes alias. This ensures that only the aliased frequencies are removed, not the signal of interest.
In conclusion, noise is a pervasive and challenging problem in nearly all electrical design and debug work. Modern oscilloscopes incorporate a number of tools to help the designer reduce, understand, and characterize noise in measurements. Variable low-pass filters add a powerful and flexible tool, with fewer compromises, offering another way to address noise issues in designs.
RELATED ARTICLES
For more, see the following articles at www.electronicdesign.com:
• “Test Instruments Stay Ahead of the Curve”
• “The Fifth Harmonic: Tradeoffs Between Sampling And Real-Time Oscilloscopes”