Measurement of very small signals can be challenging as the ability to view small signals is impacted not only by the noise of the scope, but also by scope settings and probing. Part I of this two-part series presented three techniques that can help your scope see smaller signals than you've previously seen with it. In Part 2, Agilent Technologies discusses four other methods that are applicable.
4. Use math functions such as magnify to vertically zoom
Once you’ve mastered using as much of the scope’s ADC bits as possible, the next challenge is to zoom in on the event of interest. In the case of signals with high dynamic range, a helpful technique is to create a function to show magnification of the portion of the waveform that is of interest.
Nearly all of today’s scopes come equipped with one or more math functions, or allow zoom windows where the zoom window includes both vertical and horizontal scaling independent of the main window.
For example, to zoom using a function, assign a function to magnify channel 1 of the scope. The user can change the position and scaling of the magnify function as shown in figure 4. This allows the main window to show the entire waveform, and the magnify function to zoom in on the area of interest. The math function (zoom) won’t have any better resolution than what the scope produced with the channel waveform, but you can see detail by zooming that would have been impossible to see when the full waveform was displayed.
Figure 4: In this example with an Infinium DSO9104A, the user has zoomed on a small portion of the channel waveform to allow viewing of a detail impossible to see on the original waveform. Infiniium scopes have 16 available functions enabling zooming of up to 16 areas of interestClick on image to enlarge
5. Use high-resolution mode to reduce noise
All major vendors offer high-resolution mode in their scopes. This is a mode that both increases bits of resolution as well as reduces noise. In high-res mode, the ADC oversamples and then applies a filter. A boxcar filter is typically used to average intermediate hypersamples. On many scopes, a user in high-res mode can specify bits of resolution and this choice determines the length of each boxcar filter. For example, if a scope with an 8 bit ADC is set to high-res mode and a value of 11 bits of resolution, each boxcar will consist of 8 (23
) ADC sample points.
A boxcar DSP filter will average these samples and store the resulting average vertical value to the scope’s acquisition memory as the sample rate shown on the scope’s screen. This allows for statistical elimination of a good portion of internal scope noise as shown in figure 6
. The most important attribute of high-res mode is its ability to reduce overall scope noise. High-res mode can reduce internal scope noise by up to a factor of 3. Tradeoffs associated with high-res mode are reduced bandwidth and a decrease in throughput. As the scope needs to perform additional processing with the DSP filters, update rate slows.
Real-time mode with high-res
Figure 5: If your scope has excess sample rate relative to needed bandwidth, turn on high-res mode to increase bits of resolution and reduce scope noise as shown. Using oversampling and digital filtering, and 8-bit scope can achieve 12 bits of resolution
Click on images to enlarge
6. Bandwidth limit to reduce noise
Reducing scope front-end bandwidth eliminates broadband noise that can impact the scope’s ability to see small signals. Ideally, bandwidth would be set at just 1 Hz above what is needed for an oscilloscope. If all other scope specifications were equal, a 11MHz oscilloscope would do a much better job at visualizing a signal detail from a 10 MHz sine wave than would a 50 GHz oscilloscope. Why? Because the higher bandwidth comes with additional broadband noise.
Many scopes come with bandwidth adjustment settings. For viewing small signals, adjust the bandwidth to the smallest value that meets your measurement criteria to minimize scope noise. If your scope doesn’t offer adjustable bandwidths, turning the scope to high-res mode is another technique that provides bandwidth limiting.
Other the scope will have some choice of HW-based and SW-based bandwidth limiting. HW-based bandwidth limiting is preferable as it allows the scope to remain as responsive a possible while SW-based techniques require additional processing.
Figure 6: Bandwidth limit to reduce noise. Use a lower bandwidth scope, or change the settings on your scope to bandwidth limit to minimize broadband noise
Click on image to enlarge
7. Average when signals are repetitive
Oscilloscopes include another mode called averaging. Turning averaging causes the oscilloscope to average vertical values along each captured waveform with vertical values from successive waveforms. Oscilloscopes allow the user to specify how many waveforms to average. Averaging is another technique that allows users to reduce noise from the oscilloscope. Unlike high-res mode using oversampling techniques, average doesn’t work for single shot acquisitions. Averaging requires a repetitive signal, and will make infrequent anomalies that may occur. Averaging reduces oscilloscope noise enabling better viewing of small signals.
Figure 7: If viewing a repetitive signal, turn on averaging
to reduce noise and increase visibility of small changes to the signal you are