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25 June 2015

Don't Leave Oscilloscope Performance on the Table

Figure 1: In this screenshot, four signals are displayed
on a single grid. Each signal is only using
64 counts of its ADC, which amounts to 6-bit resolution.
As test and measurement companies add to their products' capabilities, digital oscilloscopes serve a larger and more sophisticated set of measurements as vendors have added to their capabilities. And even though many of those additions come at the behest of the user community, many oscilloscope users don't even scratch the surface of what their instrument can do.

Here are a few ways to get more out of your digital oscilloscope on an everyday basis. These techniques and capabilities apply to many different use cases. By using them, you'll enjoy improvements in signal capture and viewing (on any digital oscilloscope) as well as improved measurements and analysis of circuit behavior.

Starting From the Ground Up

All digital oscilloscopes are based on the same fundamental building blocks. For the front-end set of amplifiers/attenuators, users can choose from among several amplifier/attenuator combinations by changing the volts/division knob’s setting. The amplifier is a crucial element in the oscilloscope’s bandwidth rating. The vendor guarantees that signal energy up to the rated bandwidth will pass through the amplifier (and the rest of the signal path) with an attenuation of less than 3 dB (about 30%).

Following the amplifier stage is an analog-to-digital converter (ADC). The amplifier passes a continuous voltage-vs.-time waveform to the ADC, which then samples the voltage level and stores a set of voltage measurements in the instrument’s acquisition memory. This happens “live” during signal acquisition. The sample clock controls the time between ADC samples.

When it comes to sampling rate, faster is better. yielding capture of more detail in the shape of the signal. Most of today's oscilloscopes have an 8-bit ADC (Teledyne LeCroy offers numerous HDO models with 12-bit HD4096 technology), so each measurement of the voltage is made with 8-bit resolution. In terms of oscilloscope specifications, longer memory is better. An instrument with a longer memory can maintain a high sampling rate for a longer signal length than one that has a shorter memory.

Using The Full Range Of The ADC

The first thing a user will do with an oscilloscope is to capture a signal and look at it. Ensuring that you're doing this correctly will benefit you no matter what kind of signal is involved.

Let’s look at what is perhaps the most common mistake (Fig. 1). The overwhelming majority of oscilloscope users have four-channel instruments, and at times they'll want to look at four signals on screen at once. To see each signal clearly, the user typically sets the volts/division knob and offset so each signal occupies one quarter of the vertical range of the display grid.

The drawback of this technique is that each signal is only using one-fourth of the range of its ADC. The user gets 6-bit resolution (64 counts) instead of 8 bits (256 counts). Some readers may recall the oscilloscopes with 6-bit ADCs of bygone days. Despite being less expensive than 8-bit models, they sold poorly because users quickly learned that 6-bit resolution is not very accurate. Yet, many oscilloscope users today are paying for 8-bit scopes and then using only 64 counts of the ADC.

Split the Grid For Accuracy's Sake

Figure 2: In contrast to Figure 1, here are four signals captured
and viewed using four separate grids. Each grid corresponds
to the full range of the ADC.
Here, then, is a better technique for capturing and viewing four signals. In this approach, we display each signal on its own grid. The signal occupies a large part of the range of the ADC. If you did not look closely at the display, Figures 1 and 2 look very similar. However, the quality of the data is very different. In Figure 2, at 250 mV/div, each 2-V signal occupies all eight vertical divisions of each grid. Any type of measurement made using the “6-bit technique” will be much less accurate.

Of course, not all oscilloscopes offer quad-display grids. Most Teledyne LeCroy scopes include the choice of single, dual, quad, or even octal display (in case the user wants to view that many signals and zoom/math traces). Many oscilloscopes have only a single or dual grid, though. But there’s still a way to make sure you use the ADC’s full scale.

Set up each signal to use the full range of the grid. You can get a good clean view of each signal one at a time by toggling off the view of the other signals. If you want to view several signals simultaneously, you can turn on several traces. The screen may be a bit messy because all the signals are full scale, but you can see things such as relative timing of edges and signal shapes.

Keep in mind that you do not have to view signals to characterize them using the parameter measurements of the oscilloscope. Parameters are calculated using the data in the memory, not by using the pixels on the screen.

Next, we'll take a look at how use of an oscilloscope's persistence mode and exclusion triggers help nail down intermittent glitches.

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