6 Tips for Making More Accurate Measurements with Your High-Performance Oscilloscope
By Robert Lashlee, Agilent Technologies, Inc.

Oscilloscopes are a test and measurement device and as such, making accurate measurements is a vital component of their usability. Unfortunately, users are often unaware of some of the features that can significantly improve the accuracy of their measurements. This article will address some of these features as well as discuss measurement accuracy/precision in general.

Measurement Accuracy and Statistics
To confidently predict the reliability of a digital system, you must know the statistics of its behavior. Singular measurements of setup time, hold time, propagation delay, and skew do not allow you to adequately predict the probability of errors due to timing violations. However, a design based on worst-case measurements can be too conservative. A worst-case measurement does not show how frequently or under what conditions the worst case will occur.

In these instances, measurement statistics can offer valuable insight. Typically, a measurement is performed on a waveform over numerous acquisitions. The results from each acquisition are then averaged and appropriate statistical data is calculated. Examples of statistical values typically calculated by oscilloscopes include the mean, the standard deviation, and the minimum/maximum of a measurement.

• The standard deviation is an indication of the amount of variability in the measurement (how much individual values vary from the mean). If the standard deviation is small compared to the mean, then that implies that most of the individual measurements used to calculate the mean are very close to the mean value. If the standard deviation is large compared to the mean, then that implies that the measurement results are more spread out. Keep in mind that when analyzing whether the standard deviation is small or large, it needs to be compared to the mean. A standard deviation of 20 ns may be considered small if the mean is one second, but large if the mean is 25 ns.

Having a small standard deviation means your results are precise (closely grouped, repeatable, etc.), but it does not mean your results are accurate. You could very well have results that are tightly grouped around the “wrong” result.

• The minimum and maximum values are the minimum and maximum results for the particular measurement. These values give you an idea of the range of results computed from your measurement. From analyzing how far these outliers are from your mean value, you can determine the absolute maximum deviations in your measurement. However, this statistical calculation is the most prone to measurement errors and outliers. If the outliers are real in that they were not produced by errors in the measurement process or were not a result of anomalies in the waveform, they need to be accounted for. However, if they were the result of errors, then the minimum and maximum values of a measurement can show large absolute maximum deviations when in fact the results do not possess this large of a range.

Now that you understand some of the statistical values that your oscilloscope can provide, let us look at how you can maximize the accuracy of your measurements.

Tip 1 - Use the Full Dynamic Range of the Analog to Digital Converter
Digitizing oscilloscopes utilize an analog-to-digital converter (ADC) to convert analog signals into digital ones that can be processed, analyzed, displayed, and saved on the oscilloscope.
In order to obtain the most accurate measurements, a waveform must be displayed across the entire dynamic range of the ADC. This means that the waveform should take up as much vertical space on the display as possible. Oscilloscopes in the past used 1-3-5 controls, where the voltage per division settings could be incremented in a 1-3-5 fashion. However, smaller increments are often necessary to fine-tune the amplitude of a waveform to fill up the entire dynamic range of the ADC. The Agilent Infiniium 90000A Series and the new Agilent Infiniium 9000A Series oscilloscopes feature vernier controls for just this purpose. The user can still use the traditional 1-3-5 scaling, but by pushing a knob on the front panel, the corresponding control goes into vernier scaling.

Ensuring the waveform uses the entire dynamic range of the ADC is also important when making measurements on multiple waveforms. Many people are tempted to display multiple waveforms on the same grid. It is usually difficult to discern one signal from the others, so often people will alter the vertical scale and translate the signals vertically such that each waveform can be seen separately. If you are just viewing the waveforms, then overlapping them or translating them to different vertical areas of the display is fine. However, if you are making measurements, this can significantly decrease their accuracy as you are reducing the vertical resolution and increasing the signal to noise ratio. A better method is to split the display into separate grids. Each one of these grids represent the entire dynamic range of the ADC so you can make each waveform take up an entire grid.

Tip 2 – Set the Measurement Thresholds Correctly
Oscilloscopes typically default to the 10%-90% threshold. This means that for measurements such as rise time, the rise time is calculated as the amount of time it takes for the waveform to get from 10% of the amplitude to 90% of the amplitude. However, many standards use 20%-80% thresholds. When making measurements on a system to see if it meets a certain standard, it is important to match the thresholds to the standard. Also, if the waveform you are analyzing has excessive ringing or overshoot, you may obtain better measurements by using the 20%-80% definition for the lower and upper measurement thresholds.

Tip 3 – Limit the Bandwidth to the Requirements of the Measurement
Many users assume that it is always better to have more bandwidth. If you make a measurement on a 4 GHz oscilloscope, then the same measurement on an 8 GHz oscilloscope should be even more accurate, correct? This turns out to not always be the case. By limiting the bandwidth of an oscilloscope to that required by the specific measurement being made, the noise is limited and, therefore, the accuracy is increased.
The Agilent Infiniium 90000A Series oscilloscopes allow you to limit the bandwidth via the Bandwidth Limiting filter found in the Acquisition dialog box. This filter is easily turned on by checking a box and then a corresponding field enables you to set the bandwidth to one of several different choices.

Additionally, you can buy whatever bandwidth oscilloscope meets your current needs and then upgrade if a measurement requires more bandwidth later. The Agilent 90000A Series allows you to upgrade bandwidth in this manner. For instance, if you are currently working with PCI-Express Gen 2, you can buy a lower bandwidth oscilloscope now and then upgrade to a higher bandwidth oscilloscope when you start working with PCI-Express Gen 3 (due to Gen 3’s higher bandwidth requirements).

Tip 4 – Measure All Edges
Typically when you perform a measurement on a waveform edge, the measurement is made on a single edge and statistics are calculated over a series of runs. However, if your oscilloscope has a mode where timing measurements are made on all edges in a single acquisition, this can greatly improve the accuracy of your measurements on periodic waveforms. For example, the Agilent Infiniium 90000A and 9000A Series oscilloscopes offer the Measure All Edges feature which allows you to measure up to two million edges. This enables you to perform statistical analysis on the entire waveform (up to two million edges) as opposed to a single edge.

Tip 5 – Select the Appropriate Probe/Probe Head
The probe you use has some non-ideal transient response that contributes to the overall system error. Additionally, the probe and the circuit under test form a circuit that behaves differently from the circuit without the probe (the waveform at the probe tip is not the same as the waveform that is present when the probe is removed). Probe loading often has a more significant effect on the measurement than the transient response of the oscilloscope and must be included in the analysis of measurement errors. There are several things to keep in mind regarding probing and measurement accuracy:

1. Check the probe input impedance and consider the impact it will have on your circuit. Remember, loading becomes increasingly important at high frequencies, and use a short ground lead when possible.

2. The probe will introduce distortion into your measurements unless it has a flat transmitted response throughout the bandwidth of the probe. A flat transmitted response will closely track the signal at the probe tip and pass it to the oscilloscope with minimal degradation.

3. Your best measurements can never be better than your probe and connection, so wisely choosing and using probes with properly damped accessories will improve your measurement results and their repeatability.

4. The oscilloscope and probe work together to form a cohesive measurement system. Understanding the impact of your probe on your overall system bandwidth provides you with the information you need to pick the right oscilloscope and probe for your application. The best a probe can do is to minimize the impact it has on the circuit under test and transmit the voltage at its input to its output with minimal distortion.

Tip 6 – Reduce the Noise
One method for reducing the noise was discussed above (limiting the bandwidth), but there are other methods as well. For example, many oscilloscopes (such as the Agilent Infiniium 90000A and 9000A Series oscilloscopes) offer High-Resolution acquisition modes. Most digital oscilloscopes offer 8 bits of vertical resolution in normal acquisition mode. High-Resolution mode, on the other hand, can offer much higher resolution (typically up to 12 bits), which reduces vertical noise and increases vertical resolution. The trade-off with high-resolution mode is that it reduces the bandwidth and the sample rate of your oscilloscope.
You can also reduce noise by using averaging modes. However, the signal must be periodic or DC. Averaging modes take multiple acquisitions of a periodic waveform and create an average to reduce the noise. However, both the bandwidth and sample rate are also reduced.

Conclusion
Understanding how various controls and devices impact your measurements can have a dramatic impact on the accuracy of your measurements. Using all of the features available with your oscilloscope and probe will ensure that you always obtain accurate results.

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