How does an oscilloscope ensure accuracy in parameter measurement?

Introduction

"Parameter measurement" is a powerful tool for oscilloscopes to analyze waveforms. Engineers can easily obtain various parameters without turning on the cursor. However, some engineers may be a little worried: how can the oscilloscope ensure the measurement accuracy? This article will take you step by step to understand the algorithm behind the oscilloscope parameter measurement.

The ZDS series oscilloscopes provide a wide range of measurement functions, with up to 51 measurement items. The problems encountered by engineers when using them are mostly due to insufficient understanding of details and principles. The following content will take you step by step to dig deeper and solve your doubts.

1. How to use parameter measurement

Turning on the measurement is relatively simple, just remember two points:

1. Which channel do I want to measure?

2. What do I want to measure?

Figure 1 Open measurement

Summary: There are as many as 51 measurement items, and 24 measurement items can be displayed on the same screen.

2. Analysis of parameter measurement algorithm

The items measured in an oscilloscope can be roughly divided into two categories. One category is related to voltage, such as maximum value, minimum value, top value, bottom value, etc. The other category is related to time, such as frequency, period, rise time, fall time, duty cycle, etc. Top value and bottom value are two very important measurement items and are the basis of time measurement.

Measurements related to voltage are relatively simple, and the maximum value (Vmax) and minimum value (Vmin) can be obtained by traversing all sample points. To solve the top value (Vtop) and bottom value (Vbase), it is necessary to first map all sample points into a histogram and then find the voltage value with the highest probability of occurrence.

• Top value (Vtop): The voltage with the maximum probability relative to the upper part of the waveform, and the probability reaches more than 5% of the total number of sample points.

• Bottom value (Vbase): The voltage with the maximum probability relative to the bottom of the waveform, and the probability reaches more than 5% of the total number of sample points.

Figure 2 Measurement of voltage-related items

For time-related measurement items, the top value (Vtop) and the bottom value (Vbase) are used. The positions of the three threshold lines (high, medium and low) are calculated using Vtop and Vbase. Finally, the intersection of the threshold line and the waveform is obtained to obtain the time-related measurement results, as shown in Figure 3. The positions of the three threshold lines (high, medium and low) are adjustable, with the default values ​​being 90%, 50% and 10%.

Figure 3 Measurement of time-dependent items

Summary: For some special waveforms (such as sine waves), Vtop and Vbase may fail to be solved (with a probability of less than 5%). In this case, Vmax and Vmin will be used as the new top and bottom values, and a ? sign will be added after the values ​​of Vtop and Vbase to indicate the abnormality, as shown in Figure 4.

Figure 4: Top value, bottom value, maximum value, and minimum value are the same

3. Measurement and Statistical Algorithm Analysis

The principle of measurement and statistics is very simple. First, we need to understand a concept. The same measurement item may be encountered multiple times in the same measurement, such as the period. A waveform may have N periods. This raises a new question: which period of the waveform does the measurement result of the period correspond to? In order to solve this mismatch problem and make the measurement results more meaningful, we use 6 values ​​in statistics to describe the measurement results, which are as follows:

• Current value (Current): Indicates the first measured value, corresponding to position ① in Figure 5.

• Maximum value (Max): indicates the maximum value among all measured values, corresponding to position ② in Figure 5.

• Minimum value (Min): represents the minimum value among all measured values, corresponding to position ③ in Figure 5.

• Average value (Avg): represents the cumulative average value of all measured values, corresponding to the cumulative average values ​​of the three locations in Figure 5.

• Standard deviation (Stdev): represents the standard deviation of all measured values, corresponding to the standard deviation of the three locations in Figure 5.

• Count: indicates the number of measured values, corresponding to the three positions in Figure 5.

Figure 5 Measurement statistics function cycle

Summary: The measurement statistics function is off by default and can be turned on in the menu. When statistics are off, only the current value (Current) is displayed. When statistics are turned on, 6 statistical results are displayed. In Figure 6, the positive pulse width statistics function is used, and the maximum pulse width can be obtained as 3.0028us, and the number of pulses is 42.

Figure 6 The statistics function can be turned on or off manually

4. Customize the measurement range

When using measurements, a common problem is that there are many waveforms captured, but I only want to measure and analyze a part of the waveform. In the ZDS3000/4000 series oscilloscope, we provide a method to customize the measurement range through hardware acceleration, which can be completed in just two simple steps.

Step 1: Set the measurement range to the cursor area;

Step 2: Adjust the cursor position and specify the measurement range.

Figure 7 Customized measurement area

Summarize:

The measurement function is very powerful, but also very complicated. If you are not sure about the result in actual application, you can first check whether the measurement result is voltage type or time type. For voltage type, focus on checking whether the input channel settings are correct, such as the probe ratio; for time type, focus on checking whether the three thresholds, top value, and bottom value are correct.

The measurement function of the ZDS oscilloscope uses full hardware acceleration processing, which can analyze all the original (unsampled) sample points on the full screen and perform 51 parameter measurements at the same time. The processing speed is very fast. It is the only oscilloscope in the world with 51 "true" parameter measurement and statistical functions.