Take you to understand the secret behind oscilloscope parameter measurement

"Parameter measurement" is a great tool for oscilloscopes to analyze waveforms. Engineers can easily get various parameters without turning on the cursor. However, some engineers will be a little worried: How does the oscilloscope guarantee the measurement accuracy? This article will take you step by step to understand the oscilloscope parameters. The algorithm behind the measurement.

The ZDS series oscilloscope provides a very rich measurement function, with up to 51 measurement items. Most of the problems encountered by engineers during use are due to insufficient understanding of the details and principles. Here are the contents to take you step by step to dig deeper and solve your doubts.

1. How to use parameter measurement

It is relatively simple to open the measurement, remember two points:

1. Which channel do I want to measure?

2, what do I want to test?



 

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. Parameter measurement algorithm analysis

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

The measurement related to voltage is relatively simple. The value (Vmax) and value (Vmin) can be obtained by traversing all sample points. To solve the top value (Vtop) and bottom value (Vbase), it is necessary to map all the sample points first, and then find the voltage value of the probability of occurrence.

Top value (Vtop): The voltage relative to the probability of 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 relative to the probability of the lower part 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, you need to use the top value (Vtop) and the bottom value (Vbase), and then use Vtop and Vbase to calculate the positions of the three high, medium and low threshold lines, and find the intersection of the threshold line and the waveform to get the time-related The measurement results are shown in Figure 3. The positions of the three threshold lines of high, medium and low are adjustable, and the default values are 90%, 50%, and 10%. 

Figure 3 Measurement of time-related items

Summary: There are some special waveforms (such as sine waves) that will fail to solve Vtop and Vbase (with a probability of less than 5%). At this time, Vmax and Vmin will be used as the new top and bottom values, and will be in the Vtop and Vbase. After the value, an additional? Sign is displayed to indicate an abnormality, as shown in Figure 4. 

Figure 4 The top value, bottom value and value are the same

3. Measurement and statistical algorithm analysis

The principle of measurement and statistics is very simple. To understand a concept first, the same measurement item may be encountered multiple times in the same measurement, such as a period, a waveform may have N periods. In this way, a new question arises. Which period of the waveform corresponds to the measurement result of the period? In order to solve this non-correspondence problem and make the measurement results more meaningful, we use 6 kinds of values in statistics to describe the measurement results, respectively as follows:

Current value (Current): represents a measured value, corresponding to the position of ① in Figure 5.

value (Max): indicates the value of all the measured values, corresponding to the position of ② in Figure 5.

value (Min): indicates the value of all the measured values, corresponding to the position ③ in Figure 5.

Average (Avg): Represents the cumulative average of all measured values, corresponding to the cumulative average of the three positions in Figure 5.

Standard Deviation (Stdev): Represents the standard deviation of all measured values, corresponding to the standard deviations of the three positions 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 closed by default and can be opened in the menu. When statistics are turned off, only the current value (Current) is displayed. When statistics are turned on, 6 types of statistics results are displayed. Figure 6 uses the positive pulse width statistics function, the pulse width that can be obtained is 3.0028us, and the number of pulses is 42. 

Figure 6 Statistics function can be manually turned on or off

Four, custom measurement range

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

Step: Set the measurement range to the cursor area;

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

Figure 7 Custom measurement area

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, check whether the input channel setting is correct, such as probe ratio; time type is important Check 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 (non-sampled) sample points of the full screen, and perform 51 parameter measurements at the same time. The processing speed is very fast. One has 51 "true meanings". Oscilloscope with parameter measurement and statistics function.