In-depth analysis of oscilloscope threshold setting and pulse number statistics

Threshold

In fact, we ignored the setting of threshold in the measurement menu. The threshold can be divided into lower threshold, medium threshold and upper threshold. The lower threshold, medium threshold and upper threshold of the ZDS2022 oscilloscope are set to 10%, 50% and 90% by default respectively.

The lower and upper thresholds together determine the measurement parameters such as rise time, fall time and slope, while the middle threshold affects parameters such as pulse width and duty cycle. Therefore, when it comes to the above parameters, we should all consider the relevant settings of the thresholds.

The threshold setting applies to all signal channels and also to the search function.

Figure 1 Threshold Description

Analysis of Error Measurement Results

Figure 2 Default positive and negative pulse number measurement results

As shown in Figure 2, when the positive and negative pulse counts are directly selected, the threshold is set to the default value, and the medium threshold is 50%. The relationship between the level display and the threshold is:

• When the waveform is higher than TH, the statistical pulse level is displayed as 1;

• When the waveform is lower than TL, the statistical pulse level is displayed as 0;

As shown in Figure 2, the first positive pulse on the left is higher than the default T¬H, so it can be displayed as level 1; at this time, because the second positive pulse is also higher than T¬H, the level is still displayed as level 1 (the statistics at this time have lost a positive pulse), until the first negative pulse appears, the waveform is lower than TL, and level 0 is displayed, that is, only one positive pulse is counted at this time; when the third positive pulse is detected, the number of negative pulses is counted as 1, and so on. The final measurement statistical result is shown in the schematic diagram in Figure 2, with 4 positive pulses and 3 negative pulses.

The right way to measure

Measure the number of positive pulses

The fundamental reason why the number counting result in Figure 2 is incorrect is that there is no waveform lower than TL before counting the second positive pulse on the left, so the first positive pulse and the second positive pulse are displayed as a level 1. Therefore, in order to obtain the correct measurement result, there should be a waveform lower than TL between the first positive pulse and the second positive pulse, so that the number of two positive pulses can be counted. Therefore, at this time we should adjust the medium threshold upward, as shown in Figure 3.

Figure 3: Appropriate increase in the medium threshold

Obviously, after the medium threshold is properly adjusted, for each positive pulse, the rising edge of the waveform is higher than T¬H, and the falling edge of the waveform is lower than TL. Therefore, such an operation should be performed for the measurement of positive pulses so that the measurement result will be correct. At this time, it should be noted that the position of the medium threshold should be properly adjusted. Since the threshold adjustment is performed for the positive pulse number statistics at this time, the negative pulse number statistics at this time have no reference value.

Measure the number of negative pulses

So how should we adjust the threshold for the statistics of negative pulses? We can lower the medium threshold appropriately. The statistical principle is the same as that of positive pulses. The falling edge waveform of negative pulses is lower than TL, and the rising edge waveform is higher than TH. By analogy, every negative pulse can be accurately counted. Of course, only negative pulses are counted at this time, and the statistics of positive pulses have no reference value.

Figure 4: Appropriate lowering of the medium threshold

summary

When we count the number of positive pulses, we should adjust the middle threshold appropriately upward so that each rising edge of the positive pulse is higher than TH and the falling edge is lower than TL; when we count the number of negative pulses, we should adjust the middle threshold appropriately downward so that each falling edge of the negative pulse is lower than TL and the rising edge is higher than TH.

Only with a full understanding of the measurement parameters can we adopt the correct measurement method and ultimately obtain correct and valuable measurement results!