An oscilloscope is not a vertical measurement tool

1. The concept of quantization error       
We all know that the A/D of the oscilloscope has only 8 bits, which means that for any voltage value, there are only 256 0s and 1s to reassemble. If the +/- sign bit is included, the digital range of the oscilloscope is -128-+127. Figure 1 clearly shows the principle of this digital sampling. The top of the oscilloscope screen represents +127, the middle represents 0, and the bottom represents -128. This principle gives rise to the first principle of using an oscilloscope: minimize quantization error. This principle tells us a common sense of using an oscilloscope. In order to obtain a voltage value that is closest to the true value, the vertical resolution should be as small as possible so that the displayed waveform fills the oscilloscope screen as much as possible. Figure 1 Figure 2 and Figure 3 show the effect of testing the same signal at 1V/div and 200mv/div respectively. At 1V/div, the minimum quantization error of the oscilloscope is (1V*8)/256=31.25mv, which means that signals less than 31.25mV cannot be accurately measured. For high voltage measurement, assuming the range is 100V/div, the quantization error of the oscilloscope is 800V/256=3.125V, which means that signals less than 3.125V cannot be accurately measured. Figure 2  Figure 3  I often give the following more impressive example to illustrate the quantization error: directly connect the ground and signal pins of the probe and hang it in the air, and compare the pk-pk values ​​when the range is 20mV/div and 100V/div. How big is the difference? The difference is tens of volts! ! You can do this experiment now. This means that the 20V signal tested at 100V/div may actually be only 20mV!  When measuring power supply ripple with very small output voltage, you need to use a 1:1 probe and set the oscilloscope range to 2mv/div.  The following is an example of testing a power supply ripple of about 10mV. 2. Flatness of the amplitude-frequency characteristic curve.  As far as the industry standard of oscilloscopes is concerned, the deviation of the amplitude-frequency curve of an oscilloscope from the ideal response is allowed to reach +/-2dB, which seems to be an unacceptable error range for some measurements with high precision requirements. Therefore, the oscilloscope is not defined as a measurement tool in the measurement industry. It can only be said to be a debugging tool.   　In addition, the response curves of the front-end amplifiers of different models and brands of oscilloscopes are also different. Some are Gaussian responses, some are rectangular responses, and some are fourth-order Bessel responses. For the same input frequency, the measurement results of the vertical parameters of different oscilloscopes are definitely inconsistent. At the same frequency, the measurement results under different vertical channel settings should also be inconsistent.  I learned from our maintenance and calibration center that when calibrating the oscilloscope, we generally calibrate the flatness to within +/-1.5%, which is a relatively strict standard. This means that for an ideal 1V signal, it is normal for the test result to deviate by 15mv. The 　　　above figure shows the amplitude-frequency characteristic curve of 6GHz.　  It can be seen that the maximum deviation is less than +/-2dB.  After the oscilloscope has been used for a period of time, the flatness will change and need to be sent to a calibration agency for calibration. However, many third-party agencies are usually unable to calibrate the flatness to the level that the manufacturer can calibrate. Therefore, some customers now often require the oscilloscope manufacturer to provide calibration services.