Calibration of analog oscilloscopes

Oscilloscopes, like other instruments (such as multimeters, etc.), must be calibrated before use. The so-called calibration of an oscilloscope is to correctly debug the original waveform of the oscilloscope before testing. In other words, the calibrated waveform must be consistent with the parameters set by the oscilloscope itself (these parameters are usually marked at the calibration test points). Take the GW GOS-602 oscilloscope as an example (left picture): In the lower left corner of its panel are the parameters required to calibrate the waveform, such as the voltage value of 2V, the frequency of 1KHz, etc. (right picture), which requires the voltage peak-to-peak value of the oscilloscope's calibration waveform (or sine, cosine, square wave) to be 2V and the frequency to be 1KHz. However, oscilloscopes usually cannot directly display the frequency of the waveform, but convert the frequency into a period based on the conversion of frequency and period (T=1/f), and then use periodic waves to represent the frequency (the equivalent period of a frequency of 1KHz is 1mS).

In the process of waveform correction, in order to facilitate the observation of the waveform, the center position of the waveform should be adjusted first, which requires turning the switch of the connection mode signal between the inputs to the GND position (lower left picture). At this time, if the power is turned on normally, a horizontal bright line should be displayed; if it is not displayed, then the POSITION, DC BALT and INTER should be adjusted up and down. Among them, POSITION is the waveform up and down adjustment button (middle picture), DC BAL is the center adjustment of the horizontal bright line, and INTER is the brightness adjustment. If the bright line is unbalanced (relative to the X-axis), use a non-sensing screwdriver to adjust the TEACE ROTATION near FOCUS (lower right picture), and then adjust the convergence to the best state through the adjustment of FOCUS.

After completing the first step, switch GND to AC (Figure a); when inputting the calibration waveform, adjust the attenuation or expansion button to the original position. If it is dialed incorrectly, it will seriously affect the accuracy of the measured waveform value; the selection of input track is completely controlled by the MODE selection key (Figure b); if the debugged waveform is flickering, then you must consider the synchronization function keys, namely LEVEL (horizontal synchronization adjustment) (Figure C) and TRIG. ALT, ALT.CHOP (Figure d).

Figure a Figure b Figure c Figure d

What usually needs to be calibrated is the adjustment of the peak-to-peak voltage and the number of cycles, which is also the content of our waveform test. These adjustments are completed by the buttons VOLTS/DIV, TIME/DIV, SWP.VAR, and VOLTS/DIV. The value pointed to by the mark on each button indicates that this value is the unit value of each grid on the coordinate axis of the display screen. The horizontal axis represents the period, and the vertical axis represents the voltage amplitude. For example: the white designated point of VOLTS/DIV is at 1V (lower left figure), which means that the voltage amplitude of each small grid of the vertical axis is 2V; on TIME/DIV, the designated point is pointed to 1mSV (lower right figure), which means that the period of each small grid of the horizontal axis is 1mS. Then, according to the number of unit grids occupied by the waveform, the amplitude and period of the waveform can be directly read (or read after empirical calculation), and then used to judge the right and wrong and analyze the cause of the fault. Therefore, the calibration work before use is very important.

Before formally calibrating, according to the reference value of the calibration in the lower left corner of the oscilloscope, the voltage gear should be set to the unit of 1V and the period gear should be set to the position of 1mS (of course, you can also choose other unit values), and at the same time, confirm which CHANNEL (which trace) to use or two CHANNELs to use together (which one to use depends on which position you set the MODE selection function to): CH1 (first trace), CH2 (second trace), DUAL (two traces are used at the same time), ADD (double trace superposition). Press POWER to start adjustment, and set the input coupling mode to GDN (input to ground), which is used to calibrate the center position of the waveform. This function key is also used with POSTION (waveform up and down adjustment button). Since the waveform we measure is often a pulse signal waveform, when the center position is adjusted, the gear will generally be set to AC (AC input), and the DC gear (DC input) is rarely used.

After setting the input mode to AC, connect the probe of the signal transmission line to the calibrated test port (lower left picture), and you can see the square wave on the display. But the square wave at this time is not necessarily standard (correct). It is possible that the peak-to-peak value of the voltage is insufficient and the period is incorrect. This is a test of your familiarity with the various functions of this oscilloscope. There is a button in the center of the axis of the voltage button, which is used to compensate the voltage value. Under normal circumstances, turn it right to lock it (middle picture), and it can be used normally. If the calibration parameter value cannot be restored after locking, you need to use this voltage amplitude compensation potentiometer to compensate. The cycle adjustment button is not so hidden. It is on the left side of the large button for setting the cycle unit. It is marked as SWP VAP, which can adjust the waveform period. At the same time, there is a POSMCN button on the left side of SWP VAPR, which is used to horizontally translate the waveform (lower right picture). It is used to coordinate WSP VAP, so that we can observe or adjust the waveform period more accurately and conveniently. These can set the original waveform of the oscilloscope to meet the calibration reference value. If you encounter this situation: the waveform cannot be still when the probe is connected to the calibration test port. It may be because the LEVEL on the right side of the large cycle button has not been adjusted. The name of LEVEL is "tracking level", and its actual function is to supplement the horizontal synchronization control. When two tracks are used at the same time, the horizontal synchronization often fails. At this time, you should consider the TRIC. ALT button on the top of the LEVEL, which is a mandatory lock. If you are familiar with using these buttons, it is not difficult to correct the original waveform of the oscilloscope.

One thing that must be paid special attention to when correcting the waveform is the signal attenuation position of the signal transmission line (see Figure 15). When it is dialed to *1, it means no attenuation (normal setting point); when it is dialed to *10, it means 10 times attenuation. Usually when the frequency of the input signal is too low, its corresponding period will become very large. At this time, it is necessary to attenuate it before testing. However, it is still necessary to increase the test result by 10 times, so that it is the original waveform value. Another one is the expansion button (*10 disk MAC) located between SWP VAP and POSMCN (see Figure 16). When the waveform cannot be seen clearly when the cycle unit number is set to the lowest microsecond value, or when the frequency of the waveform is very high, this expansion button must be used. In other words, the so-called expansion and attenuation only apply to the cycle, but not to the voltage amplitude. Moreover, whether it is expansion or attenuation, the multiple of the cycle must be reduced or enlarged accordingly after the waveform is adjusted. In order to make the waveform reading more accurate and clear, when calibrating the waveform, the waveform must be adjusted to be the most accurate, clearest, and the line to the finest. Only in this way can the reading be the most accurate and the error be minimized, which often plays a decisive role in fault analysis. Finally, there is one more thing to note: after the calibration waveform is adjusted, all compensation buttons cannot be adjusted or changed (i.e. SWP VAP and voltage compensation), otherwise the oscilloscope will have to be recalibrated again.