How to use and work principle of oscilloscope

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How to use and work principle of oscilloscope [Copy link]

How to use an oscilloscope and its working principle How to use an oscilloscope: An oscilloscope, as its name suggests, is a machine that displays waveforms. It is also known as the "eyes of electronic engineers." Its core function is to display the actual waveform of the measured signal on the screen so that engineers can find and locate problems or evaluate system performance, etc. Its development has also gone through two eras: analog and digital. A more accurate name for a digital oscilloscope is a digital storage oscilloscope, or DSO (Digital Storage Oscilloscope). This "storage" does not mean that it can store waveforms on media such as USB flash drives, but refers to the instant display characteristics of analog oscilloscopes. Analog oscilloscopes rely on cathode ray tubes (CRTs, commonly known as electron guns) to emit electron beams, and this beam of electrons is deflected under the magnetic field formed by the measured signal, thereby reflecting the waveform of the measured signal on the screen. This process is instant, without any storage process in between. The principle of digital oscilloscope is as follows: first, the oscilloscope uses the front-end ADC to quickly sample the measured signal. The sampling speed can usually reach hundreds of M to several G times per second, which is quite fast; and the back-end display component of the oscilloscope is an LCD screen, and the refresh rate of the LCD screen is generally only tens to more than one hundred Hz; in this way, the data sampled by the front end cannot be reflected on the screen in real time, so the storage link is born: the oscilloscope temporarily stores the data sampled by the front end in the internal memory, and reads the data from this memory when the display is refreshed, using this level of storage link to solve the speed difference between the front-end sampling and the back-end display. When many people see an oscilloscope for the first time, they may be intimidated by the numerous buttons on its panel, and the oscilloscope is generally expensive, so they have a fear of using it. This is unnecessary, because although the oscilloscope looks complicated, in fact, to use its core function-displaying waveforms, it is not complicated, and it can be done in three or four steps. The complexity of the current oscilloscope is caused by the addition of many auxiliary functions. These auxiliary functions naturally have their value. Skillful and flexible application of them can achieve twice the result with half the effort. As beginners, we don't care about these for now. We just need to use its most core and basic functions. The use of an oscilloscope is similar to that of a multimeter. To use an oscilloscope, you must first connect it to the system under test, using an oscilloscope probe, as shown in 20-4. An oscilloscope generally has 2 or 4 channels (usually marked with numbers 1 to 4, and the extra probe socket is an external trigger, which is generally not used). Their low bits are equivalent, and you can choose any one of them. Plug the probe into one of the channels, and the small clip at the other end of the probe connects to the reference ground of the system under test (here you must pay attention to one problem: the clip on the oscilloscope probe is directly connected to the earth, that is, the ground wire on the three-pin plug, so if there is a voltage difference between the reference ground of the system under test and the earth, it will cause damage to the oscilloscope or the system under test), the probe contacts the point under test, so that the oscilloscope can collect the voltage waveform at that point (ordinary probes cannot be used to measure current, and a special current probe must be selected to measure current). Next, you need to adjust the buttons on the oscilloscope panel so that the waveform under test is displayed on the screen in an appropriate size. You only need to adjust the parameters of the oscilloscope according to the two major elements of a signal - amplitude and period (frequency and period are conceptually equivalent). The knob above each channel socket is used to adjust the amplitude of the channel, that is, the adjustment of the vertical size of the waveform. By turning them, you can change the voltage value represented by each vertical grid on the oscilloscope screen, so it can be called "volt grid" adjustment, as shown in the following two comparisons: the left is 1V/grid, the right is 500mV/grid, the amplitude of the left waveform occupies 2.5 grids, so it is 2.5V, and the amplitude of the right waveform occupies 5 grids, also 2.5V. It is recommended to adjust the waveform to the right, because at this time the waveform occupies a larger space in the entire measurement range, which can improve the accuracy of waveform measurement, as shown in 3. In addition to the volt grid knob above, you will usually find a knob of the same size on the panel (not necessarily in the position shown in 20-6). This knob is used to adjust the period, that is, the adjustment of the horizontal size of the waveform. By turning it, the time value represented by each horizontal grid on the oscilloscope screen can be changed, so it can be called "second grid" adjustment, as shown in the following two comparisons: the left is 500us/grid, and the right is 200us/grid. The left cycle occupies 2 grids, and the cycle is 1ms, that is, the frequency is 1KHz. The right cycle occupies 5 grids, which is also 1ms, that is, 1KHz. There is no more reasonable question here. Specific problems should be treated specifically. They are all very reasonable. The oscilloscope uses a narrow electron beam composed of high-speed electrons to hit the screen coated with fluorescent material to produce tiny spots of light. Under the action of the measured signal, the electron beam is like the tip of a pen, which can draw a curve of the change of the instantaneous value of the measured signal on the screen. The oscilloscope can be used to observe the waveform curve of the amplitude of various electrical signals changing with time, and it can also be used to test the electrical quantity of various signals, such as voltage, current, frequency, phase difference, amplitude modulation, etc. The dual-trace oscilloscope is composed of two channels of y-axis preamplifier circuit, gate circuit, electronic switch, hybrid circuit, delay circuit, y-axis post-amplifier circuit, trigger circuit, scanning circuit, x-axis amplifier circuit, z-axis amplifier circuit, calibration signal circuit, oscilloscope and high and low voltage power supply circuit. When observing the signal waveform, the measured signals UA and UB are input into the oscilloscope through the two input terminals of CHA and CHB, and are first sent to the y-axis preamplifier circuit yA and yB for amplification. Because both channel yA and channel yB are controlled by the electronic switch, the UA and UB signals are alternately transmitted to the hybrid circuit, delay circuit, y-axis post-amplifier circuit behind, and added to the vertical deflection plate of the oscilloscope. In order to meet various test needs, the electronic switch can have five different working states, namely CHA, CHB, alternating, intermittent, ADD, etc. These five working states are controlled by the display mode switch. When the display mode switch is in the alternating position, the electronic switch is a bistable circuit. It is controlled by the gate signal from the scanning circuit, so that the two front channels of the y-axis follow the scanning circuit. There are two trigger modes: internal trigger and external trigger, which are selected by the trigger source selection switch. When the switch is in the internal position, the trigger signal comes from the measured signal sent through the y-axis channel. When the switch is in the external position, the trigger signal is sent from the outside. This signal should be in an integer ratio with the frequency of the measured signal. In the use of oscilloscopes, most of them adopt the internal trigger working mode. The scanning circuit generates a scanning signal (sawtooth wave circuit). It is connected to the x-axis amplifier circuit through the x-axis selection switch, and is sent to the x-axis deflection plate of the oscilloscope after amplification. The z-axis amplifier circuit adjusts the brightness of the light spot on the fluorescent screen and erases the unnecessary light spot track. When the gate signal of the scanning circuit comes to the z-axis amplifier circuit, the z-axis amplifier circuit outputs a positive brightness enhancement pulse signal and adds it to the control pole of the oscilloscope. That is to say, when the scanning signal is in the positive process, the light spot on the fluorescent screen is brightened. During the conversion process of the electronic switch, the electronic switch circuit also adds the output pulse signal to the z-axis amplifier circuit. At this time, the z-axis amplifier circuit outputs a negative pulse signal and adds it to the control pole of the oscilloscope. In this way, during the conversion process of the electronic switch, the excessive light spots when the two channels work alternately are eliminated to improve the clarity of the displayed waveform. The correction signal generating circuit generates a rectangular signal with a certain frequency and amplitude. It is used to correct the sensitivity of the y-axis amplifier circuit and the scanning speed of the x-axis. High and low voltage power supply, of which the high voltage is supplied to the oscilloscope display system. The low voltage is supplied to the various levels of the oscilloscope circuit.