Oscilloscope is the window to observe the waveform, it allows designers or maintenance personnel to see the electronic waveform in detail, to achieve the effect of seeing is believing. Because the human eye is the most sensitive visual organ, it can see clearly the end of autumn, reflect objects to the brain very quickly, and make comparison and judgment. Therefore, oscilloscope is also known as waveform multimeter.
Early oscilloscopes only showed the change of voltage with time for qualitative observation. Then, the improved oscilloscope has quantitative function, measuring amplitude and time, as well as their changes. At the same time, in order to record and compare the accidental events, we need to use the long afterglow effect of camera and oscilloscope.
The frequency characteristic of analog oscilloscope is determined by vertical amplifier and cathode oscilloscope. In the 1980s, digital processing and microprocessor were introduced into oscilloscopes, and digital oscilloscopes appeared. Now analog oscilloscopes are called analog real-time oscilloscopes (Art), and digital oscilloscopes are called digital storage oscilloscopes (DSO).
Art needs amplifiers and cathode ray oscilloscopes which are suitable for the bandwidth. With the increase of frequency, the process requirements of cathode ray oscilloscopes are strict, the cost is increased, and there is a bottleneck. As long as DSO has a high speed A / D converter suitable for bandwidth, other memory, D / a converter and display are low speed components, and display can use LCD flat array and color screen.
DSO uses microprocessor for control and data processing, so that DSO has some functions that art does not have, such as advanced trigger, combined trigger, burr capture, waveform processing, hard copy output, floppy disk recording, long-time waveform storage, etc.
The bandwidth of DSO depends on the sampling rate, which is 4 times of the bandwidth. The reconstructed waveform is compensated by interpolation algorithm, and the waveform will be distorted; The A / D conversion speed is fast, but the D / a conversion speed is slow, so the waveform update rate is low, and the occasional signal will be missed; The vertical resolution is generally 8 bits, which is obviously low; There are many knobs on the panel, the menu is complex, and it is not convenient to use; No brightness modulation, no three-dimensional graphics can be observed; The storage capacity of the waveform is not enough to process the waveform.
At present, the shortcomings of DSO have been basically overcome, but not all good performance is reflected in the same oscilloscope, that is, each DSO will have certain characteristics, but also some shortcomings, so we should pay attention to the comparison when selecting the model. Some DSOs have the same waveform update rate as art, but some don't. one DSO has the ability of three-dimensional graphics display on the fluorescent screen of art, while most DSOs don't. Most of the real-time bandwidth of DSO is the same as single time bandwidth, but there are some DSOs that only guarantee real-time bandwidth.
The DSO mentioned above includes a / D converter and microprocessor. In this way, adding a plug-in card to a PC can also form a DSO, but generally the sampling rate is low, the function is less, and the price is cheap. There are also DSO modules based on VXI bus and rack type DSO plug-ins.
DSO's memory is the only component of oscilloscope components next to a / D converter. It stores the sample of the measured signal for the subsequent D / a converter to recover the waveform. Now the storage capacity can reach more than 1m.
Ordinary DSO has 8-bit vertical resolution, that is, each scan has 256 samples, which requires 256 points of storage, equivalent to 256 bytes. If the resolution is increased and the horizontal axis is expanded 10 times, it is equivalent to 20K bytes; The vertical axis is also expanded 10 times, equivalent to 40K bytes. It can be seen that DSO should have at least 2K bytes, and medium DSO should have more than 40K bytes. If you want to record 10 times the above waveform, you need at least 400k bytes. Therefore, the size of storage capacity is very important.
Conversely, the storage capacity also affects the scanning speed. For example, a memory with only 50K points per scan track can record 100 μ If the sampling rate is equal to 4 times the bandwidth, the real-time bandwidth is equal to 125MHz. Obviously, if it is necessary to increase the sampling rate to 1000ms / s, record 100 μ It needs 100k memory.
In order to store a complete graph, let the pixel be 1024 × 512 = 0.5m bit, four graphics, 2M bit storage. In FFT analysis, additional storage is needed to compare the components of the new waveform with the reference waveform or stored waveform. For waveform storage, some DSOs also provide floppy disk or hard disk for data recording.
1. Differences in simple repetitive signals
There is almost no difference between the performance of dos and analog oscilloscope for displaying the fast edge of the repetitive signal with high enough repetition rate. Both oscilloscopes can be used to observe the signal waveform well. Electric welding machine
When measuring signal parameters, the advantage of DSO is that it has the ability of automatic measurement. When using the analog oscilloscope, the user must set the cursor, analyze and understand the displayed waveform to get the measurement results.
If want to undertake adjusting job, so general had better use analog oscillograph. This is because the real-time display ability of analog oscilloscope enables it to display the input voltage at any time. Its waveform update rate (the number of times of scanning on the screen per second) is very high, which can be far more than 100000 scans per second at high scanning speed. So any change in the signal will be displayed immediately. And some of the changes will be displayed in the form of waveform brightness changes on the display screen.
Contrary to the analog oscilloscope, DSO displays the waveform reconstructed from the collected waveform data. The number of flashes per second is far less than 100. On some oscilloscopes, more than five times of waveform acquisition per second are needed. As a result, there is a significant time delay between the change of the signal and the display of the changed signal on the screen. When adjusting the system, this is a major disadvantage of using DSO.
2. Differences in more complex repetitive signals
The above simple repetitive signals can be seen in many electronic fields, but they are often used as information carriers. For this purpose, there may be various forms, such as sine wave, impulse wave, ramp wave or step wave, and multiple modulation signals and modes may often be used at the same time.
A common example of complex analog signal is all TV signal. The signal is composed of a variety of components with different frequencies and amplitudes, including both pulse and sine wave, plus another sine wave which is phase shifted to represent color information. In this case, the DSO of analog oscillometry has its own special advantages, each of which can make the best observation on different parts of the signal.
When using DSO, because of its limited sampling points and no brightness change, many waveform details can not be displayed. Although some DSO may have two or more brightness levels, it is only a relative difference. Coupled with the limited display resolution of oscilloscope, it still can not reproduce the effect of analog display.
The use of analog oscilloscope and DSO can usually be a good observation of simple repetitive signal. But both have their advantages and limitations. For analog oscilloscopes, it is difficult to display signals with low frequency due to the short afterglow time of CRT. Because the brightness of the trace on the oscilloscope is inversely proportional to the scanning speed, it is difficult to see the low repetition rate signal with fast rising and falling time. The brightness and speed of DSO are independent of the repetition rate. With the different situation of the measured signal, this feature may be an advantage or a disadvantage.
