There are many definitions of waveform, such as time domain waveform or frequency domain waveform. For oscilloscopes, most of the time it is the change of voltage with time, that is, the time domain waveform. Therefore, oscilloscopes can analyze the voltage changes of the measured points, so they are widely used in various electronic industries and fields.
Generally, the classification of oscilloscopes in our industry is only based on analog oscilloscopes and digital oscilloscopes. Some manufacturers may give them other names, such as digital fluorescent oscilloscopes, in order to highlight some functions of their oscilloscopes. But its essential principle can't escape from these two kinds of oscilloscopes.
Analog oscilloscope belongs to the early oscilloscope, mainly cathode ray tube (also known as kinescope, widely used in early TV sets and monitors). The electron beam emitted by the cathode ray tube passes through the horizontal deflection and vertical deflection system, and hits the fluorescent substance on the fluorescent screen to display the waveform.
However, up to now, the remaining advantage of analog oscilloscope seems to be only the price. It has no ability to store data and analyze waveforms, limited trigger function, and poor ability to capture single and accidental signals. Moreover, because it uses a large number of analog devices, these devices will change with the change of time and temperature, so its performance is unstable. Analog oscilloscope has almost been eliminated in modern electronic measurement, so today we mainly talk about digital oscilloscope.
Due to the limitation of display technology, early digital oscilloscopes still used CRT (cathode ray tube) display screen on analog oscilloscopes. The biggest difference between digital oscilloscope and analog oscilloscope is that the input signal is not directly typed on the display screen, but sampled and digitized by ADC (analog-to-digital converter) and stored in the buffer, and then the data is read out by the signal processing circuit.
Because the early digital oscilloscopes were displayed by CRT, it is necessary to convert digital quantities into analog quantities through DAC digital-to-analog converters and display them on CRT display screen. Most modern digital oscilloscopes no longer use CRT display screen, but use LCD display screen, which not only reduces the size a lot, but also provides more convenient touch function, and no longer needs to convert digital sampling points into analog signals. Because there is no essential difference in functional structure between the two, there is generally no difference between CRT oscilloscope and LCD oscilloscope in the industry.
Digital oscilloscope is often called digital storage oscilloscope, because an important part of digital oscilloscope is to store the data collected by ADC. Through the main board of Maikexin STO 1 104C intelligent oscilloscope, we can intuitively understand the main process of collecting data by modern digital oscilloscope:
(1) signal is attenuated to a proper proportion by the probe and sent to the front end of the oscilloscope. How much voltage an oscilloscope can measure generally depends on the probe, which can attenuate tens of thousands of volts into tens of volts.
(2) The signal reaches the front-end attenuator and amplifier through the coupling circuit. The characteristic of the oscilloscope software is to adjust the vertical gear to make the waveform occupy the whole screen as much as possible, thus improving the vertical accuracy and making the measurement more accurate. The front-end part largely determines the first indicator of oscilloscope: bandwidth.
③③ARM processor controls FPGA to adjust the sampling rate of ADC analog-to-digital converter, and the characteristic of oscilloscope software is to adjust the time base. Because the storage depth is fixed, the sampling rate = storage depth ÷ waveform recording time, and the time base setting is usually changed by changing the sampling rate. Therefore, the sampling rate marked by the manufacturer is often effective under a specific time base setting, but in a large time base, the sampling rate has to be reduced due to the influence of storage depth. ADC analog-to-digital converter and RAM high-speed memory affect two other indicators of oscilloscope: sampling rate and storage depth.
④ Next, FPGA drives ADC to sample synchronously, and ADC binarizes the collected data and writes it into cache. Memory cache is the storage depth. Generally speaking, the size of the memory is four times the storage depth indicated by the oscilloscope. Because FPGA can't control the trigger of the oscilloscope, the collected signal must be twice as deep as the storage depth of the tag, and then a waveform is filtered according to the trigger, so that the oscilloscope can see the waveform before the trigger position. Because the oscilloscope can't stop collecting the waveform collected before screening, otherwise the waveform capture rate will be too low, so it is necessary to continue collecting sampling points with the same length at the same time, and so on, which will take four times.
⑤ After receiving the trigger instruction, the memory hands over the data to the ARM processor for processing.
⑥ The ⑥⑥ARM processor processes the data and outputs it to the display screen for display to users through the display interface. Through calculation, the oscilloscope can also imitate the multi-level brightness display similar to analog oscilloscope, as well as the unique color temperature display effect and afterglow display effect of digital oscilloscope.
⑦ After the oscilloscope processes the data, it can save the current waveform image or data into the memory. It should be noted that the storage here is completely different from the high-speed storage with storage depth. Most oscilloscopes use external memory such as USB flash drive, SD card and computer. Now some modern oscilloscopes have built-in large storage, which can be directly stored in the oscilloscope.
In this process, 234 are all processed in parallel.
Due to the limitation of the processing speed of digital oscilloscope, it is impossible to ensure that the waveform of the measured signal can be displayed on the screen continuously in real time, and there will be waveform data loss between the two displayed waveforms, which is the so-called dead time, which is the biggest disadvantage of digital oscilloscope compared with analog oscilloscope. However, with the enhancement of oscilloscope computing power and the increase of waveform capture rate, this shortcoming is slowly being made up.