The Differences and Working Principles of Digital Oscilloscopes and Analog Oscilloscopes[Copy link]
An oscilloscope is an electronic measuring instrument with a wide range of uses. As the saying goes, electricity is invisible and intangible. However, an oscilloscope can help us "see" electrical signals, making it easier for people to study the changing processes of various electrical phenomena. Therefore, the core function of an oscilloscope, just like its name, is to display the waveform of electrical signals, so that engineers can find and locate problems or evaluate system performance, etc.
Waveforms also have many definitions, such as waveforms in the time domain or frequency domain. For oscilloscopes, most of the time, what is measured is the change of voltage over time, that is, the waveform in the time domain. Therefore, oscilloscopes can analyze the voltage changes at the measured point, and are widely used in various electronic industries and fields.
Generally, we in the industry only classify oscilloscopes into analog oscilloscopes and digital oscilloscopes. Some manufacturers may give other names to highlight a certain function of their oscilloscopes, such as digital phosphor oscilloscopes, etc. However, their essential principles still cannot escape these two categories of oscilloscopes.
Analog oscilloscopes are early oscilloscopes that are mainly based on cathode ray tubes (also called picture tubes, which were widely used in early televisions and monitors). The electron beam emitted by the cathode ray tube passes through the horizontal deflection and vertical deflection systems and hits the fluorescent material on the screen to display the waveform.
However, nowadays, the only advantage of analog oscilloscopes seems to be the price. It has no ability to store data and analyze waveforms, has limited triggering functions, and is not capable of capturing single and sporadic signals. In addition, since it uses a large number of analog components, these components will also change over time and temperature, so the performance is unstable. Analog oscilloscopes have almost been eliminated in modern electronic measurements, so today we will mainly talk about digital oscilloscopes.
Early digital oscilloscopes, due to limitations in display technology, still used the CRT (Cathode Ray Tube) display screen on analog oscilloscopes. The biggest difference between digital oscilloscopes and analog oscilloscopes is that the input signal is no longer directly displayed on the display screen, but is sampled and digitized by an ADC (Analog to Digital Converter) and stored in a high-speed cache, and then the data is read out through a signal processing circuit.
Since early digital oscilloscopes used CRT displays, it was necessary to use a DAC digital-to-analog converter to convert digital quantities into analog quantities and display them on the CRT display. Modern digital oscilloscopes no longer use CRT displays, but instead use LCD displays, which are not only much smaller in size, but some also provide more convenient touch functions, and no longer need to convert digital sampling points into analog signals. Since there is no essential difference between the two in terms of functional structure, the industry generally does not call them CRT oscilloscopes and LCD oscilloscopes.
Digital oscilloscopes are often called digital storage oscilloscopes, because an important part of digital oscilloscopes is to store the data collected by ADC. We can intuitively understand the main process of modern digital oscilloscopes collecting data through the motherboard of the Microsonic STO1104C smart oscilloscope:
①The signal is attenuated into a suitable ratio through the probe and sent to the front end of the oscilloscope. The voltage that the oscilloscope can measure generally depends on the probe. The probe can turn a voltage signal of tens of thousands of volts into tens of volts through attenuation.
②The signal reaches the front-end attenuator and amplifier through the coupling circuit. The oscilloscope software adjusts the vertical gear so that the waveform fills the entire screen as much as possible, thereby improving the vertical accuracy and making the measurement more accurate. The front-end part largely determines the first indicator of the oscilloscope: bandwidth.
③The ARM processor controls the FPGA to adjust the ADC analog-to-digital converter sampling rate, which is expressed as adjusting the time base in the oscilloscope software. Since the storage depth is a fixed value, the sampling rate = storage depth ÷ waveform recording time. Usually, the change of the time base setting is achieved by changing the sampling rate. Therefore, the sampling rate marked by the manufacturer is often valid under a specific time base setting. Under a large time base, the sampling rate has to be reduced due to the influence of the storage depth. The ADC analog-to-digital converter and RAM high-speed memory affect the other two major indicators of the oscilloscope: sampling rate and storage depth.
④ Next, the FPGA drives the ADC to synchronously sample, and the ADC converts the collected data into binary data and writes it into the cache. The memory cache is the storage depth. Generally, the size of the memory is four times the size of the oscilloscope's identification storage depth. Because the FPGA cannot control the trigger of the oscilloscope, the collected signal must first be twice the identification storage depth, and then a section of the waveform is filtered according to the trigger, so the oscilloscope can see the waveform before the trigger position. And because the oscilloscope cannot stop collecting the previously collected waveforms when filtering, otherwise the waveform capture rate will be too low, so it is necessary to continue to collect sampling points of the same length at the same time, and repeat this process, which is four times.
⑤After receiving the trigger instruction, the memory will hand over the data to the ARM processor for processing
⑥The ARM processor processes the data and outputs it to the display screen through the display interface to show it to the user. Through calculation, the oscilloscope can also imitate the multi-level brightness display similar to the analog oscilloscope, as well as the color temperature display effect and afterglow display effect unique to the digital oscilloscope.
⑦ After the oscilloscope has processed the data, it can save the current waveform image or data to the memory. It should be noted that the storage here is completely different from the high-speed cache with storage depth. Most oscilloscopes use external storage such as USB flash drives, SD cards, computers, etc. Now some modern oscilloscopes have built-in large storage that can be directly saved in the oscilloscope.
In this process, ②③④ are processed in parallel.
Due to the limitation of the processing speed of digital oscilloscope, it cannot guarantee that the waveform of the measured signal can be displayed on the screen continuously and in real time. There will be waveform data loss between the two displayed waveforms, which is also called dead time. This is also the biggest disadvantage of digital oscilloscope compared with analog oscilloscope. However, with the enhancement of oscilloscope computing power and the continuous increase of waveform capture rate, this disadvantage is also being gradually made up.
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