Related knowledge and selection plan of oscilloscope 
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Since its introduction, the oscilloscope has been one of the most important and commonly used electronic test instruments. Due to the development of electronic technology, the capabilities of oscilloscopes are constantly improving, and their performance and prices are also varied, and the market is uneven. Oscilloscopes seem simple, but there are many problems in how to choose them. Based on many years of experience, combined with the selection guide of Beijing Ocean Industrial Technology Co., Ltd., this article tells you from several aspects what you should pay attention to when choosing an oscilloscope:
1. Understand the signal you need to test
Do you need to know what to observe with an oscilloscope? What is the typical performance of the signal you want to capture and observe? Does your signal have complex characteristics? Is your signal a repetitive signal or a single signal? What is the bandwidth, or rise time, of the signal transition you want to measure? What signal characteristics do you intend to use to trigger short pulses, pulse widths, narrow pulses, etc.? How many signals do you plan to display at the same time? What do you do with the test signal?
Second, choose the core technology difference of the oscilloscope: analog (DRT), digital (DSO), or digital and analog (DPO)
The traditional view is that analog oscilloscopes have a familiar control panel and are inexpensive, so they always feel that analog oscilloscopes are "easy to use." However, with the speed of A / D converters increasing year by year and the price continuously decreasing, as well as the increasing measurement capabilities and practically unlimited measurement functions of digital oscilloscopes, digital oscilloscopes have taken the lead. However, digital oscilloscopes display three-dimensional defects and slow processing of continuity data. They require oscilloscopes with both digital and analog technologies, such as DPO digital fluorescent oscilloscopes.
3. Determine the test signal bandwidth
The bandwidth is generally defined as the frequency at which the amplitude of the sine wave input signal attenuates to -3dB, which is 70.7% of the amplitude. The bandwidth determines the basic oscilloscope's ability to measure the signal. If there is not enough bandwidth, the oscilloscope will not be able to measure high-frequency signals, the amplitude will be distorted, the edges will disappear, and the detailed data will be lost; if there is not enough bandwidth, all the characteristics of the obtained signal, including ringing and ringing, etc. Meaningless.
An effective experience to determine the bandwidth of the oscilloscope you need-"5 times the experience criterion": multiply the highest frequency component of the signal you want to measure by 5 to make the measurement result more than 2% accurate.
In some applications, you do not know your signal bandwidth of interest, but you know its fastest rise time. At this time, the frequency response uses the following formula to calculate the associated bandwidth and the rise time of the instrument: Bw = 0.35 / signal The fastest rise time.
There are two types of digital oscilloscope bandwidth: repetitive (or equivalent time) bandwidth and real-time (or single) bandwidth. The repetitive bandwidth only applies to repetitive signals, showing samples from multiple signal acquisitions. The real-time bandwidth is the highest frequency that can be captured in a single sampling of the oscilloscope, and it is more important when the captured events are not frequent or transient signals. The real-time bandwidth is closely related to the sampling rate.
The higher the bandwidth, the better, but higher bandwidth often means higher prices, so you should choose the signal frequency component you want to observe according to the budget.
4. Sampling rate (or sampling speed) of A / D converter
The unit is the number of samples per second (S / s), which refers to the frequency at which the digital oscilloscope samples the signal. The faster the oscilloscope's sampling rate, the higher the resolution and clarity of the displayed waveform, and the lower the probability of losing important information and events.
If you need to observe slow-changing signals or low-frequency signals in a long time range, the minimum sampling rate plays a role. In order to maintain a fixed number of waveforms in the displayed waveform record, you need to adjust the horizontal control knob, and the displayed sampling rate is also It will change as the level adjustment knob changes.
How to calculate the sampling rate? The calculation method depends on the type of waveform being measured and the signal reconstruction method used by the oscilloscope, such as sinusoidal interpolation and vector interpolation. In order to accurately reproduce the signal and avoid confusion, Nyquist's theorem states that the sampling rate of the signal must not be less than twice its highest frequency component. However, the premise of this theorem is based on infinitely long and periodic continuous signals. Since an oscilloscope cannot provide a record length of unlimited time, and by definition, low-frequency interference is discontinuous and not periodic, so a sampling rate that is twice the highest frequency component is usually not sufficient.
In fact, the accurate reproduction of the signal depends on its sampling rate and the interpolation method adopted by the signal sampling point, that is, waveform reconstruction. Some oscilloscopes provide operators with the following options: sinusoidal interpolation for measuring sinusoidal signals, and linear interpolation for measuring rectangular waves, pulses, and other signal types.
There is a rule of thumb that is useful when comparing the sampling rate and signal bandwidth: If the oscilloscope you are looking at has interpolation (through filtering to regenerate between sampling points), then the ratio of (sampling rate / signal bandwidth) should be at least 4 : 1; when there is no sinusoidal interpolation, a ratio of 10: 1 should be adopted.
Fifth, the screen refresh rate is also called the waveform update speed
All oscilloscopes will blink. The oscilloscope captures the signal a specific number of times per second. No measurement will be taken between these measurement points. This is the waveform capture rate, also known as the screen refresh rate, expressed as the number of waveforms per second (wfms / s). The difference between the waveform capture rate and the A / D sampling rate must be distinguished. Sampling rate means the frequency that the oscilloscope A / D samples the input signal in a waveform or period; waveform capture rate refers to the speed at which the oscilloscope acquires the waveform. The waveform capture rate depends on the type and performance level of the oscilloscope and has a wide range of changes. Oscilloscopes with a high waveform capture rate will provide more important signal characteristics and greatly increase the probability that the oscilloscope will quickly capture transient anomalies such as jitter, runt pulses, low frequency interference, and instantaneous errors.
Generally speaking, analog oscilloscopes have a relatively high screen refresh rate due to their simple circuits, and digital storage oscilloscopes (DSOs) use a serial processing structure to capture 10 to 5000 waveforms per second. In order to change the problem of low screen refresh rate of digital oscilloscopes, digital fluorescent oscilloscopes use a parallel processing structure, which can provide a higher waveform capture rate, some up to millions of waveforms per second, greatly increasing the possibility of capturing intermittent and difficult events Sex, and allows you to discover problems with signals faster.
Six, choose the appropriate storage depth, also known as the record length
Storage depth is a measure of how many sampling points the oscilloscope can store. If you need to continuously capture a burst, you need the oscilloscope to have enough memory to capture the entire event. Divide the length of time to be captured by the sampling rate required to accurately reproduce the signal to calculate the required memory depth.
The storage depth is closely related to the sampling rate. The storage depth you need depends on the total time span to be measured and the required time resolution.
Modern oscilloscopes allow users to select the record length in order to optimize details in some operations. Analyzing a very stable sinusoidal signal only requires a record length of 500 points; but if you want to parse a complex digital data stream, you need a record length of one million points or more.
Capturing the effective trigger of the signal at the correct location can usually reduce the amount of storage that the oscilloscope actually needs.
7. Choose different trigger functions according to your needs
The trigger of the oscilloscope can make the signal scan horizontally and synchronously at the correct position, so that the signal characteristics are clear. Trigger control buttons can stabilize repeated waveforms and capture single waveforms.
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