Introduction to oscilloscope

One 、 The basic principle
The earliest oscilloscopes were analog oscilloscopes , But now the market is mostly digital oscilloscope , Analog oscilloscopes have almost disappeared . For all that , It is still necessary to understand the structure of analog oscilloscope , In order to be impressed by the shortcomings of the popular digital oscilloscope .
1.1 Analog oscilloscope

chart 1 It is the architecture block diagram of analog oscilloscope . In essence , The working mode of analog oscilloscope is to directly measure the signal voltage , And draw the voltage in the vertical direction through the electron beam passing through the oscilloscope screen from left to right . The electron beam is subjected to the combined action of horizontal time base and vertical deflection , The horizontal time base causes the electron beam to move from left to right at a constant rate , Vertical deflection controls the position of the electron beam in the vertical direction . The voltage signal introduced through the probe acts on the vertical deflection plate after attenuation or amplification , Cause the electron beam to move up and down . As the electron beam moves up and down, it also moves from left to right , such , One voltage - The time chart is drawn .
The display screen is a cathode ray tube （CRT）, The electron beam is projected somewhere on the screen , There will be bright substances behind the screen . The higher the frequency of electron beam projection at the same position , The brighter the display . therefore , The signal measured by the analog oscilloscope is not a two-dimensional plan , But there is a third dimension —— Brightness level .
Two 、 Digital oscilloscope

chart 2 It is the structure block diagram of digital oscilloscope . Digital oscilloscope through analog-to-digital converter （ADC） Convert the measured voltage into digital information . It captures a series of sample values of the waveform , The discrete points are connected by interpolation algorithm and displayed . These discrete points can be stored , It can also be displayed after the waveform disappears . You can also do operations , Add, subtract, multiply and divide FFT wait .
3、 ... and 、 The difference
1） Analog oscilloscope collects and displays continuous signals , Digital oscilloscope collects discrete points , The curve between discrete points is supplemented by interpolation algorithm
2） The waveform captured by the digital oscilloscope can be stored , operation （ Add , reduce ,FFT etc. ）, The analog oscilloscope can only display the current signal , Unable to record past signals .
3） frequency range .CRT It limits the frequency range displayed by the analog oscilloscope . At very low frequencies , The signal shows bright and slowly moving dots , And it is difficult to distinguish the waveform . At high frequencies , The limiting factor is CRT Write speed . When the signal frequency exceeds CRT Write speed of , The display is too dim , Difficult to observe . The limit frequency of analog oscilloscope is about 1GHz. The sampling rate of digital oscilloscope depends on ADC, Its sampling rate can be as high as 100G.
Four 、 Introduction to trigger system
For periodic signals , Need to display stably , Can't shake left and right . Imagine playing a movie , Play scene by scene . The same is true of oscilloscope display , All points sampled once form a scene of signals displayed on the oscilloscope screen , Then sample another scene of signal display . Playing a movie requires continuous action in each scene , The oscilloscope displays periodic signals, and it hopes that the signals of each scene coincide . Suppose you test a sinusoidal signal , The point displayed at the beginning of the first act is the peak position , The point displayed at the beginning of the second act is the peak valley position , That is, the difference between the sine waves displayed twice 180 Degree phase . The waveform of the oscilloscope is updated quickly , Because of the visual persistence effect , When the second act signal is displayed , You can also see the signal of the first act . If you observe the two signals together, it will be chaotic , It looks like it's moving left and right . Pictured 3 Shown

The sampling of oscilloscope is not always carried out , Instead, after sampling a scene of signals, rest for a period of time before starting the next sampling . As long as the position of the first point of each sampling in the vertical direction is the same , The waveforms naturally coincide . Trigger is when the input voltage value is fixed XX When sampling starts .
The above is a periodic signal , That's not the time for periodic signals ？ In fact, most signals are not periodic . Imagine self driving ,boy driving ,girl Take scenery . If boy All the way without a stop ,girl Can you take a good view ？ It's best to have a plan in advance , Stop at a scenic place to take photos . The trigger of oscilloscope is similar , For example, the normal rise time of a signal is 4n~5ns, But due to some interference , Its rise time occasionally becomes 2ns. here , The trigger mode is set to rise time less than 3ns Trigger when , The oscilloscope will display the signal that meets the conditions , Other signals will not show . therefore , The function of trigger is to find out the signal of concern , Filter out signals you don't care .