Basic principle and measuring method of analytical oscilloscope
Oscilloscope is a widely used electronic measuring instrument. It can transform the invisible electric signal to the naked eye into the visible image, which is convenient for people to study the change process of various electric phenomena. A oscilloscope USES a narrow beam of high-speed electrons, struck on a surface coated with fluorescent material, to produce tiny points of light (this is how a traditional analog oscilloscope works). Under the action of the measured signal, the electron beam ACTS like the tip of a pen and can plot the instantaneous value of the measured signal on the screen. Use oscilloscope to be able to observe all sorts of different signal amplitude the waveform curve that changes with time, still can use it to test all sorts of different electric quantity, be like voltage, current, frequency, phase difference, amplitude modulation.
(1) Presetting: rotate the brightness knob counterclockwise to the bottom, move the vertical and horizontal position to the middle, and the attenuation is placed in the highest level, and the scanning is placed in the "outer X gear";
(2) Turn on the power again, and wait for one or two minutes after the light is on for preheating before carrying out relevant operations;
(3) First adjust the grayscale, then focus, and then adjust the horizontal and vertical displacement to make the highlights in the center of the appropriate area;
(4) Adjust scanning, scanning fine tuning and X gain, observe scanning;
(5) Unplug the outer X gear to the appropriate position in the scanning range file, and observe the voltage waveform in the vertical direction changing according to the law of sines and cosines provided by the machine;
(6) To study the external voltage from Y input and indirectly into the oscilloscope, adjust each gear to the appropriate position, you can observe the voltage waveform (and time changes of the image)(synchronous polarity switch can make the starting point of the image from the positive half cycle or negative half cycle start;
(7) If you want to observe the vertical offset of bright spot (such as when applied with a constant current voltage), you can adjust the scan to "outer X gear".
(different oscilloscopes may operate in different ways), etc.
(1) Insert the oscilloscope probe into the socket of channel 1, and put the attenuation on the probe into "1" gear;
(2) Put channel selection in CH1 and coupling mode in DC file;
(3) Insert the probe into the pinhole of the calibration signal source, and the light trace appears on the oscilloscope screen;
(4) Adjust the vertical knob and horizontal knob to stabilize the waveform displayed on the screen, and put the vertical fine-tuning and horizontal fine-tuning in the calibration position;
(5) The number of cells occupied by the waveform in the vertical direction is multiplied by the indicating value of the vertical attenuation knob to obtain the amplitude of the calibration signal.
(6) The number of bars occupied by each period of the waveform in the horizontal direction is multiplied by the indicating value of the horizontal scanning knob to obtain the period of the calibration signal (the inverse of the period is the frequency).
(7) In general, the frequency of the calibration signal is 1kHz and the amplitude is 0.5v, which is used to calibrate the internal scanning oscillator frequency of the oscilloscope. If it is abnormal, the corresponding potentiometer of the oscilloscope (internal) should be adjusted until it is consistent.
How to choose oscilloscope?
You should know what you plan to use the oscilloscope to observe. What is the special feature of the signal you want to capture? Does your signal have complex characteristics? Is your signal repeat signal or 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 are you going to display at the same time? Analog or digital?
In all, the traditional view thinks that the analog oscilloscope has the familiar panel control, the price is low, therefore always thinks that the analog oscilloscope "USES conveniently". However, with the increasing speed and price of A/D converter year by year, as well as the increasing measuring capacity and virtually unlimited functions of digital oscilloscope, digital oscilloscope has become the leader.
How is the bandwidth?
Bandwidth is generally defined as the frequency when the amplitude of sinusoidal input signal attenuates to -3db, that is, 70.7%. Bandwidth determines the basic measuring ability of oscilloscope to signal. With the increase of the signal frequency, the oscilloscope's ability to display the signal accurately will decline, if there is not enough bandwidth, the oscilloscope will not be able to distinguish the high-frequency change. The amplitude will be distorted, the edges will disappear, and the details will be lost. Without sufficient bandwidth, all the properties of the signal obtained, such as ringing and singing, are meaningless.
A rule of thumb for determining the bandwidth effectiveness of the oscilloscope you need is the "5x rule"; Multiply the highest frequency component of the signal you want to measure by 5. This will give you an accuracy of more than 2% in your measurements
In some applications, you do not know the bandwidth of the signal you are interested in, but you know the fastest rise time, most digital oscilloscopes use the following formula to calculate the associated bandwidth and the rise time of the instrument: bandwidth = 0.35 ÷ the fastest rise time of the signal.
There are too kinds of bandwidths: repeat (or equivalent time) bandwidth and real time (or single time ) bandwidth. Repeat bandwidth is only applicable for repeat signals to display comes from sampling during multiple signal acquisition. Real-time bandwidth is the highest frequency that can be captured in a single sample of oscilloscope, and the requirements are quite demanding when the captured events do not often occur. Real-time bandwidth is associated with sampling rates.
Since wider bandwidth tends to mean higher prices, evaluate the frequency components of the signals you normally look at against your budget.
How is the sampling rate?
Defined as sampling times per second (Sa/s), the frequency of signal sampling by exponential oscilloscope. The higher the sampling rate of the oscilloscope, the higher the resolution and clarity of the waveform displayed, and the lower the probability of important information and events being lost.
The minimum sampling rate becomes important if the slow signal over a long time range is observed. In order to maintain a fixed waveform number in the displayed waveform record, the horizontal control button needs to be adjusted, and the displayed sampling rate will also change with the adjustment of the horizontal control button.
How to calculate sampling rate? Measuring method depends on the waveform measured and the signal reconstruction method adopted by oscilloscope.
To reproduce the signal accurately and avoid confusion, the Nyquist theorem states that the sampling rate of the signal must be no less than twice its highest frequency component. However, the premise of this theorem is based on infinitely long and continuous signals. Since no oscilloscope can provide a record length of infinite time and, by definition, low-frequency interference is discontinuous, sampling rates of twice the highest frequency components are usually insufficient.
In fact, the accurate reproduction of signal depends on its sampling rate and the interpolation method used in the gap of signal sampling points. Some oscilloscope will provide following choice to operator: sinusoidal interpolation method for measuring sinusoidal signal, and linear interpolation method for measuring rectangular wave, pulse and other signal types.
There is a useful rule of thumb for comparing sample rates and signal bandwidths: if the oscilloscope you are looking at has interpolation (filtered to regenerate between sampling points), the ratio (sampling rate/signal bandwidth) should be at least 4∶1. Without sine interpolation, the ratio of 10∶1 should be adopted.
How fast does the screen refresh?
All oscilloscopes flash. That is to say, the oscilloscope captures the signal at a certain number of times per second, and the measurement between these measuring points is no longer carried out. This is the waveform capture rate, also known as screen refresh rate, expressed as the number of waves per second (WFMS /s). Sampling rate represents the frequency of input signal sampled by oscilloscope in a waveform or period.Waveform acquisition rate refers to the oscilloscope acquisition waveform speed. Waveform acquisition rate depends on the type and performance level of oscilloscope, and has a large range of changes. The oscilloscope with high wave acquisition rate will provide more important signal characteristics and greatly increase the probability of the oscilloscope to quickly capture instantaneous abnormal conditions, such as jitter, short pulse, low frequency interference and instantaneous error.
Digital storage oscilloscope (DSO) can capture 10 to 5,000 waveforms per second using a serial processing structure. DPO digital fluorescence oscilloscope USES parallel processing structure, can provide higher waveform capture rate, some up to millions of waves per second, greatly improve the possibility of intermittent capture and difficult to capture events, and allow you to find the signal problems faster.
What is the storage depth?
Storage depth is a measure of how many sampling points an oscilloscope can store. If you need to capture a pulse string continuously, you need the oscilloscope to have enough memory to capture the entire event. The required storage depth, also known as record length, can be calculated by dividing the length of time to be captured by the sampling speed required to accurately reproduce the signal.
Capturing the effective trigger of the signal in the right position can usually reduce the storage capacity of the oscilloscope.
Storage depth is closely related to sampling speed. The depth of storage you need depends on the total time span to be measured and the required time resolution.
Modern oscilloscopes allow users to select record lengths to optimize the details of some operations.Analysis of a very stable sinusoidal signal requires only 500 points of record length;But to parse a complex stream of digital data, you need a million points or more of record length.
What kind of trigger do you need?
The trigger of the oscilloscope can synchronize the horizontal scanning of the signal at the right position, which determines whether the signal characteristic is clear or not. Trigger control buttons stabilize repeated waveforms and capture single waveforms.
Most users of universal oscilloscopes only use edge triggering, and you may find it useful to have other triggering capabilities in some applications. Especially for the fault search of new design products.Advanced triggering allows the event of interest to be isolated, making the most efficient use of sampling speed and storage depth.
Today there are many oscilloscopes with advanced triggering capabilities: you can trigger based on pulses defined by amplitude (such as short pulses), time-limited pulses (pulse width, narrow pulse, conversion rate, build/hold time), and pulses (logical trigger) described by logical state or graph.The combination of extended and regular triggering functions also helps to display video and other hard-to-catch signals, so advanced triggering capabilities provide a great deal of flexibility in setting up the test process and greatly simplify the job.
How many channels do you need?
The number of channels you need depends on your application. For the general economic fault - finding applications, the need is a dual - channel oscilloscope. However, if you want to observe the interrelationship of several analog signals, you will need a 4-channel oscilloscope. Many engineers working in analog and digital systems are also considering 4-channel oscilloscopes.A newer option, called a mixed-signal oscilloscope, combines the logic analyzer's channel counting and triggering capabilities with the oscilloscope's higher resolution into a single instrument with a time-dependent display.
What does an oscilloscope do?
Oscilloscope is a kind of instrument which is widely used to display the trace change of instantaneous value measured.It can transform the electric signal invisible to the naked eye into visible image, which is convenient for people to study the change process of various electric phenomena. An oscilloscope USES a narrow beam of high-speed electrons to produce tiny points of light by hitting a surface coated with fluorescent material. Under the action of the measured signal, the electron beam ACTS like the tip of a pen and can plot the instantaneous value of the measured signal on the screen.Use oscilloscope to be able to observe all sorts of different signal amplitude the waveform curve that changes with time, still can use it to test all sorts of different electric quantity, if voltage, current, frequency, phase difference, adjust amplitude to wait a moment.