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.
The response characteristic of digital oscilloscope and its choose and buy skill
Another reason why high speed digital oscilloscope chooses brick wall reaction type is to avoid or minimize Aliasing phenomenon. When using digital oscilloscopes to measure high speed signals, graphics confusion can occur, mainly because some signals are mixed with unnecessary waveforms when reproducing the sampled high speed signals. These mixed signal frequency components can distort the original signal waveform and, in severe cases, cause measurement errors.
Most of the pattern confusion occurs in the analog to digital converter (adc) continuous signals, which contain components that exceed the Nyquist frequency, or half the sampling frequency.This component retraces in the Nyquist frequency domain and appears in the oscilloscope measurement bandwidth. It can be clearly seen from the frequency characteristic diagram that the figure confusion effect of the brick wall reactive oscilloscope is negligible.
Under the same conditions, it can be clearly seen that in the field beyond the Nyquist frequency of 2GHz, there is almost no signal, which can inhibit the occurrence of confusion.
In addition, if 20GHz, 10GHz and 5GHz sampling frequencies are used to measure waveforms with a period of 2.2ns and a rise time of about 90ps, different results will be obtained.The lower the sampling frequency is, the longer the actual measurement value of rising time is, and the waveform cannot be faithfully presented.
At present, the high-speed serial interface measurement USES the real-time sampling broadband digital oscilloscope, and the sampling frequency of the high performance machine equipped with the analog to digital converter is up to about 20GHz. Generally, in order to reduce the occurrence of graphics confusion, the sampling frequency of a gaussian reactive oscilloscope should be 4-6 times of the input signal, while that of a brick wall reactive oscilloscope only needs 2.5 times.
Generally, the frequency band is lower than 1GHz, so gaussian reaction system is mostly adopted, while the instruments higher than 1GHz are mostly brick wall reaction system.Table 2 shows the advantages and disadvantages of the two types of reactive oscilloscopes.
Choose oscilloscope according to performance requirement
So, how to choose the most suitable oscilloscope? There are four simple steps:
Calculate the highest frequency component fmax of the measured signal.The upper limit of the signal frequency component can be calculated by measuring the rising time of the signal.Assuming that the rise time is migrated from 20% to 80%, the approximate value can be estimated using the mathematical formula (0.4/ signal rise time) rather than directly from the data transmission rate.Take the popular third-generation bus PCIExpress, which in most cases has a rise time of about 100ps.
Select the response characteristics of the oscilloscope. That is to choose a suitable one between the gaussian reaction system and the brick wall reaction system.
The necessary input bandwidth must be secured. It is related to the measurement error of rise time. An instrument company has done simulation experiments: if the brick wall reactive system allows 3% error, the bandwidth can be calculated with (1.4 fmax); If the error is limited to 10%, use (1.2 fmax) to calculate. When the tolerance error is 20%, it is calculated by (1.0 fmax).
Estimate the lowest sampling frequency. This value takes advantage of the above bandwidth value, which is a minimum (2.5 bandwidth) for a brick wall reactive oscilloscope.
The above four points can be used to illustrate a case: the rise time of 100ps digital signal, its fmax is 4GHz, select the brick wall reaction oscilloscope, assuming that the error of the rise time is limited to 3%, then the bandwidth of the input signal is 5.6ghz, therefore, the minimum sampling frequency also needs 14GHz.
If the sampling frequency of 14GHz is applied to the gaussian reaction system, the input bandwidth becomes 3.5ghz, and the rising time of the measured signal is 220ps, which is half of the difference with the brick wall reaction system.Some wide-band real-time oscilloscopes rely on the active application of digital signal processing to realize the characteristics of brick wall reactive system.After all, circuit technology alone is unlikely to achieve desirable characteristics.
In a word, whether the bandwidth and sampling frequency are suitable or not is an important pointer when choosing expensive oscilloscope.In addition, understanding the characteristics of the test instrument is also the key to mastering the correct measurement.
Principle of handheld oscilloscope
A hand-held oscilloscope USES a narrow beam of high-speed electrons to produce tiny points of light on a screen coated with fluorescent material. Under the action of the measured signal, the electron beam ACTS like the tip of a pen and can draw the curve of the instantaneous value of the measured signal on the screen.The use of oscilloscope can observe various different signal amplitude changes with time waveform curve, but also can use it to test a variety of different electricity, such as voltage, current, frequency, phase difference, adjustment amplitude and so on.
How to choose handheld oscilloscope?
Select handheld oscilloscope mainly pay attention to several indicators, bandwidth, storage depth, sampling rate, waveform update rate, isolation channel, display, language, easy operation, and some other kinetic energy (waveform recording, 232/422/485/CAN/I2C/SPI bus decoding) bandwidth: bandwidth is 100M, 200M commonly now, since buy at least buy 100M, each manufacturer is about the same; Storage depth: this various manufacturers vary from 10K to dozens of M have, in general, a few hundred K is enough, too small waveform expansion can not see, square wave are changed into triangular wave. Sampling rate: as long as this index reaches more than 5 times of the bandwidth, all manufacturers have basically reached; Waveform update rate: this index is also a very important index, waveform update rate is too low will lead to some abnormal signals can not be seen; Isolation channel: in the handheld oscilloscope, this is very important, can reduce the risk of accidental short circuit, electric shock, etc., can also reduce the parasitic capacitance during testing, etc., but fortunately, most manufacturers are isolated.