Basic principle and measuring method of oscilloscope
Oscilloscope is one kind of electrical measurement equipment with wide application. It could transfer invisible electrical signal to visible image to facilitate people research change processes of all kinds of electricity phenomenon. Oscilloscope use narrow electron beam which consists high-speed electronics and project onto screen coated with fluorescent substance to produce small spot (this is original working principle of analog oscilloscopes. Under the function of measured signal, electronics like the point of a pen which could on screen describe change curve of measured signal during instantaneous value. An oscilloscope can be used to observe the waveform curves of various signal amplitudes varying with time. It could also be used to measure various electricity, such as voltage, current, frequency, phase, amplitude modulation and etc.
(1) pre-adjusting: counterclockwise rotate the brightness button to the bottom, shift vertical and horizontal positions to the middle, set the attenuation gear at the highest level, place scan “outside X gear”
(2) turn on the power, do following steps waiting for one or two minutes after the indicator light is on
(3) adjust luminance first, then adjust the focus, adjust vertical and horizontal displacement to make the light point in center of suitable area.
(4) adjust scan, scan fine tuning and X gain, observe scan.
(5) adjust outside X gear to suitable scan range, observe the voltage waveform according to sine and cosines law on vertical direction inside the device.
(6) Connect the applied voltage to be studied to oscilloscope through Y input and earth. Adjust all gears to suitable position and observe waveform (the picture changes with time) (the synchronous polarity switch can make the starting point of the image start from positive half cycle or negative half cycle.)
(7) If you want to observe the vertical migration of light spot (with one added DC voltage), could adjust scan to “outside X gear”.
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.
The development of oscilloscope probe
In the past 50 years, the interface design of various oscilloscope probes has been evolving continuously to meet the requirements of increased instrument bandwidth speed and measurement performance. In the earliest days, banana plugs and UHF connectors were commonly used. In the 1960s, the common BNC connector became the common probe interface type because BNC was smaller and more frequent. Currently, the BNC probe interface is still used for test and measurement instrument design, and the current higher quality BNC connector provides a maximum available bandwidth function of nearly 4GHz.
Later, some manufacturers put forward the common workarounds, BNC type probe interface design in the use of the BNC connector at the same time, additional provides a simulation code detection scale coefficients of stitching, as part of the mechanical and electrical interface design, which makes the oscilloscope is compatible with automatic detection and change the oscilloscope display of vertical attenuation range.
Requirements for oscilloscope current probe
Wide frequency range: from dc to tens or even hundreds of megabytes.
The range is large: from milliampere to kiloampere.
Small size: with the improvement of integration and the increase of signal frequency, the external size of components is getting smaller and shorter and pins are getting shorter and shorter.
Easy operation and high accuracy.