bandwidth
Bandwidth is the main parameter index of oscilloscope. When the bandwidth is insufficient, the waveform will be seriously distorted, and the square wave may even become a sine wave. We use an oscilloscope with a bandwidth of 20M and a bandwidth of 100M to observe the 20M square wave signal, and the results are shown in the following figure:
As can be seen from this figure, the square wave shape can hardly be observed by 20M oscilloscope, and the observation effect of 100M oscilloscope is better than that of 60M oscilloscope. What caused this? The figure below shows the spectrum of a 20M square wave after FFT. We can see that the square wave is composed of fundamental wave and 3rd, 5th, 7th, 9th ... harmonics. So the square wave of 20M contains the fundamental wave of 20M, the third harmonic of 60M, the fifth harmonic of 100M and the seventh harmonic of 140M. ...
If the waveform is to be measured accurately, the bandwidth of the oscilloscope should be greater than the main harmonic components of the waveform. Therefore, for sine wave, the bandwidth of oscilloscope can be required to be greater than the frequency of waveform, while for non-sine wave, the bandwidth of oscilloscope can be required to be greater than the maximum main harmonic frequency of waveform.
In the choice of bandwidth, generally 5 times of the maximum frequency of the measured signal is the most suitable bandwidth. For a basic oscilloscope, an oscilloscope with a bandwidth of 100MHz can be used in many measurement scenarios.
Sampling rate and storage depth
Sampling rate and storage depth are two other important indicators of oscilloscope. Why talk about them together? Because they are closely related to waveform recording time: sampling rate = storage depth ÷ waveform recording time.
The waveform displayed by a digital oscilloscope is actually a sampling point, just like a photo. The more sampling points, the closer the signal is to reality. The sampling rate indicates the ability of the oscilloscope to capture these sampling points per second, such as the sampling rate of 1GSa/s, indicating that the oscilloscope can collect 1 100 million sampling points per second. The storage depth indicates how many sampling points an oscilloscope screen can contain at most, for example, the storage depth of 28Mpts indicates that an oscilloscope screen can have at most 28 million sampling points. We should ensure that the sampling rate is at least twice the bandwidth of the measured signal, in fact, we suggest that it is more than 3-5 times, so that it is easier to capture the abnormal information of the waveform.
It should be noted that the sampling rate is also divided into equivalent sampling and real-time sampling. At present, oscilloscopes are basically marked with real-time sampling rate, because equivalent sampling is only applicable to periodically changing signals. If you find that the sampling rate is different from the advertised price when buying an oscilloscope, be sure to ask the seller whether it is real-time sampling or equivalent sampling.
port number
Oscilloscopes usually have two channels and four channels. Considering that several channels of the oscilloscope are suitable for you, mainly consider whether it is necessary to measure 3-4 signals at the same time. If not, you can choose a two-channel oscilloscope.
Waveform capture rate
Digital oscilloscope can't collect, store, process and display signals at the same time, so it will miss some information when processing and displaying, which is also called oscilloscope dead time. The waveform capture rate indicates how many times the oscilloscope can collect waveforms per second. The greater this ability, the smaller the dead time, and the easier it is to capture abnormal waveforms.
It should be noted here that the waveform capture rate is not fixed, it will change with the change of various settings of the oscilloscope, and the waveform capture rate marked by general manufacturers is the maximum that can be achieved.
Trigger mode
If the oscilloscope captures a waveform and is taking a picture, the trigger will determine when the oscilloscope presses the shutter. The trigger determines how the sampling points of the waveform we want to capture are displayed on the screen.
The common trigger modes of oscilloscopes are edge trigger and pulse width trigger. Some oscilloscopes will support more trigger modes besides this, so you can know how to choose according to your own needs.
Portability and operability
If you need to take an oscilloscope out, then the portability of the oscilloscope is also a factor worth considering, and whether the oscilloscope supports battery power supply, because you may have the need to work outdoors. Some modern oscilloscopes also support touch screen function, which will be faster and easier to operate.
Screen size and display mode
The display screen size of the oscilloscope and whether it supports color are also factors to be considered. Some oscilloscopes support fluorescence display and color temperature display, which is equivalent to adding a Z axis. For some complex signals, such as video signal, modulation signal and jitter signal, an oscilloscope with fluorescence display and color temperature display can help reveal the details of the signal more quickly.
accurate
The vertical resolution of ADC analog-to-digital converter of oscilloscope is the vertical resolution of digital oscilloscope, and its digits represent the accuracy of converting input voltage into digital value by oscilloscope.
It should be noted here that the resolution of the oscilloscope display screen in the vertical direction is limited. In addition, when measuring high-frequency signals, the amplitude itself is inaccurate, and there is even a 30% error at the upper limit frequency. Moreover, too high vertical resolution will increase the analog-to-digital conversion time, affect the sampling rate, and then affect the bandwidth, which is not worth the loss. The vertical resolution of general oscilloscopes is 8 bits, and that of high-resolution oscilloscopes is 12 bits. However, if the precision of the oscilloscope analog circuit is not improved, it is meaningless to simply pursue the resolution of ADC. If you want the accuracy of voltage, you should use a multimeter. The main function of oscilloscope is to observe the shape of waveform, and the measurement accuracy is generally within 2-3%, which is completely comfortable for most applications.
Measurement and analysis
Oscilloscopes generally support the display of common measurement items, such as signal frequency, period and amplitude. Some oscilloscopes will also support additional mathematical operations, FFT and advanced mathematical functions. If there is any calculation demand in this area, it should also be considered.
Automatic function
The automatic function of oscilloscope can replace multimeter to measure voltage. It can measure the DC component, AC component, frequency, peak value, effective value and other information of the signal at one time, even the waveform, so that you can see the signal at a glance, which is much stronger than measuring only an average voltage with a multimeter, reducing misjudgment and improving efficiency. However, the speed of the car is very important. The automatic shift speed of McCormick oscilloscope is 1 sec, which is faster than that of general multimeter.
Serial bus analysis
Serial bus is widely used in today's data communication design, so what kind of serial bus analysis the oscilloscope supports is also a consideration. We need to know clearly whether the decoding mode of oscilloscope is hardware decoding or software decoding, and whether it supports decoding trigger and decoding text display.
Interoperability and waveform data
Some oscilloscopes can be controlled remotely by mobile phone or computer, and the waveform data can be saved in the computer and analyzed by computer software. I once met a friend who needed to measure for a long time in a room with a high temperature of 36 degrees. The remote function control of the newly bought oscilloscope phone helped him a lot. In the past, the oscilloscope he used had to be operated in a room with an oscilloscope. This time, he can operate directly outdoors.
Selection of probe
The probe will introduce resistance load, capacitance load and inductance load, which will affect the accuracy of measurement data. In order to minimize this influence, it is best to use the probe with the oscilloscope of the same brand. When purchasing a probe, make sure that the bandwidth of the probe is greater than or equal to that of the oscilloscope. It is also necessary to know whether to measure voltage or current, how big the signal amplitude is, and whether differential measurement signals are needed, which will affect the choice of probe.
Price–performance ratio
In addition to the probe, the oscilloscope is a consumable, and the host itself is very stable. It is entirely possible that an oscilloscope can be used for 10 years. The elimination of oscilloscopes is often caused by iterative updating of technology, so it is very valuable to buy an easy-to-use, convenient and excellent oscilloscope. At present, domestic oscilloscopes in the low-end field are no worse than those imported, and some are even better.