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How to Choose an Appropriate Digital Oscilloscope
When choosing a digital oscilloscope, we mainly consider whether it can truly output the measured signal source, that is, the consistency between the output signal source and the measured signal source. The performance of digital oscilloscope greatly affects its ability to achieve signal source integrity. The following is a detailed analysis based on its main performance indicators.
1 bandwidth
The bandwidth of digital oscilloscope refers to the frequency value (f-3dB) when the amplitude of output waveform decreases to 70.7% of the actual amplitude with the change of frequency. Bandwidth determines the basic measurement ability of digital oscilloscope to signal source. With the increase of the frequency of the signal source, the precise output ability of the digital oscilloscope to the signal source decreases. In the actual test, we will find that when the frequency of the measured signal source is close to the bandwidth of the digital oscilloscope, the digital oscilloscope will not be able to distinguish the high-frequency change of the signal source, and the output signal source will be distorted. For example, a digital oscilloscope with a bandwidth of 100 MHz is used to test a signal source with a frequency of 100 MHz and a voltage amplitude of 1 V. The output voltage of the oscilloscope is only about 0.7 V. Figure 2 shows the measurement results of a digital oscilloscope with bandwidth of 4 GHz, 1.5 GHz and 300 MHz for the same step signal source. As can be seen from the figure, the higher the bandwidth of the digital oscilloscope, the steeper the rising edge of the signal source, the more high-frequency components of the output, and the more accurate the reproduced signal source. Considering the price factor in practical application (the higher the bandwidth of digital oscilloscope is, the more expensive the price is), through the accumulation of practical experience, we find that as long as the bandwidth of digital oscilloscope is 3-5 times of the maximum frequency of the signal source under test, the accuracy of (+3%) to (+2%) can be obtained, which meets the general test requirements.
2 rise time.
The rise time is defined as the period during which the pulse amplitude rises from 10% to 90% (as shown in Figure 3), which reflects the transient characteristics of the vertical system of the digital oscilloscope. The digital oscilloscope must have a fast enough rise time to accurately capture the details of the fast-changing signal source. The faster the rising time of digital oscilloscope is, the more accurate the acquisition of fast transformation of signal source is.
3 frequency response.
Frequency response is the response performance of a sinusoidal wave source with equal amplitude when different frequencies are input. It includes the amplitude response in all frequencies ranging from a sinusoidal signal source with DC or AC low frequencies of several hertz to a frequency with no output amplitude. In actual measurement, only considering the bandwidth performance is not enough to ensure that the digital oscilloscope can accurately reproduce the original signal source. In the measurement of digital oscilloscope, we find that the frequency response curve of some digital oscilloscopes is not flat in the low frequency band, and there will be a large fluctuation. If the signal source of the corresponding frequency is tested, it will appear. Distortion. At this time, even if the bandwidth of the digital oscilloscope is much higher than the frequency of the signal source under test, the signal source can not be reproduced. Therefore, when choosing digital oscilloscope, the frequency response of different signal sources is also one of the key performance indicators.
4 sampling rate.
The sampling rate of digital oscilloscope is expressed as sample points per second (S/s). The faster the sampling rate is, the more real the original signal source can be reproduced. According to Nyquist's theorem, only when the sampling rate is greater than or equal to twice the frequency of the measured signal source, can the original signal source be restored without distortion. But the premise of this theorem is based on infinite time and continuous signal source. In practical test, the technology of digital oscilloscope can not meet this condition. According to the accumulation of practical experience, in order to accurately reproduce the original signal source, the sampling rate of digital oscilloscope is generally 2.5-10 times of the maximum frequency of the original signal source.