When selecting digital oscilloscope, we mainly consider whether it can truly display the measured signal, that is, the consistency of the display signal and the measured signal.
The performance of digital oscilloscope greatly affects its ability to achieve signal integrity. The following is a detailed analysis according to its main performance indicators.
The bandwidth of digital oscilloscope refers to the frequency value of constant amplitude sine wave signal of different frequency input, when the amplitude of output waveform decreases to 70.7% of the actual amplitude with the change of frequency. The bandwidth determines the basic measuring ability of digital oscilloscope.With the increase of signal frequency, the ability of digital oscilloscope to display the signal accurately decreases. In the actual test, we will find that when the frequency of the measured signal 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, and the display signal will appear distortion. For example, a signal with a frequency of 100 MHz and a voltage amplitude of 1 V is tested with a digital oscilloscope with a bandwidth of 100 MHz, and the displayed voltage is only about 0.7 V. The higher the bandwidth of digital oscilloscope, the steeper the rising edge of the signal, the more high-frequency components displayed, the more accurate the reproduced signal.Considering the price factor in practical application (the higher the bandwidth of digital oscilloscope, the more expensive the price is), through the accumulation of practical experience, we found that as long as the bandwidth of digital oscilloscope is 3-5 times of the highest frequency of measured signal, the accuracy of ±3% to ±2% can be obtained, meeting the general testing requirements.
Rise time is defined as the time when the pulse amplitude rises from 10% to 90%, which reflects the transient characteristics of the vertical system of digital oscilloscope.Digital oscilloscope must have a fast enough rise time to accurately capture the rapidly changing signal details.The faster the rise time of digital oscilloscope, the more accurate the capture of the fast transformation of the signal.
The rise time and bandwidth of general digital oscilloscope satisfy the following formula:
Where, tr is rise time, f-3db is frequency band width, k is a constant between 0.35-0.45 (different types of digital oscilloscopes have different values, you can refer to the corresponding manual).Through calculation and referring to the selection principle of bandwidth, it can be concluded that as long as the rise time of digital oscilloscope is less than one third to one fifth of the measured signal, it can meet the general test requirements.
The frequency response is the response performance when input sinusoidal signal of different frequency. It includes the amplitude response from the low frequency sinusoidal signal of a few Hertz in dc or ac to the frequency at which the amplitude cannot be displayed. Only considered in practical measurement bandwidth performance will not be enough to guarantee the accuracy of the digital oscilloscope can reconstruct the original signal in digital oscilloscope measurement work, we found some digital oscilloscope frequency response curve in low frequency band is not flat, there will be a big ups and downs, if the test corresponding frequency distortion will occur.At this point, even if the bandwidth of digital oscilloscope is much higher than the measured signal frequency, it cannot reproduce the signal.Therefore, in the selection of digital oscilloscope, the frequency response of different measured signals is also one of the key performance indicators.
The sampling rate of digital oscilloscope is expressed as sample points per second (S/ S).The faster the sampling rate, the better the reproduction of the original signal. According to Nyquist's theorem, the sampling rate should be more than 2 times the measured signal frequency to restore the original signal without distortion. However, the premise of this theorem is based on infinite time and continuous signals, which cannot be satisfied by digital oscilloscope technology in practical test.According to the accumulation of practical experience, in order to accurately reproduce the original signal, the sampling rate of digital oscilloscope is generally 2.5-10 times of the highest frequency of the original signal.