Method for determining bandwidth of analog and digital oscilloscopes

 　　As long as you are involved in electronic circuit design, you will definitely come into contact with an oscilloscope when debugging the circuit. An oscilloscope is a measuring instrument. It can convert electrical signals that are imperceptible to the naked eye into recognizable images. The most important parameter of an oscilloscope is the bandwidth, which is related to the frequency range that the oscilloscope can measure. So how to determine the measurable bandwidth of an oscilloscope? This article will introduce it to you.

　　
　　The most important parameter of an oscilloscope, bandwidth, determines the measurement frequency range of the oscilloscope. When you get a digital oscilloscope, how do you measure whether the bandwidth meets the required nominal indicators? Let's take the SDS1072CML oscilloscope as an example to test its bandwidth. According to the data, its nominal bandwidth is 70MHz.
　　
　　Bandwidth of oscilloscope
　　
　　In this article, the bandwidth of digital simulation is used, which is often called analog bandwidth. Analog bandwidth is the bandwidth of the input amplifier at the front end of the oscilloscope, which is equivalent to a low-pass filter. As shown in Figure 1, in the amplitude-frequency characteristic curve, as the frequency of the sine wave increases, the amplitude of the signal drops to 3dB (70.7%). The frequency point at this time is called the bandwidth of the oscilloscope.
　　

　　Bandwidth detection
　　
　　The detection is mainly achieved through a linear slow scanning signal, and the slow scanning signal is adjusted to a range that exceeds the bandwidth of the oscilloscope. In order to ensure the accuracy of bandwidth detection, attention should be paid to impedance matching during detection. The signal generator selects a 50ohm output impedance, a 50ohm BNC coaxial cable is selected, and a 50ohm impedance matcher is added to the input end of the oscilloscope. 　　As shown in Figure 2, the frequency band range is 1MHz-240MHz, the scanning time is 80s, and the frequency difference of each grid time base is 15MHz, and the center frequency of the screen is 105MHz. The oscilloscope time base gear is adjusted to 5s/div (at this time, it enters the scanning state, there is no dead time, so all data points are displayed on the screen). When the amplitude of the signal drops to 3dB (70.7%), the frequency is about 117MHz, that is, BW≈117MHz, which is much higher than the nominal bandwidth of 70MHz of the oscilloscope. 　　By observing the linear sweep signal, the bandwidth of the oscilloscope is quantitatively analyzed. Next, another parameter of the oscilloscope (rise time) is used to analyze the oscilloscope bandwidth. Use a signal generator to input a fast rising edge signal to the oscilloscope, measure its rise time with the oscilloscope, and finally calculate the bandwidth. 　　Calculation process 　　First, give formula (1):
　　



　　

　　

　　

　　
　　In order to facilitate the subsequent calculation and input, the formula (1) is simplified to obtain:
　　
　　Formula (2): 　　The actual rise time of the signal is: 　　In order to make the measurement results more accurate, the oscilloscope should be adjusted to the equivalent sampling mode, and the sampling rate can reach 50 GSa/s. As shown in Figure 4, the parameters are obtained: 　　From formula (2), it can be obtained: 　　From formula (1), it can be obtained: 　　From the calculation results, the bandwidth of this oscilloscope should be between 110 MHz and 120 MHz, and the correctness can be further verified through actual operation. This article provides a method for determining the bandwidth of an oscilloscope to help people confirm the measurable range of the oscilloscope. Avoid the situation where the actual measured value is greater than the measurable range, and ensure the accuracy of the oscilloscope measurement.