Problems that must be paid attention to when using

Attention must be paid to the use of digital oscillograph

digital oscillograph is increasingly popular because of its unique advantages such as waveform triggering, storage, display, measurement, waveform data analysis and processing. Due to the large performance difference between digital oscilloscope and analog oscilloscope, if it is used improperly, it will produce large measurement error, which will affect the test task

distinguish between analog bandwidth and digital real-time bandwidth

bandwidth is one of the most important indicators of oscilloscope, and share with the industry the latest developments, development trends and utilization in this market field. The bandwidth of analog oscillograph is a fixed value, while the bandwidth of digital oscillograph has a certain deflection tensile coefficient D analog bandwidth and digital real-time bandwidth. The highest bandwidth that can be achieved by the digital oscilloscope using sequential sampling or random sampling technology for repeated signals is the digital real-time bandwidth of the oscilloscope. The digital real-time bandwidth is related to the highest digital frequency and waveform reconstruction technology factor K (digital real-time bandwidth = maximum digital rate/k), which is generally not given directly as an index. From the definition of the two bandwidths, it can be seen that the analog bandwidth is only suitable for the measurement of repetitive signals, while the digital real-time bandwidth is suitable for the measurement of repetitive signals and single signals at the same time. The manufacturer claims that the bandwidth of the oscilloscope can reach megabytes, which actually refers to the analog bandwidth, and the digital real-time bandwidth is lower than this value. For example, Tek's tes520b has a bandwidth of 500MHz, which actually means that its analog bandwidth is 500MHz, while the highest digital real-time bandwidth can only reach 400MHz, which is far lower than the analog bandwidth. Therefore, when measuring a single signal, we must refer to the digital real-time bandwidth of the digital oscilloscope, otherwise it will bring unexpected errors to the measurement

relevant sampling rate

sampling rate, also known as digital rate, refers to the sampling times of analog input signals in unit time, which is often expressed in ms/s. Sampling rate is an important index of digital oscilloscope

1. If the sampling rate is not enough, it is easy to appear aliasing phenomenon

if the input signal of the oscilloscope is a 100kHz sinusoidal signal, and the signal frequency displayed by the oscilloscope is 50KHz, what is going on? This is because the sampling rate of the oscilloscope is too slow, resulting in aliasing. Aliasing means that the waveform frequency displayed on the screen is lower than the actual frequency of the signal, or even if the trigger indicator on the oscilloscope is on, the displayed waveform is still unstable. The generation of aliasing is shown in Figure 1. Then, for a waveform with unknown frequency, how to judge whether the displayed waveform has been aliased? You can slowly change the sweep speed t/div to a faster time base to see whether the frequency parameters of the waveform change sharply. If so, it indicates that waveform aliasing has occurred; Or the shaking waveform stabilizes at a faster time base, which also indicates that waveform aliasing has occurred. According to Nyquist's theorem, aliasing will not occur until the sampling rate is at least 2 times higher than the high-frequency component of the signal. For example, a 500MHz signal requires a sampling rate of at least 1gs/s. There are several methods to simply prevent aliasing:

adjust the scanning speed

adopt AUTOSET

try to switch the collection mode to the envelope mode or peak detection mode, because the envelope mode is to find the extreme value in multiple collection records, while the peak detection mode is to find the maximum and minimum value in a single collection record. Both methods can detect faster signal changes

if the oscilloscope has an insta Vu acquisition method, it can be selected because this method has a fast acquisition speed. The waveform displayed by this method is similar to that displayed by an analog oscilloscope

2. Relationship between sampling rate and t/div

the maximum sampling rate of each digital oscilloscope is a fixed value. However, at any scanning time t/div, the sampling rate FS is given by the following formula:

fs=n/(t/div) n is the sampling point per grid

when the number of sampling points n is a certain value, FS is inversely proportional to t/div, and the larger the scanning speed is, the lower the sampling rate is. The following is a set of data of scanning speed and sampling rate of tds520b:

Table 1 scanning speed and sampling rules can reduce carbon dioxide emissions by 7million tons in one year. Sample rate

t/div (NS) FS (gs/s) 50.25

in the analog oscilloscope, the rise time is an extremely important indicator of the oscilloscope. In the digital oscilloscope, the rise time is not even given as an indicator. Due to the measurement method of digital oscilloscope, the rise time automatically measured is not only related to the position of the sampling point, as shown in Figure 2, a shows that the rise edge falls just in the middle of the two sampling points, and the rise time is 0.8 times of the digitization interval. If there is a sampling point in the middle of the rising edge of B in Figure 2, the rising time of the same waveform is 1.6 times of the digitization interval. In addition, the rise time is also related to the sweep speed. Here is a group of data of sweep speed and rise time when tds520b measures the same waveform:

t/div (MS) tr（ μ s)

it can be seen from the above data that although the rise time of the waveform is a fixed value, the results measured with a digital oscilloscope are very different due to different scanning speeds. The rise time of the analog oscilloscope has nothing to do with the scanning speed, while the rise time of the digital oscilloscope is not only related to the scanning speed, but also related to the position of the sampling point. When using the digital oscilloscope, we cannot deduce the rise time of the signal according to the measured time as with the analog oscilloscope

precautions for the use of oscilloscope

(1) hot electronic instruments should generally avoid frequent startup and shutdown, as is the case with oscilloscopes

(2) if the waveform is found to be disturbed by the outside world, the oscilloscope shell can be grounded

(3) the voltage of "Y input" should not be too high to avoid damaging the instrument, and it should not exceed 400 V at the maximum attenuation. When the "Y input" conductor is suspended, it will be disturbed by the external electromagnetic interference, which should be avoided

(4) before turning off the machine, turn the brightness adjustment knob counterclockwise to the end to minimize the brightness, and then turn off the power switch

(5) when observing and adjusting the bright spots on the screen, the brightness of the bright spots should be moderate and should not be too bright