What should be paid attention to when using an oscilloscope?

What should be paid attention to when using an oscilloscope?

The oscilloscope is an indispensable measurement tool used by maintenance engineers, but various problems will be encountered during oscilloscope use. This article sorts out the common problems in using the oscilloscope for you to solve the doubts of the operation and maintenance personnel.

1) Q: What is the fixed relationship between bandwidth and sampling frequency?

A: At least 2 points need to be sampled in each cycle of the highest frequency signal of the measured signal; otherwise, aliasing will be caused. But in practice, it depends on many other factors. Generally speaking, the sampling rate is 4-5 times the bandwidth, and the waveform can be reproduced more accurately.

2) Q: How to understand the bandwidth in the oscilloscope indicator?

A: Bandwidth is the basic indicator of an oscilloscope. Like the definition of amplifier bandwidth, it is the so-called -3dB point, the frequency point at which a sine wave is added to the oscilloscope's input, and the amplitude is attenuated to 70.7% of the actual amplitude is called the bandwidth. Therefore, when choosing an oscilloscope, we should choose a bandwidth 5 times the highest signal frequency to achieve a certain measurement accuracy.

3) Q: Under the condition of a certain bandwidth, is the sampling frequency as high as possible?

A: Bandwidth is the basic condition to limit the captured high-frequency components of the measured signal. Because the FNIRSI-1014D Dual Channel Oscilloscope adopts an advanced waveform reconstruction algorithm and provides a real-time sampling rate up to 1GSa/s, it ensures the perfect capture and accurate quantification of the trigger signal and can finally accurately reproduce the acquired signal.

4) Q: What factors affect the operating speed of the oscilloscope?

A: Two main aspects affect the speed of the oscilloscope. One is the data transmission from the front-end data acquisition to the back-end processing, which is generally transmitted by bus, and the other is the back-end processing method.

5) Q: How to eliminate glitches when using an oscilloscope?

A: If the glitch is inherent in the signal, and you want to use edge triggering to synchronize the signal, you can use the high-frequency suppression trigger method, which usually synchronizes the signal. If the signal glitches, making the oscilloscope remove the glitch and not display it is usually challenging. You can try to limit the bandwidth, but if you are not careful, you may also lose some information in the signal itself.

6) Q: Under what circumstances should the sampling rate be considered?

A: It depends on the object to be measured. On the premise that the bandwidth is satisfied, it is hoped that the minimum sampling interval (the inverse of the sampling rate) can capture the signal details you need. In practical applications, it is best not to use an oscilloscope to measure signals with bandwidth frequencies. Using an oscilloscope, you can verify whether the sampling rate is sufficient by the following methods: Stop the waveform and zoom in on the waveform. If you find that the waveform changes, the sampling rate is not enough; otherwise, the measurement accuracy cannot be met. It can also be used to analyze whether the sampling rate is sufficient.

7) Q: When observing the details of the waveform, which one has the advantage of analog and digital oscilloscopes?

A: The vertical accuracy of analog oscilloscopes we used in the early days was generally +/-3%, while the vertical accuracy of digital oscilloscopes was as high as +/-1%. In this regard, digital oscilloscopes have significant advantages.

8) Q: How to capture and reproduce the lost momentary signal?

A: To capture the instantaneous signal, please refer to the following settings: select the edge trigger type, set the trigger mode to single mode, set the signal to rise trigger, and adjust the trigger level to an appropriate value. In addition, some oscilloscopes are equipped with EasyZoom window expansion technology, which can magnify and observe local details while observing the global signal.

9) Q: What type of oscilloscope should I choose to improve design efficiency effectively?

A: With the development of oscilloscopes, data analysis and processing have been greatly improved. Using an oscilloscope is not only to observe waveforms during debugging but more importantly, to find problems in the design, analyze and calculate device parameters, and help you optimize design solutions. Choosing what kind of oscilloscope is best depends on the signal you want to observe and analyze.

10) Q: What should be paid attention to when using the oscilloscope probe?

A: Probes are generally ignored in the use of oscilloscopes. Passive probes are widely used because of their wide measurement range and low price, and they can meet most measurement requirements. The selection of passive probes should be consistent with the oscilloscope bandwidth consistent. When the probe is replaced and the probe is exchanged for channels, the probe compensation adjustment must be performed to match the input channel. The easiest and most intuitive way to adjust probe compensation is to use the probe waveform.

11) Q: What is the real-time sampling rate of the oscilloscope?

A: Real-time sampling refers to sampling the waveform at equal time intervals, performing A/D conversion according to the sampling sequence, and storing it in the memory. Real-time sampling is the most prominent and intuitive sampling method. This type of sampling requires simply distributing the sample points in time, all of which are acquired in response to a single trigger on the oscilloscope.

12) Q: What is the equivalent time sampling of an oscilloscope?

A: Equivalent time sampling refers to the oscilloscope piecing together the waveforms collected by multiple acquisitions (multiple triggers) into one waveform. The sampling rate may be very slow, and the trigger points of the two acquisitions have a certain offset. The inverse of the minimum sampling interval between two points is called the equivalent sampling rate. Its indicators can reach very high, such as 1ps.

13) Q: When viewing the waveform on the oscilloscope, what is the difference between the external trigger and the self-trigger?

A: The usual trigger of an oscilloscope is an edge trigger, which has two trigger conditions, trigger level and trigger edge; that is when the rising edge (or falling edge) of the signal reaches a certain level (trigger level), the oscilloscope triggers. Oscilloscopes only use external triggering when there is a problem with the signal self-triggering. In addition, the signal is more complex, and many points meet the trigger conditions, so it is impossible to trigger at the same position every time to obtain a stable display. Then you need to use an external trigger. The FNIRSI-1014D two-in-one oscilloscope and signal generator provides a two-channel desktop oscilloscope to meet user testing needs.

14) Q: How to obtain the total bandwidth of the measurement system?

A: When measuring digital signals, the signal's rise time determines the system's total bandwidth, and the measurement system's total bandwidth = 0.35/rise time.

15) Q: How to apply trigger holdoff in measurement? How does it work?

A: The meaning of trigger holdoff is to temporarily close the trigger circuit of the oscilloscope for a while. During this period, the oscilloscope will not trigger even if there is a signal waveform point that meets the trigger conditions. The function of the trigger part of the oscilloscope is to display the waveform stably, and trigger holdoff is also a function set to display waveforms stably.

16) Q: The oscilloscope is normal, but the waveform is disorganized when observing the measured signal with the oscilloscope.

A: This is because the ground end of the signal under test and the oscilloscope ground wire do not share the same ground. Usually, the self-test signal of the oscilloscope is used to check whether the probe and the oscilloscope are normal. If the oscilloscope and the probe are normal, the measured waveform is abnormal. When measuring a signal with a small amplitude, the probe's ground wire can be unplugged, and the ground wire equipped with the oscilloscope can be used to connect to the ground for testing.

17) Q: The oscilloscope is normal, but no signal waveform is generated.

A: There are three reasons:

a. Input signal from channel 1, but accidentally open channel 2;

b, the signal coupling method (AC-GND-DC) selects the grounding position.

c. Confirm that the signal has been generated and is normally input to the BNC interface of the oscilloscope.

18) Q: How to measure DC voltage?

A: First, you need to set the coupling mode to DC, adjust the vertical scale to an appropriate value according to the approximate range, and then compare the displacement of the offset line with the channel mark. During use, press "AUTO" to automatically measure to complete the test results.

19) Q: Why the waveform storage has already stored the settings but also need to store the settings separately?

A: First, the main difference between the two is that the storage space occupied by waveform storage is much larger than that of setting storage. Therefore, considering the storage space and cost, it is necessary to save the two separately. Secondly, there are also differences in the tune-up of the two. When the waveform is recalled, the oscilloscope is in the STOP state, and the saved running state is not changed when the setting is recalled, which can facilitate the direct observation of the waveform.