The oscilloscope is still an indispensable tool on the workbench. Here are six common problems based on oscilloscope users to solve the doubts of laboratory researchers.
Q1: In the high-speed serial test, what are the requirements for the oscilloscope required for the test? Which indexes are the most critical?
A: Basically, the bandwidth and sampling rate must meet the requirements of serial signals. Next, we need to investigate whether it is a differential signal, and the oscilloscope's analysis function for serial testing, such as pattern triggering and decoding.
Q2: When measuring high-speed digital signals, must the bandwidth of the oscilloscope be more than 5 times the signal frequency? Why?
A: Select the bandwidth of the oscilloscope, which is generally 2.5 times the rate of the signal under test or 5 times the highest frequency of the signal, so that you can see the 5th harmonic of the high-speed signal.
Q3: How does the bandwidth during the test affect the test results? What are the requirements for the bandwidth of the test instrument?
A: First of all, insufficient bandwidth will lose the high frequency harmonic components of the signal, resulting in inaccurate time and amplitude testing. However, even if oscilloscopes with the same bandwidth will show different rise times, it is very critical for the application to measure the errors that occur on the rising edge. In addition, in the data signal, it also has a great influence on the eye opening. Because of this, rise time specifications are very important for devices (oscilloscopes) that perform measurements in the time domain.
Q4: Is the higher the bandwidth, the better?
A: As mentioned earlier, the rise time of currently widely used circuit boards, connectors, cables and integrated modules is very limited, so that high-frequency components are seriously lost after transmission of high-speed signals. Many new third-generation standards (USB3.0, PCIE Gen3, 10G-KR) have taken this into account and require much lower bandwidth than before. Of course, there are some exceptions that require higher bandwidth. For example, the 100G Ethernet solution uses complex modulation technology (DP-QPSK), which requires four analog inputs and a bandwidth exceeding 20 GHz for analysis. In view of these applications, Tektronix has announced that its oscilloscope with a bandwidth of more than 30GHz will be launched later this year.
Q5: How can I increase the sensitivity of the test instrument?
A: Select the appropriate bandwidth. Excessive bandwidth will increase the noise. In the vertical setting, let the signal fill the screen as much as possible to make full use of the AD digits of the oscilloscope. You can use the waveform average, the appropriate bandwidth of the probe, and select high resolution (Hi-res) Acquisition mode, etc.
Q6: When debugging the system design, confirm abnormal phenomena and clarify the operating conditions of the circuit in a short time, how to increase the chance of catching abnormal phenomena?
A: Using DPX technology and turning on the infinite afterglow, you can see the abnormal signal that may not be seen for hours in a few seconds. This performance increases the probability of witnessing transient events that occur in digital systems, including transient pulses, glitches, and conversion errors. (Organized by China Educational Equipment Purchasing Network)
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