Analysis of Six Common Issues When Using Oscilloscopes
Oscilloscopes are an indispensable measurement tool for engineers, but during the use of oscilloscopes, various issues may arise. This article compiles the six most common problems faced by oscilloscope users to help laboratory personnel address measurement concerns. The oscilloscope is currently an essential tool on the workbench, and here we summarize six common questions based on oscilloscope users to resolve the doubts of laboratory researchers.
Q1: What are the requirements for the oscilloscope needed for high-speed serial testing? What are the key indicators? A: Generally, the bandwidth and sampling rate must meet the requirements of the serial signal. Next, it is necessary to examine whether it is a differential signal and the oscilloscope’s analysis functions for serial testing, such as triggering and decoding of code patterns.
Q2: When measuring high-speed digital signals, must the oscilloscope’s bandwidth be more than five times the signal frequency? Why? A: The bandwidth of the oscilloscope is generally 2.5 times the rate of the measured signal or five times the highest frequency of the signal, which allows observation of the fifth harmonic of high-speed signals.
Q3: How does bandwidth affect test results? What are the requirements for the bandwidth of testing instruments? A: Firstly, insufficient bandwidth will lose high-frequency harmonic components of the signal, leading to inaccuracies in time and amplitude measurements. However, even oscilloscopes with the same bandwidth can exhibit different rise times. For applications, errors occurring at the rising edge are critical, and the eye diagram’s opening in data signals is also greatly affected. Therefore, the rise time specification is very important for devices (oscilloscopes) performing measurements in the time domain.
Q4: Is higher bandwidth always better? A: As mentioned earlier, the rise times of widely used circuit boards, connectors, cables, and integrated modules are very limited, causing significant loss of high-frequency components after high-speed signal transmission. Many new third-generation standards (USB3.0, PCIEGen3, 10G-KR) have taken this into account, requiring much lower bandwidth than before. Of course, there are exceptions that require higher bandwidth. For example, the 100G Ethernet solution employs complex modulation techniques (DP-QPSK) and requires four analog inputs and bandwidth exceeding 20GHz for analysis. In light of these applications, Tektronix has announced that oscilloscopes with bandwidth exceeding 30GHz will be launched later this year.
Q5: How can we improve the sensitivity of testing instruments? A: Choose the appropriate bandwidth; excessive bandwidth increases noise. In vertical settings, try to make the signal fill the screen to fully utilize the oscilloscope’s AD resolution. Waveform averaging, appropriate probe bandwidth, and selecting high-resolution (Hi-res) acquisition mode, etc., can also help.
Q6: When debugging system designs, how can we confirm anomalies and understand circuit operating conditions in a short time to increase the chances of capturing anomalies? A: Use DPX technology and enable infinite persistence, which allows you to see anomalies that may take hours to observe in just a few seconds. This feature increases the likelihood of witnessing transient events in digital systems, including short pulses, glitches, and transition errors.
Top 10 Common Questions About Oscilloscope Use
1. What is oscilloscope bandwidth? How do we decide what bandwidth oscilloscope to buy?
A: Oscilloscope bandwidth refers to the frequency value at which the sine input signal attenuates to 70.7% of its actual amplitude, that is, the -3dB point, based on a logarithmic scale. (The higher the bandwidth, the more accurately the signal is reproduced. Without sufficient bandwidth, the oscilloscope will not be able to discern high-frequency changes. Amplitude will become distorted, edges will disappear, and detailed data will be lost. Without sufficient bandwidth, all characteristics of the signal, such as ringing and overshoot, are meaningless.)
2. Is a higher real-time sampling rate always better?
A: The higher, the better.
3. Under what circumstances does testing require disconnecting the oscilloscope’s power ground line?
A: When the ground of the measured signal is shared with the ground of the oscilloscope’s external power line.
4. What types of signals are suitable for various triggering methods of the oscilloscope?
A: Video triggering is suitable for testing standard video signals, edge triggering is suitable for testing stable signals with rising or falling edges, and pulse triggering is suitable for testing synchronized signals with severe interference.
5. How to test small glitch signals with an oscilloscope?
A: Set the oscilloscope to automatic testing mode, and compatible oscilloscopes can automatically detect small glitch signals.
6. Is the bandwidth of an oscilloscope the maximum testing frequency?
A: No, the oscilloscope bandwidth should be at least (2-5) times the frequency of the fastest signal being measured.
7. Is the voltage of the waveform tested by the oscilloscope the true RMS value?
A: No.
8. What is the relationship between oscilloscope storage depth and sampling rate?
A: Storage depth = sampling rate × sampling time.
9. Is the storage depth of the oscilloscope the same as the instrument’s memory?
A: No.
10. What is the minimum bandwidth requirement for a 1M sine wave and triangular wave for the oscilloscope?
A: The compatible oscilloscope should be at least 25M.
