Frequently Asked Questions about Oscilloscope Use

　　An oscilloscope is an electronic measuring instrument with a wide range of uses. It can transform invisible electrical signals into visible images, making it easier for people to study the changing processes of various electrical phenomena. The oscilloscope uses a narrow electron beam composed of high-speed electrons to hit a screen coated with fluorescent material to produce tiny spots of light. Under the action of the measured signal, the electron beam is like the tip of a pen, which can draw a curve of the instantaneous value of the measured signal on the screen. The oscilloscope can be used to observe the waveform curves of various signal amplitudes changing over time, and it can also be used to test various electrical quantities, such as voltage, current, frequency, phase difference, amplitude modulation, etc.

　　The oscilloscope is an indispensable tool on the workbench today. Here are six common questions from oscilloscope users to answer the questions of laboratory researchers.

　　Q1: What are the requirements for the oscilloscope required for high-speed serial testing? Which indicators are the most critical?

　　A: Basically, the bandwidth and sampling rate must meet the requirements of serial signals. Next, we need to examine whether it is a differential signal and the oscilloscope's analysis function for serial testing, such as code triggering and decoding, etc.

　　Q2: When measuring high-speed digital signals, does the bandwidth of the oscilloscope have to 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 being measured or 5 times the highest frequency of the signal, so that the 5th harmonic of the high-speed signal can be seen.

　　Q3: How does the bandwidth during testing affect the test results? What are the requirements for the bandwidth of the test instrument?

　　A: First, insufficient bandwidth will lose the high-frequency harmonic components of the signal, resulting in inaccurate time and amplitude testing. However, even oscilloscopes with the same bandwidth will show different rise times. For applications, measuring errors on the rising edge is very critical. In addition, in data signals, it also has a great impact on the opening of the eye diagram. For this reason, the rise time indicator is 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 widely used circuit boards, connectors, cables and integrated modules is very limited, so that the high-frequency components of high-speed signals are seriously lost after transmission. Many new third-generation standards (USB3.0, PCIEGen3, 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) and requires four analog inputs and a bandwidth of more than 20GHz for analysis. In view of these applications, Tektronix has announced that its oscilloscopes with a bandwidth of more than 30GHz will be launched later this year.

　　Q5: How can we improve the sensitivity of the test instrument?

　　A: Choose a suitable bandwidth. Too large a bandwidth will increase noise. In vertical settings, try to fill the screen with the signal as much as possible to make full use of the AD bits of the oscilloscope. You can use waveform averaging, suitable probe bandwidth, high-resolution (Hi-res) acquisition mode, etc.

　　Q6: When debugging a system design, how can you increase the chances of capturing abnormal phenomena and clarifying the operating conditions of the circuit in a short period of time?

　　A: Using DPX technology and turning on infinite persistence, you can see abnormal signals in seconds that might not be seen for hours. This performance increases the chances of witnessing transient events that occur in digital systems, such as short pulses, glitches, and conversion errors.