Things you need to know about oscilloscopes

Q1: What are the requirements for the oscilloscope required for testing during 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 specification 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 the 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, 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, which uses complex modulation technology (DP-QPSK), requires four analog inputs and a bandwidth of more than 20 GHz 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 an appropriate bandwidth. Too large a bandwidth will increase noise. In the vertical setting, 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, appropriate probe bandwidth, high-resolution (Hi-res) acquisition mode, etc.

Q6: When debugging a system design, how can I increase the chances of catching an abnormal phenomenon and finding out the circuit's operating conditions in a short time?

A: Using DPX technology with infinite persistence, you can see anomalies in seconds that might not be visible for hours. This feature increases the chances of witnessing transient events in digital systems, such as short pulses, glitches, and conversion errors.