Analysis of common problems in using oscilloscopes
Q1:What are the requirements for oscilloscopes for high-speed serial testing? Which indicators are the most critical?
A:Basically, the bandwidth and sampling rate should meet the requirements of serial signals, and then we need to examine whether it is differential signals, as well as oscilloscopes on the serial test analysis functions, such as code triggering and decoding and so on.
Q2:When measuring high-speed digital signals, does the bandwidth of the oscilloscope have to be more than 5 times of the signal frequency? Why? A:The bandwidth of the oscilloscope is generally chosen to be 2.5 times the rate of the signal under test 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 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 measurements. However, even oscilloscopes with the same bandwidth will exhibit different rise times, and it is critical to the application to measure the errors that occur on the rising edge, and in the case of data signals, the effect on the spread of the eye diagram. Because of this, the rise time metric is very important for devices (oscilloscopes) that perform measurements in the time domain.
Q4:Is higher bandwidth better?
A:As mentioned earlier, the widely used circuit boards, connectors, cables, and integrated modules have very limited rise times, so that high-speed signals are transmitted with significant loss of the high-frequency component. Many of the new third-generation standards (USB 3.0, 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, which uses a complex modulation technique (DP-QPSK), requires four analogue inputs and a bandwidth of more than 20GHz for analysis. With these applications in mind, Tektronix has announced that its oscilloscopes with bandwidths in excess of 30GHz will be available later in the second half of this year.
Q5:How can I increase the sensitivity of my test instruments?
A:Select the appropriate bandwidth, bandwidth is too large will increase the noise, in the vertical settings, as far as possible, so that the signal to fill the screen, so as to make full use of the oscilloscope's AD bits, you can use the waveform averaging, the appropriate bandwidth of the probe, select the high-resolution (Hi-res) acquisition mode and so on.
Q6:When debugging a system design, how can I increase the chances of capturing anomalies when confirming anomalies and figuring out the operating conditions of the circuit within a short period of time?
A:By using DPX technology and turning on infinite afterglow, anomalous signals that might not normally be seen for hours can be seen in seconds. This capability increases the chances of witnessing transient events that occur in digital systems, such as short pulses, burrs, and conversion errors.
