Since the oscilloscope is a crucial tool for design (especially signal integrity), designers should be familiar with the indicators of the oscilloscope and the way they affect measurement. Let's take a closer look at the three most important factors: rise time, sampling rate, and bandwidth.
Rise time: the faster the rise time of oscilloscope, the more accurate the rise time measured. But what happens when the bandwidth or rise time of the oscilloscope and the rise time of the signal are closer to each other?Some people have used the rule of thumb (e.g., 0.35/ rise time) to calculate the required oscilloscope bandwidth, but this rule of thumb only applies to some oscilloscope front-end designs, and generally does not apply to today's front-end designs optimized for high speed serial data rates and accompanying fast rise time.
It should be noted that two oscilloscopes with the same bandwidth performance can have completely different rise time, amplitude and phase response. Therefore, only knowing the bandwidth of oscilloscope cannot reveal its measuring performance reliably.In addition, the calculation of the rise time may not be accurate.The most reliable way to know the time response of the oscilloscope is to measure it with an ideal step signal which is much faster than the signal of the oscilloscope under test.
In the case of DPO/DSA73304D, the rise time of 9ps was determined using this method. But what does it mean that the signal velocity can be measured?According to the correct rule of thumb, the ratio of signal rise time to oscilloscope rise time is 2x or >18ps.This, it turns out, is a specified rise time for the 28Gb/s SerDes used in today's fastest FPGA designs.