Evaluating Oscilloscope Vertical Noise Characteristics
All oscilloscopes exhibit one undesirable characteristic: vertical noise in the scope's analog front-end and digitizing process. Measurement system noise will degrade your actual signal measurement accuracy, especially when you are measuring low-level signals and noise. Since oscilloscopes are broadband measurement instruments, the higher the bandwidth of the scope, the higher the vertical noise will be—in most situations. Although engineers often overlook vertical noise characteristics when they evaluate oscilloscopes for purchase, these characteristics should be carefully evaluated as they can impact signal integrity measurements in several ways.
Unfortunately, not all scope vendors provide vertical noise specifications/characteristics in their data sheets. And when they do, the specs are often misleading and incomplete. This document compares vertical noise characteristics of oscilloscopes ranging in bandwidth from 500 MHz to 1 GHz made by Agilent Technologies, Tektronix, and LeCroy. In addition, this document provides valuable hints on how to more accurately perform noise and interference measurements on low-level signals in the presence of relatively high levels of measurement system noise (oscilloscope noise).