Technical Insights > Benchtop

How to Evaluate Oscilloscope Noise for Low-Level Measurements

2019-09-30  |  4 min read 

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.

Vertical noise

  • Induces amplitude measurement errors
  • Induces sin(x)/x waveform reconstruction uncertainty
  • Induces timing errors (jitter) as a function of input signal edge slew rates
  • Produces visually undesirable “fat” waveforms

When you need to make low-level signal measurements, consider the noise characteristics of your oscilloscope carefully to make sure you aren’t missing crucial waveform information.

Two components of noise come from oscilloscopes

  1. A fixed level of noise primarily caused by the oscilloscope’s front-end design
  2. A relatively low level of noise based on the oscilloscope’s dynamic range. This changes with your vertical scale settings

Base-line noise floor

Vertical noise and random jitter should be measured and specified as an RMS (one standard deviation) value. This is because random noise, or white noise, is unbounded, meaning that the more data you collect the higher the peak-to-peak value will grow.

A specification often referred to in oscilloscope datasheets is “base-line noise floor.”  This is the level of noise when the scope is set to its lowest volts/division setting. Check that the oscilloscope bandwidth settings are the same when the measurement is taken, or it won’t be a direct comparison. Since oscilloscopes are broadband measurement instruments, the higher the bandwidth of the scope, the higher the vertical noise will be. Therefore, many oscilloscopes limit the amount of bandwidth available at the most sensitive V/div setting to provide clean measurements.

To make an apples-to-apples noise comparison, take the base-line noise floor measurement at the most sensitive V/div setting available at full bandwidth to fully understand how different oscilloscopes compare.

Be sure to check how the noise changes at different vertical settings.  Oscilloscope noise changes with dynamic range. The base-line noise floor measurement accounts for the most sensitive vertical settings.  But when you zoom out and set the oscilloscope to higher volts/division settings, there could be additional noise.

Figure 1. Noise measurement with a 1:1 passive probe

Measurement tips in the presence of noise

Here are some handy tips if you are making low-level measurements on an oscilloscope:

  • Try using a 1:1 probe to reduce the effect of oscilloscope noise in your measurement
  • Differential active probes can be used to achieve cleaner measurements at higher bandwidths
  • Use the square-root-of-the-sum-of-the-squares formula to back out the measurement system noise to get a better approximation of your signal

Summary

Inherent random oscilloscope noise can mask real signal measurements. The first step to combating this is using a low-noise oscilloscope.  You can learn more details around measuring oscilloscope noise and making clean measurements in the white paper, Evaluating Oscilloscope Vertical Noise Characteristics. The second step is to use the right probe for your measurements. The 7 Common Oscilloscope Probing Pitfalls to Avoid eBook is a useful guide to achieving clean measurement results.

To learn more about low-noise oscilloscopes, check out the S-Series webpage.