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Douglas C. Smith

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Technical Tidbit - January 2011
Using Averaging to Cleanup Scope Waveforms

noisy scope waveform

Figure 1.
Current Waveform from ESD Simulator
(Vertical = 1/2 Amp/div, Horizontal = 10 ns/div)

Abstract: Noise can contaminate oscilloscope waveforms for a variety of reasons. ESD generated noise is among the worst in this respect. The waveform averaging function in many oscilloscopes can be used to minimize some types of noise in the waveform, especially ESD generated noise. Use of averaging for cleaning up scope waveforms is discussed as well as limitations of the method.

Discussion: Figure 1 shows a waveform of the current from an ESD simulator into a large plane of metal as sensed by a Fischer F-65 current probe. Notice the "hash" that starts about 7 nanoseconds before the ESD current waveform and gives the waveform a rough appearance for the first few tens of nanoseconds. In addition, digitizing noise in the scope also contributes to the slightly rough appearance on the tail of the ESD generated current. This scope is an older one and newer scopes are generally better in both respects. But what can one do if noise contaminates your waveform?

If the contaminating noise is not related to the desired waveform, and if the desired waveform is repeatable, then using a scope's averaging function can substantially clean up the waveform in many cases. This is illustrated in Figure 2 which shows the discharge current of Figure 1 but using an average of eight individual waveforms. The noise on the waveform from both sources is substantially reduced.

Averaged waveform

Figure 2.
Current Waveform from ESD Simulator using Averaging of Eight Waveforms
(Vertical = 1/2 Amp/div, Horizontal = 10 ns/div)

This is a simple technique that I use often in the lab to cleanup waveforms. Be careful though as averaging can hide changes in the desired waveform or even cause an inaccurate display. An example of poor accuracy is a mis-trigger caused by using a triggering level that is too low. This can result in an amplitude being displayed that is of lower amplitude than the actual waveform by contributing a nearly flat trace to the average.

Summary: With allowances for some limitations, waveform averaging can be used to cleanup oscilloscope waveforms that are contaminated by noise not related to the desired signal.

I would like to thank RMV Technology Group at NASA Ames Research Center for use of their facilities to generate the data for this Technical Tidbit.

Additional articles on this website related to this topic are:
  1. December 2010 Technical Tidbit, Comparing "IEC 61000-4-2 Compliant" ESD Simulators
  2. November 2010, Comparison of Current Waveforms from 150 Ohm and 330 Ohm Networks in an IEC 61000-4-2 Simulator
  3. July 2005, Radiated Interference to High Frequency Signal Measurements from the Signal Source
  4. September 2004, Mobile Phone Response to EMI from Small Metal ESD
    (for an example of ESD radiated noise into a scope measurement)
Equipment used in this Technical Tidbit:
  1. Fischer Custom Communications F-65 Currrent Probe
  2. Agilent Infinium 54845a scope
I would like to thank RMV Technology Group for the use of their facilities to perform the tests for this article.

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Copyright 2011 Douglas C. Smith