Finding the Decay Time Constant: Tips and Tricks for Using an Oscilloscope

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Discussion Overview

The discussion revolves around determining the decay time constant of a waveform observed on an oscilloscope. Participants explore methods for estimating this constant based on oscilloscope settings and waveform characteristics, focusing on the interpretation of time measurements from the displayed traces.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Homework-related, Mathematical reasoning

Main Points Raised

  • One participant asks how to determine the decay time constant from the oscilloscope waveform.
  • Another participant suggests focusing on the 100 ns/div oscillograph and estimating the time it takes for the waveform to reach 63% of its maximum value, noting potential nonlinearities or multiple time constants.
  • A participant requests clarification on the method for estimating the decay time constant from the waveform.
  • There is a suggestion that an estimate of 8-10 ns might be correct for the decay time constant.
  • One participant agrees with the estimate and recommends using a 10 ns/div setting for better accuracy instead of the 100 ns/div setting.
  • Another participant points out that the 100 ns setting is the sweep time, implying a need for a different approach.
  • A participant mentions following a textbook procedure for oscilloscope settings and calculates a decay time of 9.2 ns, seeking validation for this result.
  • One participant responds that the calculated value seems acceptable, suggesting that the oscilloscope may not have had a 10 ns setting available.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate oscilloscope settings and methods for estimating the decay time constant. While some agree on the estimation process, there is no consensus on the best approach or the accuracy of the measurements.

Contextual Notes

Participants mention potential nonlinearities in the waveform and the possibility of multiple time constants, which may affect the accuracy of the decay time constant estimation. There are also references to specific oscilloscope settings that may limit measurement precision.

MissP.25_5
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What is the decay time constant of the waveform? How to determine it from this oscilloscope?
The waveforms are in the pictures below:
 

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Look at the 100 ns/div. oscillograph (the orhers are useless).

How long would you estimate it takes the bottom trace to get to 63% of its way to the top?

(This would be just an estimate since there may be nonlinearities involved, or two or more time constants. But a pretty good estimate anyway).
 
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rude man said:
Look at the 100 ns/div. oscillograph (the orhers are useless).

How long would you estimate it takes the bottom trace to get to 63% of its way to the top?

(This would be just an estimate since there may be nonlinearities involved, or two or more time constants. But a pretty good estimate anyway).

Thanks for replying but could you explain this method, please?
 
MissP.25_5 said:
Thanks for replying but could you explain this method, please?

Your trace begins at the bottom and ends at the top. How much time along the time axis to reach 63% of the distance from the bottom to the top? Use the 100 ns/div. trace.
 
rude man said:
Your trace begins at the bottom and ends at the top. How much time along the time axis to reach 63% of the distance from the bottom to the top? Use the 100 ns/div. trace.

Should 8~10ns be correct?
 
MissP.25_5 said:
Should 8~10ns be correct?

Yes.
To get a better estimate you should have run 10 ns/division instead of just 100 ns.
 
rude man said:
Yes.
To get a better estimate you should have run 10 ns/division instead of just 100 ns.

Actually, 100ns is the sweep time.
 
MissP.25_5 said:
Actually, 100ns is the sweep time.

Actually, I know.

You should have chosen 10 ns/div. sweep time
 
rude man said:
Actually, I know.

You should have chosen 10 ns/div. sweep time

Well, I just followed the procedure in the textbook. It says to set the sweep time to 2.5mircrosec, 250nsec and finally 100nsec.
I tried calculated the decay time and taking the best point that I could and I got 9.2ns. Is this ok? By the way, thanks for helping me out.
 
Last edited:
  • #10
Maybe your oscilloscope didn't have a 10 ns.etting.

Anyway, I think your number is fine.
 

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