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

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The discussion focuses on estimating the decay time constant of a waveform using an oscilloscope, specifically analyzing a trace set at 100 ns/div. Participants emphasize the importance of determining how long it takes for the waveform to reach 63% of its maximum value. A suggestion is made to use a faster sweep time of 10 ns/div for better accuracy, although one participant followed textbook procedures that included 100 ns settings. The estimated decay time calculated is around 9.2 ns, which is considered acceptable by others in the discussion. Overall, the conversation highlights methods for accurately measuring decay time constants with oscilloscopes.
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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