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

In summary, the conversation discusses determining the decay time constant of a waveform using an oscilloscope with a 100 ns/div sweep time. The method involves looking at the trace and estimating how long it takes to reach 63% of the distance from the bottom to the top. The conversation also mentions using a 10 ns/div sweep time for a more accurate estimate. The final estimated decay time is 9.2 ns.
  • #1
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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  • #2
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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  • #3
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?
 
  • #4
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.
 
  • #5
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?
 
  • #6
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.
 
  • #7
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.
 
  • #8
MissP.25_5 said:
Actually, 100ns is the sweep time.

Actually, I know.

You should have chosen 10 ns/div. sweep time
 
  • #9
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.
 

1. What is a decay time constant?

A decay time constant is a measure of how quickly a decaying signal drops to a certain percentage of its original value. It is typically represented by the symbol "τ" and is calculated by taking the time it takes for the signal to drop to 1/e (approximately 37%) of its original value.

2. Why is it important to find the decay time constant?

The decay time constant is important because it provides valuable information about the behavior and characteristics of a decaying signal. It can help determine the type of decay (exponential, logarithmic, etc.), the rate of decay, and the stability of the signal.

3. How can an oscilloscope be used to find the decay time constant?

An oscilloscope can be used to find the decay time constant by measuring the time it takes for the signal to drop to a certain percentage of its original value. This can be done by adjusting the time scale and triggering settings on the oscilloscope to capture the decay portion of the signal and then using the cursor function to measure the time interval.

4. Are there any tips for accurately finding the decay time constant with an oscilloscope?

Yes, there are a few tips that can help ensure accuracy when using an oscilloscope to find the decay time constant. These include using a suitable time scale, triggering the oscilloscope correctly, and using the cursor function to measure the time interval several times and taking an average for more precise results.

5. Can the decay time constant be affected by external factors?

Yes, the decay time constant can be affected by external factors such as temperature, humidity, and noise. It is important to minimize these external factors as much as possible to obtain accurate and reliable measurements of the decay time constant.

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