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measuring rise time percentages 10%-90%

Discussion in 'General Electronics Chat' started by walters, Aug 1, 2005.

  1. walters

    walters Banned

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    I have a problem of knowing or getting the 10%,20%,30%,etc -90% percentage values from a diagonal line the rise time?

    how do i get these percentages?

    see the diagonal line on the oscilloscope where is 10% is it the first division ?
    where is 20% because the rise time is a diagonal line how do i calucalate the
    percentages of the diagonal line for 10%-90%?

    Does the Rise time have a period ?

    i need to know how to get 10%,20%,30%,etc -90% percentage values from a diagonal line the rise time?
     
  2. pebe

    pebe Member

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    The risetime of a waveform is the time period between 10% and 90% of maximum amplitude.
     
  3. walters

    walters Banned

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    Yea but how do i get the 10% time interval value?
    20% time interval value?
    30%-90% etc.
    how do i get the measured value from the oscilloscope display?
     
  4. dave

    Dave New Member

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  5. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It's not an electronics question?, or even a scope question?, just a simple maths question.

    10% is 1/10, 20% is 2/10 - it's not difficult?.

    On a scope simply count the horizontal squares, so one square high, and four squares low would be 1/5 (one high out of five), which is the same as 2/10, giving 20%.
     
  6. walters

    walters Banned

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    Yea but if i put in a 5 volt rise time ramp dialogal line how do i know the
    10%-90% math rise time interval percentage values?

    5 volts rise time input

    what is the 10% rise time percentage?
    what is the 20% rise time percentage?
    30%
    40%
    50-90

    what is the rise time period?
     
  7. Styx

    Styx Active Member

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    I think you are really mis-understanding what rise/fall time is
    10%-90% of the voltage is just a convention.

    Take a diagonal line that rises from 0V to 10V in 10seconds

    now the 0%-100% rise time would be 10sec


    now in this case 10%-90%:

    The 10% voltage in this case is 1V and it occurs at 1s
    The 90% voltage in this case is 9V and occurs at 9s

    the 10%-90% rise time would be 9sec

    This case is easy since it is nice easy numbers and a nice easy signal
     
  8. Styx

    Styx Active Member

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    Right take this waveform for instance.
    say 1 sqaure = 1V and 1us

    it is approxamitly 9 spaces high (remember engineering is knowing when to say PI =3.14)

    this is for 0% to 100%

    It is equally 10.5 devisions in the X-dimentions



    Since maximum voltage (ie 100% is 9devisions), so 10% = 0.9V.
    This occurs 1 time devision from the start of the slope = 1us

    eqaully 90% = 8.1V
    and that occurs 9.5 time time devisions from the start = 9.5us

    thus the rise time for 10%-90% = 9.5us - 1us = 8.5us rise time

    follow that
     

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  9. walters

    walters Banned

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    Thanks alot styx

    What about the 10%-90% time intervals how do i get those?

    Do i just get the rise time period from the oscilloscope Time per division
    X by 8 divisions= the rise time period

    example so if the rise time is dialogal arcross 8 divisions and equals
    50ms lets just say

    50ms divided would be me the 10% time interval?
    20% time interval ?
    30-90% intervals
     
  10. walters

    walters Banned

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    Thanks styx for the pdf picture

    So where is 10% ,20%,30% and etc

    how do i get the time intervals from the divisions and oscilloscope time per division setting
     
  11. Styx

    Styx Active Member

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    errr?

    Time intervals are taken from the scope division setting, ie 1us/Div for instance

    From that you know what 1 big square on the scope is worth.

    why would you want to know what 20%, 30% is when you wanted "rise-time" which is defined as the time taken for a signal to change it voltage form 10% to 90% of its full voltage swing

    As Nigel stated this is simple maths

    What is 10% of 88 for instance?
    What is 40% 49 for instance
    what is 90% of 3.1415926 for instance

    in this case what is 10% of 9
    what is 90% of 9

    at what time (from the start of the rise) does a voltage of 10% occur
    at what time (from the start of the rise) does a voltage of 90% occur

    What is the difference between those two time? = rise time

    IF you really want a forumla/steps
     
  12. walters

    walters Banned

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    Thanks styx for the help

    at what time (from the start of the rise) does a voltage of 10% occur
    at what time (from the start of the rise) does a voltage of 90% occur

    Yes how do i get these information, how do i do it?

    What is the difference between those two time? = rise time

    IF you really want a forumla/steps

    Yes what formulas and steps do i do for this procedure?
     
  13. walters

    walters Banned

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    Older scopes have those 10% and 90% dashed lines on their screen

    Where are they i still don't know where the percentages are

    I know where 0% and 100% is but i don't know where 10%-90% are
    don't i have count the divisions and X by the time per divisions?

    I need like a Old style oscilloscope display to help me out to "Guide" me better
    because im just guessing really where 10%,20%,30%-90% are.

    Is there a better way to have a % percentage "guide" made for me to help me out more
     
  14. Roff

    Roff Well-Known Member

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    I hesitate to get into this, because you seem to have some sort of mental block, or don't understand percentages, but oh, well, damn the torpedoes, full speed ahead!
    All scopes have an uncalibrated variable gain knob. Adjust that knob until the pulse waveform spans exactly 5 divisions vertically. Set both pulse levels on graticule lines. When the pulse rising edge is exactly 0.5 divisions above the lower level, that's 10% of the amplitude. Note the position of that point on the horizontal (time) scale. When the pulse edge is exactly 0.5 divisions below the upper level, that's 90%. Note that position on the horizontal scale. Now subtract the 10% time reading from the 90% time reading. The result is the 10-90% risetime. For falltime, do the same thing on the falling edge.
     
  15. walters

    walters Banned

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    So 5 divisions vertically= 100% rise time
     
  16. Dean Huster

    Dean Huster Well-Known Member

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    "Older scopes have ... those ... lines"?

    Now there's a statement that's relative WRT time. "Older" (i.e., ca. 1955) did NOT have a "risetime graticule" (the proper term for "those ... lines"). The newest scopes (i.e., DSOs over the last 10 years) may not have those lines because risetime is typically a calculating function of any decent DSO.

    Decent scopes (i.e., "real" analog scopes made by Tektronix) have had risetime graticules since around 1965 or 1970 if the scope had a bandwidth of around 35MHz or more. If you look carefully at the graticule, you'll see the major division lines and then two dotted lines. You use your volts/div knob and your variable volts/div knob to adjust the pulse waveform screen height so that the more positive level is on the upper dotted line and the less positive level is on the lower dotted line. The next major graticule line above the lower dotted line is the 10% point. The next major graticule line below the upper dotted line is the 90% point. Speed the time/div up so that the transition uon which you are triggering is spread out as far horizontally as you can get it WITH THE VARIABLE TIME/DIV IN THE CALIBRATED POSITION, adjust the horizontal position so that the 10% point crosses over the second vertical major graticule line and measure the time (number of divisions multiplied by the time/div setting) between this point and the point where the transition crosses the 90% point. This is your risetime.

    Note that the oscilloscope your are using must have a faster risetime that the transition which you are measuring or all you'll be measuring is the scope's risetime.

    If you have the trigger slope set for "+", you'll be measuring the risetime. If you have the trigger slope set for "-", you'll be measuring fall time (from the 990% point down to the 10% point).

    Dean
     
  17. walters

    walters Banned

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    Thanks Dean Huster for the information

    I need to get a rise time gradules display to help guide me better with
    the oscilloscope
     
  18. Roff

    Roff Well-Known Member

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    If you set it up so that the upper and lower levels span 5 divisions, yes.
    We measure 10% - 90% because, with a monotonic signal (no preshoot or overshoot), it is impossible to determine where 100% is exactly. For instance, on an exponentially rising edge (single RC circuit), the signal theoretically never reaches 100%. It will reach 99% in about 5 time constants, 99.9% in 7 time constants, 99.99% in 9 time constants, 99.999% in 12 time constants,...
     
  19. walters

    walters Banned

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    So how many Time Constants there in a 10%-90% rise time?

    how can i measure the time constants ? with the oscilloscope gradules and divisions where do i count to get the time constants in a rise time?
     
  20. Styx

    Styx Active Member

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    you are reallly starting to piss me off
    Are you fore real!!!

    How do we know!!! it depends on the signal you are looking at!!!!

    This is like hte 3rd thread on the subject and many ppl have explained it to you.

    Do you know what percentages are?
     
  21. walters

    walters Banned

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    Yea Time constants have percentages also but i don't know how to get
    the values or know how to do it with a oscilloscope sorry

    And there is Time constants percentages with rise time
     

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