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I am using a Tektronix TDS 2002c oscilloscope. I have used Oscilloscope a bit. I do know how to use the measure function in the scope to measure rise time of pulse.Do you have an understanding of time measurement using an oscilloscope? Yes, you can measure rise time of a pulse on an oscilloscope, however, you need an understanding of how to use and read the oscilloscope. You make no mention of what oscilloscope(s) are available to you or what the scope capabilities are.
I think there's some confusion over the term 'rise time'. If I understand correctly the OP is not interested in the shape of the PWM pulse itself (i.e the rise time of the pulse edge) but is trying to determine how long it takes for the PWM duty cycle to increase from 20% (i.e. 1:4 mark/space ratio) to 70%?
Edit:
What is the PWM frequency?
An RC filter to smooth the PWM could be used to give a voltage representing the duty cycle, then that voltage could be measured on the scope.
An RC filter to smooth the PWM could be used to give a voltage representing the duty cycle, then that voltage could be measured on the scope.
OK....this is confusing. Please draw a schematic or something. schematic=1000 words.You are correct. I think I confused some by mentioning the time rise time. I shouldn't have used that. As you mentioned I am only interested in the time taken for PWM duty cycle to increase from 20 to 70%.
The PWM frequency is 100Hz.
Actually I am trying to validate something. I am not sure how the development team have implemented this. The requirement we had requires the "PWM output to increase from 20% duty cycle to 70% duty cycle at the press of a particular switch in 350 ms". My understanding is that the development guys increases the pwm duty cycle in a cyclic task. And to make matters worse this more or less Black Box testing. We have not much idea of how the implementation is done in the box(system).I think something is missing! What changes the duty cycle? For example say you adjust the duty cycle by turning a POT, then the time could be seconds/minuets/hours depending on how fast you turn it. Is it adjusted by some kind of interrupt? What I am trying to say is we need to know what controls the duty cycle so we know the time frame. Personally unless this was a quick 20%-70% I would be using a deep memory Logic analyzer. That way you just run it and then scroll back to 20% place a cursor there, then scroll forward and place other cursor at 70%.
Can you give details of the system? Otherwise say for example you have a interrupt that makes the duty cycle increase every 10 seconds by 5%, your never going to capture that on a scope.
Does your scope have a post trigger setting? and a delay setting? Do you have access to a Logic analyzer? do you have say a PK2 or PK3?
The Oscilloscope does not have magnified time base option.You may not have enough scope to do what you want. It sounds like you don't have any signal to indicate the beginning of the sweep so that means if you want to see it you need to capture the entire 350 ms. If your scope has a delay time base you can just scroll thru the trace to see the duty cycle. If it doesn't have delay you could try the X10 magnifier.... It may be enough. If not go back to Alec's idea.
Using an RC filter should give me a saw tooth wave? Am I correct?I think there's some confusion over the term 'rise time'. If I understand correctly the OP is not interested in the shape of the PWM pulse itself (i.e the rise time of the pulse edge) but is trying to determine how long it takes for the PWM duty cycle to increase from 20% (i.e. 1:4 mark/space ratio) to 70%?
Edit:
What is the PWM frequency?
An RC filter to smooth the PWM could be used to give a voltage representing the duty cycle, then that voltage could be measured on the scope.
1) Is the duty cycle intended to be a constant 20% before you press the switch?The requirement we had requires the "PWM output to increase from 20% duty cycle to 70% duty cycle at the press of a particular switch in 350 ms".