Fast frequency to voltage conversion?

Discussion in 'The Projects Forum' started by Dyslexicbloke, Feb 19, 2011.

  1. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    Hi folks.
    I have been working on my CHP project ...(http://www.navitron.org.uk/forum/index.php/topic,12943.0.html) and due to the limitations of the PLC that I am using (It was free) I need to build a frequency to voltage interface board.

    I would like to convert frequency to voltage, I am currently working with 50 Hz but could change this by modifying the shaft encoder. Assuming the target stays at 50 Hz I will need a 0 -10v signal that represents 40 – 60 Hz ideally, although 0 – 60 Hz would probably work all be it with lower resolution.

    I appreciate that a simple integrator looking at the square frequency will work but if I use a cap big enough to limit the ripple to an acceptable level then the system is far to slow.

    I probably need some sort of sample and hold circuit looking at each individual pulse width but really don’t know where to start.
    Any input would be greatly appreciated.

    ( I have some analogue switch IC’s, bought to multiplex the PLC inputs, if that helps at all. )
    Thanks
    Alistair
     
  2. beenthere

    Retired Moderator

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  3. SgtWookie

    Expert

    Jul 17, 2007
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    I'm afraid that the LM2907 will also suffer from slow conversion; at 50Hz, a series of filters will be required to get a decent-looking output signal.
     
  4. bertus

    Administrator

    Apr 5, 2008
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  5. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    I have looked at the sheet for the PLL chip and I am sceptical that it will work at 50Hz, to be honest I was a little lost .....
    Any ideas RE building from descrete componants or basic IC's?
    Al
     
  6. Dyslexicbloke

    Thread Starter Active Member

    Sep 4, 2010
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    I hav found several circuit examples using an analough switch to apply voltage to a cap feeding a voltage follower.
    These seem like a good way to go but I could do with a leg up un respect of timing and how to go about things.
    I would need a sequence of events something like this.

    with a square wave of between 10 and 60 Hz ......
    Positive edge > start charging the cap, output should not change.
    Negative edge > sample new value and change output to match.
    Output latched > discharge cap to prime for next cycle.

    Am, I correct in assuming that I would actually need 2 sample and hold stages, the first feeding the second, achieve the above.

    Do you think that detecting 'current sample = output' would be a good way of determining when to switch to hold and discharge the cap.

    The switch IC's I am using will be MC14016BCP.
    The project is as much about learning a new type of componant as it is achieving the desired result.

    Any and all comments welcome.
    Thanks
    Al
     
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
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    Have you looked at this thread : http://forum.allaboutcircuits.com/showthread.php?t=48943

    The OP wanted to convert a speedometer signal to voltage then back to frequency. The thread is quite lengthy now, but the gist of it is in the first 6 pages. The OP used the NJM4151 initially, but I think has since changed to one of the very similar alternatives.

    John
     
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