Frequency measurement on dvm

Discussion in 'General Electronics Chat' started by vpoko, Jan 8, 2012.

  1. vpoko

    Thread Starter Member

    Jan 5, 2012
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    47
    I have a Velleman DVM890F multimeter and was playing around with the frequency measuring function today. To my surprise, I found that it worked if I used just one probe, whether the red or black. This is probably perfectly reasonable to everyone else, but I don't understand how can it detect the frequency without a complete circuit. How does it work? Thanks in advance!
     
  2. hgmjr

    Moderator

    Jan 28, 2005
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    What frequency were you measuring?

    hgmjr
     
  3. vpoko

    Thread Starter Member

    Jan 5, 2012
    258
    47
    A uC switching a pin high and low with various delays. The meter says its range is 10-20,000 Hz, but I didn't measure through the whole range.
     
  4. Adjuster

    Well-Known Member

    Dec 26, 2010
    2,147
    300
    Perhaps the disconnected side had enough stray capacitance to ground to give a bit of input voltage. The input impedance of such a meter would be at least 1MΩ, and probably 10MΩ in parallel with maybe 20pF.

    Depending in the input frequency, and the meter's sensitivity, the capacitance of the dangling lead to ground might do it, or even the wiring in the meter might.
     
  5. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    One thing is for sure, you will need to use both leads in order to yield the most accurate frequency measurement.

    hgmjr
     
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