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Universal Counter is not stable with a load why?

Discussion in 'General Electronics Discussion' started by Davewalker5, Sep 25, 2014.

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  1. Davewalker5

    Davewalker5

    125
    0
    Sep 20, 2014
    I use these Universal counters
    HP5315B Universal Counter
    HP5334B Universal Counter

    I'm using a Ling Function generator for a waveform generator
    Sine waveform at low frequencies

    I use the Universal counter to display a digital readout of what the frequency is of the sine waveform

    When I plug in the function generators output to the Universal counters input and it displays the frequency

    The problem I'm having is when I plug a load or to a circuits input ( board under test ) it makes the universal counter not stable, the display is jumping around and won't get stable

    Any reason why?

    I tried adjusting the Universal counters input trigger and sensitivity

    The only thing that makes the universal counter stable, is when I unplug it from t the circuit board under test when is loading the universal counters input?
     
  2. Davewalker5

    Davewalker5

    125
    0
    Sep 20, 2014
    Why doesn't this circuit work in square waveform? only works in sine waveform

    The frequency is 10 hz input

    Is Capacitor C13 blocking the low frequency Square waveform?

    Is that why the circuit works only with a sine waveform

    What does U1 , Q7, Q8 do? is this a draining off the voltage circuit, any reason to use a Logic Gate?
     

    Attached Files:

  3. Harald Kapp

    Harald Kapp Moderator Moderator

    10,309
    2,237
    Nov 17, 2011
    What do you mean by "it doesn't work"? What do you expect to happen/see? What does happen, what do you see instead? What frequency do you use?

    U1, Q6-Q8 could be a peak detector (for the output of U4, pin 14) which turns on or off (Q6) a current source (Q7, Q8). Since the rightmost part of the schematix is missing, I can't say more.
    A logic gate may have been used to control the current source from the logical AND of the peak signal and a separate control signal. This is not good practice, but can be seen sometimes, especially with CD4xxx components.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    10,309
    2,237
    Nov 17, 2011
    Measure the output voltage of the function generator without and with load. A function generator typically has an output impedance of 50 Ohm. When loaded, the output voltage will drop. Do you have acccess to an oscilloscope to observe the waveform of the function generator?

    Another possibility is hum (50 Hz or 60 Hz depending on your mains frequency) introduced e.g. by a ground loop (protective earth connection of the different devices used in the setup). DO you have access to an insulating mains/mains transformer? using this on one of the instruments can break a ground loop.
     
  5. davenn

    davenn Moderator

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    1,884
    Sep 5, 2009
    the 2 threads were directly related ... I have therefore merged them

    Dave
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    10,309
    2,237
    Nov 17, 2011
    I've seen both but thought the ycould stand each for himself since they deal with different issues. But it's o.k.

    Harald
     
  7. Davewalker5

    Davewalker5

    125
    0
    Sep 20, 2014
    Yes on the O scope , the frequency and voltage is good

    The problem is that the Universal counter display is not stable, displaying the frequency is jumping around random only when it's loaded or hooked up to the circuit , under test
     
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