I forgot

Discussion in 'General Electronics Chat' started by vineo763, Aug 16, 2006.

  1. vineo763

    Thread Starter Member

    Aug 13, 2006
    Hey, I'm using a 30 MHz Oscilloscope at home. I'm also using a Function generator. I was just curious about the AC voltage readings on the multimeter. If I connect a function generater a 10 V to my oscilloscope, it only reads about 3.63 volts on my multimeter. Why is this?
  2. Gadget

    Distinguished Member

    Jan 10, 2006
    I'm assuming the 10 volts is a Peak to Peak value..(from the top of the + half of the cycle to the bottom of the - half of the cycle). If that is correct, then your meter is out by 0.1 volt.

    Most meters are calibrated to read the RMS value of a sine wave.
    The RMS value of a sine wave is 0.707 of the peak.
    The peak voltage is Half the Peak to Peak voltage.
  3. nomurphy

    AAC Fanatic!

    Aug 8, 2005
    There is an inherent "gotcha" with function generators, in that they are usually calibrated to drive 50 ohms.

    This means that if you drive a high impedance, you will see twice the amplitude that you expect per the setting on the generator.

    For instance, if you want a 0-5V signal into a 10K load from the generator, then set the generator's output for 0-2.5V to get the signal level you actually desire.

    BTW -- if your scope has a "50 ohm" setting, then you could use that to properly load the signal and directly read the voltage.