output impedance

Discussion in 'General Electronics Chat' started by gerases, Feb 24, 2013.

  1. gerases

    Thread Starter Member

    Oct 29, 2012
    Newbie question.

    I read about output impedance here: http://www.kpsec.freeuk.com/imped.htm

    Practical example. I'm looking for a function generator. One I found has an output impedance of 50 ohms and runs on 15V DC voltage. 50 ohms is pretty low, which is good for output impedance because it means little input voltage is lost and lots of it available for the input of another device.

    Besides the fact that the device connected to the output should have an impedance of at least 10 times 50, what other information can we derive from the fact that the output impedance is 50 ohms?

    For example, if we just know the input voltage and output impedance, can we say how much of the input voltage will be left at the output with no load connected? In the case of the function generator, the amplitude range is 10 volts peak-to-peak. Are those two facts related - i.e. peak-to-peak output voltage and output impedance? Can I measure output impedance somehow?

    Sorry if the question is too vague.
  2. MrChips


    Oct 2, 2009
    50Ω output impedance is very common because this allows you to use a 50Ω coax cable to make connections.

    Why do you want to measure the output impedance?
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    The "Voltage Divider Method" is a quick way to determine output impedance.

    Online Calculator

    Rough overview, measure open circuit voltage beween + and - on signal Generator set to sinewave. Say it reads 10V on DMM or scope. Put a 50Ω resistor across + and - outputs of signal generator, if output impedance is 50Ω, your DMM or scope should show 5V across the resistor (½ the open circuit voltage).

    50Ω is common, though there are some odd ones around that have a 600Ω output impedance.
  4. crutschow


    Mar 14, 2008
    To preserve the high frequency content of the output signal, such as the edges of square-waves, without distortion or ringing, the cable and load impedance need to be matched to the 50 ohm output impedance of the generator. For low frequency signals you may not need that match. Here's some further info on impedance matching.