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Discussion in 'General Electronics Discussion' started by Chr1s, Feb 23, 2017.

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  1. Chr1s


    Dec 20, 2016
    On my scope i notice that i have 1Mohm, 13pF termination to Ground and from the menu i can change this to 50ohm?

    Coud someone explain when you would use the 50ohm termination setting and also what is the significance of the capacitance?

    Your help is much appreciated.

  2. duke37


    Jan 9, 2011
    The capacitance will affect high frequency measurements. Recently I was looking at some intermediate frequency transformers running at a modest 455kHz. They had 100pF capacitors in them. Adding 13pF would drop the resonant frequency significantly.

    50Ω is the most common impedance used in high frequency circuits and most co-axial cables are of this impedance. Terminating the cable or equipment in 50Ω means that there will be no reflections (no standing wave). A violin or geetah has a string which is twanged, the pulse runs up to the end and is reflected back to make the instrument resonant. Damping with a cloth absorbs the energy so there is no or little resonance. Putting energy into a 50Ω cable and terminating it with a 50Ω aerial (antenna) means that all the energy is transmitted.

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Sometimes datasheets for transistors and other active components include example circuits of pulse tests. Many of these examples use a 50Ω load. Your 50Ω input is useful for measurements like these. A 50Ω coax cable is used in place of your probe and cable. Prior to scope manufacturers providing the 50Ω option we had to use a coax with a 'T' connector connected to the scope's BNC. One port of the 'T' had a 50Ω termination connected to it.


    (*steve*) likes this.
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Some scopes I have that we once very expensive have selectable input impedance. The schematic shows that the 50Ω impedance is produced by simply switching a 50Ω resistor across the input.

    Chris' suggestion above is perfectly acceptable.

    Be aware that 50Ω inputs can be easily damaged by even relatively low voltage inputs. For example, if a 1/2W 50Ω resistor is used (and this is pretty typical) then it will be called on to dissipate 1/2W when the input reaches 5V.

    CDRIVE Hauling 10' pipe on a Trek Shift3

    May 8, 2012
    Oh yeah that's a damn important point! I imagine there are DUT's that wouldn't like it either. :)

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