Oscilloscope input impedance

Discussion in 'General Electronics Chat' started by cx1111, Nov 4, 2013.

  1. cx1111

    Thread Starter New Member

    Nov 4, 2013

    I have a project in which I need to detect the microbubble response to ultrasound excitation. The waves which are to be generated and therefore measured will range from 1MHz-20MHz.

    I have an old oscilloscope that only has a 1M ohm 20pF input impedance, and I am aware that high frequency signals require a 50 ohm input impedance.

    Is there any way I can use this scope for my project by using a probe or something to bypass this impedance problem? Also, any basic info on how the input impedance of the scope affects the circuit/signal would be appreciated.

    Thank You
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    If you have 10X probe for that scope, the input impedance through that will be 10M and 2pF, but you will reduce sensitivity by 10.

    It is not hard to build a "preamp" to put between your experiment and the scope input.

    You will have to specify the input impedance, gain, noise level, and frequency response required.
  3. bertus


    Apr 5, 2008

    If your scope does not have the possibility to switch the input to 50 Ohms, you could make a 50 Ohms pass through box.
    I have made one with 2 100 Ohms resistors parallel in a box with BNC connectors, like in the drawing I made:


    I made the box to use in stead of the burned 50 Ohms resistor in my scope.

  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    I suspect the OP's microbubbles will not be very happy if connected directly to 50Ω. :(

    I'm guessing the OP will need a preamp located at the experiment that will has an input impedance >100KΩ and then will need 50Ω cable to get to the scope input. Likely, the OP's scope has a switch on it which configures the input to 50Ω input impedance. (Mine does).

    The preamp may not need voltage gain; rather it could be like a emitter/source follower; high input Z and low output Z.
  5. vk6zgo

    Active Member

    Jul 21, 2012
    50Ω ls more of a specification,than anything inherent in HF signals,allowing
    standard interfaces between test equipment (including 'scopes in the 1Ghz & above class)& the DUT.

    It is not commonly used with 'scopes at around 1-20MHz.

    If you really wanted 50Ω in this frequency range,you can use external terminators.

    What is the output impedance of the detector circuit for your microbubbles?

    If it is very much less than the 'scope input impedance,you can use a plain x1 probe,with no major drama.

    Most people would use a x10 probe which will load the circuit less,but reduces the sensitivity of the overall system.