Impedance matching

Discussion in 'General Electronics Chat' started by Johnny1010, Jul 16, 2014.

  1. Johnny1010

    Thread Starter Member

    Jul 13, 2014
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    Hello everyone,
    I am facing few problems understanding the impedance matching. I have made a few concepts reading on internet please correct where wrong plus sorry for the long question:

    1)If I use the probe at 10x to measure output of my amplifier this would be the circuit fig_1 and I would get correct result as the impedance is matched.

    2)Now instead of using a probe when I place a bnc female at the output of amplifier and use a coax to connect it to oscilloscope I get this circuit fig_2
    Now this would work at low frequencies but at very high frequencies the impedance would decrease and thus we won't get the desired result.

    3)But when I connect the output from a signal generator to the oscilloscope directly using a coax (creating the scenario in case 2) increasing frequency should show deterioration in the signal but this doesn't happen I get the same results as I should with the probe.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    I don't quite understand your question.

    The purpose of the oscilloscope probe is not for matching impedance.

    You want the impedance of the scope/probe to be much higher than that of the circuit under test so as to minimize loading on the circuit.

    That is the main reason why it is preferable to use the x10 probe.

    The second reason for using the x10 probe is that it increases the effective bandwidth of the oscilloscope.
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    oaciliscope inputs are generally around 1 meg ohm. the only effets would be the cable capacitance. the 10 x probes reduce loading on the circuit under test by isolating the scope from the circuit under test, effectivly , the tip of the probe is all the circuit seesl
     
  4. Johnny1010

    Thread Starter Member

    Jul 13, 2014
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    Yes 10x would decrease the loading on the circuit but still we want the internal impedance of the test probe to be equal to the internal impedance of oscilloscope. Is it correct?
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Why do you want the impedances to match?
     
  6. Johnny1010

    Thread Starter Member

    Jul 13, 2014
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  7. MrChips

    Moderator

    Oct 2, 2009
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    You are applying impedance matching in a different context.

    1) You match impedance for maximum power transfer,

    2) to prevent reflections in transmission lines.

    In the case with a scope probe, the purpose of the x10 probe is to increase the load impedance and to flatten the frequency response of the probe + scope combination.
     
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  8. Johnny1010

    Thread Starter Member

    Jul 13, 2014
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    Now instead of measuring the output of an amplifier with a probe I have soldered a bnc female at the output and I use a coax cable to connect the output directly to the oscilloscope. Now in this case the results are different from those obtained through a probe. What could be the reason for this?
     
  9. crutschow

    Expert

    Mar 14, 2008
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    In your first post, para 3), you said there was no change is measured signal with the BNC-coax, now you say you do see a change. Which is it? :confused:

    If there is a change, the reason you gave in para 2) is valid. It depends upon the relative output impedance of the circuit as compared to the impedance of the coax and oscilloscope. Even the small impedance of a 10:1 probe can generate an error if the frequency is high enough that the probe impedance loads the circuit impedance.
     
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  10. alfacliff

    Well-Known Member

    Dec 13, 2013
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    you are supposed to be measuring the power across the load, the osciloscope is not tbe load, its test measurement equipment.
     
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  11. Johnny1010

    Thread Starter Member

    Jul 13, 2014
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    @crutschow that is exactly my point that when I use the bnc-coax combination for connecting the function generator to oscilloscope there should be changes at high frequency
    ( just as the changes I see when I use the same combination at the output of my amplifier)
    but there aren't any.
    Also in order to remove the changes in this bnc-coax connection if I myself use a
    [ 10M(resistor)||C(capacitor) ] combination
    in series to this connection would that resolve the issue.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    Now I understand my confusion. You are mixing apples and oranges. :rolleyes:

    The function generator has a low impedance (likely 50Ω output) so it is designed to match the low impedance of a coax cable. Your circuit likely has a higher impedance so it is affected by the coax load. It's all a matter of relative impedances.

    You could use an RC at the input (not output) of the coax to simulate a 10:1 probe, but what would be the purpose of that? :confused:
     
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  13. Johnny1010

    Thread Starter Member

    Jul 13, 2014
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    Thanks crustchow that makes complete sense you are a life saver :). Also just wanted to know that RC thing for my understanding. Now I have realized that I had got the concept of impedance matching all wrong. Now if you could please explain what impedance matching is what is 50 ohm termination when are these things applicable and when not. I would be really thankful.
     
  14. crutschow

    Expert

    Mar 14, 2008
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    Impedance matching is used for two reasons:

    One is to get the maximum power output from a circuit (typically RF transmitters). Normally this is not a consideration in most other types circuits. For example solid-state audio power amps have a much lower output impedance than the speaker load.

    The other reason is to match a transmission line impedance to minimize reflections and standing waves in the line. For example coax lines are typically 50Ω or 75Ω so you want either the load or source (and sometimes both) to match this impedance. This becomes important when the wavelength of the highest signal frequency is shorter than about 1/10th the length of the line or the pulse risetime is shorter than about 1/4 of the propagation delay of the wire. Thus, for example, you don't worry about matching for typical lengths of audio cable.
     
    Last edited: Jul 16, 2014
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