AC coupling and 50ohm impeance.

Discussion in 'General Electronics Chat' started by Justin Junghee Kim, Sep 25, 2015.

  1. Justin Junghee Kim

    Thread Starter New Member

    Sep 25, 2015
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    Hi all,

    I'm developing a circuit that takes ultrasonic input and transmit it to the receiver. I'm using a video transmitter to cover ultrasonic range (my target range is 40Khz) The receiver is connected to an oscilloscope for reading through BNC cable.


    When I read the output of the receiver, the oscilloscope input setting was AC coupling with 50oHm impedance. When I choose this setting, the osccliscope display the message saying :

    The combination of AC and 50 ohm produces a very high lower frequency limit.
    The table shows the maximum lower frequency cutoff for all coupling/impedance combinations.
    1Mohm 50ohm
    AC 10 hz 200khz
    DC 0 hz 0hz
    - Message end -

    In this setting, the output of the receiver is similar to input signal of transmitter with gain less than 1. It is shown figure 1. below.
    20150925_131948.jpg figure 1.

    However when I connect this to DAQ (AC coupling, and high input impedance), then the output looks like figure 2. below. The output has lots of gain and couple with the DC voltage(I think).. The output data through DAQ is same as oscilloscope reading with AC coupling with 1M ohm impedance setting ( or DC coupling with 50 ohm or DC coupling with 1M ohm setting. They are same)

    20150925_131931.jpg figure 2.

    I'd like to design an extra circuit that would be placed between output of the receiver and input of the DAQ, and works like AC and 50ohm setting..

    SO far, I tried changing input impedance by placing resister in parallel when the oscilloscope setting is 1M ohm. However, it doesnt work. It only changes gain of the output of the receiver...

    Please let me know if you have any idea to design an additional function between DAQ and the receiver (I guess filter function)

    Best Regard

    Justin
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Try the scope in DC coupling.
     
  3. Papabravo

    Expert

    Feb 24, 2006
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    That's because 50Ω AC coupling is normally used for low-level RF signals.
     
  4. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the scope is probably 1meg ohm input ut a 50 ohm resistor across the input to just read the waveform across the resistor. might have to use a coupling cap to block any dc present to prevent heating of resistor.
     
    Justin Junghee Kim likes this.
  5. recklessrog

    Member

    May 23, 2013
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    Using the 50 ohm input usually puts a limit on the maximum input level, sometimes as low as 5v rms. you should really be sure of what you are feeding into your scope with an unknown signal by always starting with with a 10:1 probe and set the attenuator to its highest setting then switch down until a usable trace is visable. I think you should refer to the user manual that came with the instrument. 50 ohm input is a specialised application usually reserved for specific R.F testing although there are many other scenarios where it's use may be applicable.
    There are many good reference guides on the internet, Tektronics and several other scope manufacturers have tuition guides downloadable for free. The manual for my Phillips/Fluke PM3394 has a whole chapter dedicated to just this subject, and that is to inform "experienced" engineers! ( I'm not suggesting for one moment that you are not experienced, but many beginners will be reading these posts so it's for general info too)
     
  6. Justin Junghee Kim

    Thread Starter New Member

    Sep 25, 2015
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    Thanks for replying. I don't think DC coupling would work.
     
  7. Justin Junghee Kim

    Thread Starter New Member

    Sep 25, 2015
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    Thanks for replying it.
    I don't think DC coupling works though.

    Right, and I'm using 40kHz. It is very low. I'm interested in how to get rid of DC coupling when I'm not using an oscilloscope for reading an output.

    I should try this.

    Yes... Hope that I can find it.. My scope is kind of old...
     
  8. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    one dumb question:

    Are you using a 10:1 probe? Do you know what frequency compensation is for said probe?

    Put the question another way: Do you know how to use or what the little screw does on the 10:1 probe?
    It can be on the body or on the probe itself?

    Ac coupling does what at the expense of what?

    I can't count today.
     
  9. Lestraveled

    Well-Known Member

    May 19, 2014
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    1,215
    KISS, the OP says he is connecting the signal directly to the BNC input connector of the scope.

    Justin
    If you want to run AC coupled into 50 ohm input at 40KHz, I suggest that make your own AC isolator. Take two BNC connectors (male/female), connect the shields together and connect the center conductors together through a 1 uF cap. The Xc of a 1 uF cap is 4 ohms at 40 KHz. Set the coupling of your scope to 50 ohms DC.
     
  10. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Convince me that 50 ohms is needed. You MIGHT have to terminate the line driver/cable in the characteristic Z.

    I'm suggesting to add a 10:1 probe and compensate the probe properly, so the edges are square. The input Z then goes to 10 Meg as well.

    I think the inability to use the scope is messing up the understanding.
     
  11. Papabravo

    Expert

    Feb 24, 2006
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    I did not use the term "low-level" with respect to frequency. Rather, it was a reference to the power of the signal. At the input of a typical receiver the power level is typically on the order of -80 dBm to -120 dBm.
     
  12. Justin Junghee Kim

    Thread Starter New Member

    Sep 25, 2015
    4
    0
    Thank you all.. I found a solution now.
    I realized from some of answers that I may not state my question clearly.

    The goal is, I want to design a circuit that will be placed between output of receiver and input of DAQ. This circuit must filter DC signal.
    And my question is.. What is 50ohm and AC coupling setting? The answer is: The oscilloscope adds internal function (I realized it was a differentiator) so it filter out all funky looking DC signal, shown in figure 2 from my original post.
    However, the oscilloscope is test device. It is very limited to take large number of data. Thus I'm not going to place between the receiver and DAQ, and it is not possible. However, I used oscilloscope to demonstrate the situation(DC coupling between receiver and DAQ), and try to find a solution.

    I tested with simple second order differentiator and it worked. However, the output amplitude dropped, but it can be boosted by amp.

    Justin
     
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