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Can't trigger on 2 channels simultaneously on oscilloscope =/

  1. Aug 1, 2012 #1
    I'm not sure where to put this so mods feel free to move it as needed.

    I'm working on this oscilloscope trying to measure two signals from the detectors. To extract the signficiant signals, I need to use the trigger mechanism. However, the oscilloscope is not letting me (or I don't know how) apply the trigger to two channels at the same time. It has only been working with one trigger at a time.

    The oscilloscope type that I am using is Tektronix DPO4104. Help would be very appreciated because I've spent many hours not being able to get this to work.
     
    Last edited: Aug 1, 2012
  2. jcsd
  3. Aug 1, 2012 #2
    You can't trigger two signals at once (with some exceptions) on oscilloscopes. The trigger can only come from a single source, either a channel or an external source. Once the scope is triggered, everything, on all channels is recorded.
     
  4. Aug 1, 2012 #3

    berkeman

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    Staff: Mentor

    You can make a single trigger signal that is the combination of the two input signals -- you just have to do that with a separate circuit. What do the two analog signals look like? What kind of combination trigger signal would work for you?

    It might be as simple as using two resistors to make a combination signal, or two diodes to OR the signals, or two comparators to detect separate trigger levels, and then OR the two comparator outputs, etc.
     
  5. Aug 1, 2012 #4
    Oh.. that would explain a lot!

    I'm assuming you are asking what the signals look like on the oscilloscope. It is a negative pulse with sort of a vertical fall followed by a rugged rise on a slope (like a triangle).

    Forgive me if I'm interpreting your second question wrong as well. The combination that I would like is straight forward -- just edge triggering on both channels (with noise reduction coupling), with negative slope. I would like the two trigger levels to differ however, one around 100 mv and the other around 28mv.

    Hmm, simple in practice of course. I am a complete beginner, I would love if you can send me a source that can guide me in this.

    Thanks a lot for your help! :approve:
     
  6. Aug 1, 2012 #5

    uart

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    Hi Nano-P. Normally you only trigger on one channel or the other. If the two signals are synchronized then they will both appear as a stable trace on the scope. If however they are asynchronous (different unrelated frequencies for example), then only the trace you trigger can be stable, and the other one will "dance" around.

    Sometimes in this situation you can use a storage scope with a 'one-shot" trigger to make more sense of what's going on.
     
  7. Aug 1, 2012 #6

    berkeman

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    Good suggestion by uart -- does your 'scope have a memory? Then you can store with one trigger and one-shot later with the other, and superimpose the two waveforms.

    Why do you want to trigger on a combination of the two signals? Are they repetitive, or do they vary? Can you say more about what you are trying to do?

    Worst case you can make a couple of amp/comparator circuits to do your own triggering on the two waveforms, and make the overall combined trigger signal as a combination of the two comparator outputs...
     
  8. Aug 1, 2012 #7
    Uart, I do apologize but I'm not familiar with some of the lingo. And bekerman, I'll expound on what I am doing in the following.

    I have a scintillator set up in connection to a photomultiplier, and I am measuring the output voltage from the photomultiplier. I am looking for a significant signal (ie a certain voltage) to tell me that I have measured a muon particle which interacts with the scintillator.

    Now what happens is, there is too much noise and I can not extract any meaningful information without setting up trigger criterias. I have two different scintillators set up, each with their own photomultiplier connected with BNC cables onto the oscilloscope.

    The signals are not repetitive. There is a lot of noise in the middle and every once in a while (I would estimate once per second) there is a pulse caused by the trigger. I have a attached an image for you to see.

     

    Attached Files:

  9. Aug 1, 2012 #8

    berkeman

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    Wow, that is challenging. Small waveforms and fast timescales. My idea of discrete amps and comparators probably is not practical for this.

    Do you have any other oscilloscopes? Some 'scopes and logic analyzer / scope combination units have a "Trigger Out" BNC port. If you had 2 other scopes with Trigger Outupts, you could logically combine those two trigger signals into one to feed into your primary scope's External Trigger input...
     
  10. Aug 1, 2012 #9
    Yes, that sounds like a good idea. Thanks.
     
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