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O-Scope Ext. Trigger used for?

Discussion in 'General Electronics Discussion' started by Simmon, Jan 30, 2014.

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  1. Simmon

    Simmon

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    Jan 30, 2014
    What is the O-scope External Input trigger used for?

    What can I use for an input to the O-scope external input trigger?

    A Trip point
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,439
    2,627
    Nov 17, 2011
    Welcome to the forum.

    An external trigger is used if the measured signal does not contain a signature that is suitable for triggering.
    Exampel: you want to display the voltage at the output of a conventional scr based dimmer. If you use zero crossing from the signal as a trigger, you will see e.g. the rising edge of the voltage centered on the trigger time. As you dim the signal, the edge will stay centered, but the 50Hz (or 60Hz) signal will shift in phase across teh screen.
    Typically you'd like to see how the phase of the dimmed signal shifts. You'd use an undimmed version of the 50Hz (60Hz) voltage to trigger the scop. The original zero crossing is now centered on the triger (usually center of the screen), baut the dimmed siganl will show how the edge of the trimmed signal shifts with respect to the zero crossing of the original sine.

    You can use a second scope channel for that purpose, but sometimes you have no spare channel left and are happy to have an external trigger.

    What you can use as input to the external trigger depends on how it is specified. See the scope manual.
     
  3. Simmon

    Simmon

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    Jan 30, 2014
    What do you mean by the signal doesn't contain a signature?

    What kind of signals is used for external triggering?

    How can you use zero volts as the trigger?

    How do I set up the O scope external trigger input to trigger on the zero crossing? I'm not sure how to do this
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,490
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    Jan 21, 2010
    Typically the scope will trigger on the signal rising or falling through a voltage. This is fine for a simple repeating waveform, but not for something more complex.

    I'm glad you asked :D

    An example might be a complex waveform such as a burst of a sine wave followed by a pause, repeating... If you just trigger on the waveform rising,you might trigger anywhere in the signal. This will result in you seeing jitter. If you can trigger on another signal which preceeds the burst,then it will look stable on the scope.

    A classical use is with analog TVs to display signals synced to the vertical or horizontal oscillator.

    you normally have an adjustment to set the trigger level. You just set it to 0. It can also be negative.

    This depends a lot on your scope. Can you tell us what it is?
     
  5. Simmon

    Simmon

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    Jan 30, 2014
    Tektronix 2221 and Tektronix 2212

    Can you list please what basic signals or circuit when using the ext. trigger input for testing or troubleshooting a circuit?

    What signals have you triggered off of?

    Logic Enable signals?
    Logic Clock signals?
    Logic Reset Signals?
    Comparator trip points? or outputs switched
     
  6. Simmon

    Simmon

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    Jan 30, 2014
    For digital logic circuits what can I use the external trigger for?
     
  7. Simmon

    Simmon

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    Jan 30, 2014
    Tindel

    What can I use as a trigger source or signal for the external trigger on the O-scope , when testing digital logic circuits?

    Trigger signals:
    1.) Reset signal, put the external trigger on the reset signal buss
    2.) Clock Signal, put the external trigger on the clock signal buss
    3.) AC Line Frequency, put the external trigger on the AC main line, to sync up the AC line frequency with the waveforms on channels on the O-scope
    4.) Video Horizontal or vertical signal, to sync up the waveforms you want to view/monitor on channels on the O-scope
    5.) Comparators outputs, when the comparator switches logic states on the output pin, this can be your trigger to monitor that event or stage of the circuits
    6.) Analog or Digital On/off switch, when you press or switch an analog switch to turn on or off a function in a circuit, this can be your trigger signal

    7.) What can you guys add please?
     
  8. gorgon

    gorgon

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    Jun 6, 2011
    The external trigger is normally used when you want to trigger on a signal you are not looking at on your normal input channels.

    Any signal that is inside the electrical spcifications of the external trigger input can be used as a trigger source.

    There is no fixed rule for what you can use as a trigger. All depends on the actual circuit you test at the moment. You use the signal that will bring forward the correct picture of the signals you want to look at, whatever they are.
     
  9. Simmon

    Simmon

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    Jan 30, 2014
    Gorgon do you have any examples from your experience of using the external trigger input?
     
  10. shumifan50

    shumifan50

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    Jan 16, 2014
    A simple description:
    You use the trigger to start the trace when the trigger fires. This allows not displaying the trace until the trigger happens.

    A very simple example could be using RTS (request to send) signal on a RS232 link as the trigger to display the CTS(clear to send) line. Then you can measure the RTS to CTS delay.

    The previous posts explain what an external trigger does quite plainly. How you use this information is up to you. Do not expect people on this forum to do all your thinking for you. Read the responses carefully, make sure you understand them, if not read again. Then apply your understanding to specific problem solving.
     
    Last edited: Feb 27, 2014
  11. Simmon

    Simmon

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    0
    Jan 30, 2014
    Can't you see the CTS signal without using the RTS signal as the trigger?

    When using the RTS signal as the trigger , what does it do different than not having the RTS signal as the trigger?

    The CTS signal will not display on the O-scope? unless you have the RTS signal as the trigger?
     
  12. shumifan50

    shumifan50

    567
    56
    Jan 16, 2014
    You are wasting time and effort = read what is written.
    Try it and see what happens.
     
    Last edited: Feb 27, 2014
  13. gorgon

    gorgon

    603
    24
    Jun 6, 2011
    More advanced scopes like the TEK2221a also allow pre/mid/post-triggering so you can see what happend before and/or after the trigger activated.

    The mid- and post-triggering is very useful when you debug code in microprocessor circuits. You can trigger on an activation signal in a piece of code, address decoder or whatever, and see what made this happen in the real world.

    Use your imagination, this is a tool to use when you need an extra channel in your scope, for some reason. There is no exact answer to when to use the resources you have at hand. The important thing is to have an idea of how to help yourself when the need is present in your work or study.

    The unexpected is always unexpectedish :D
     
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