O-scope External Input trigger used for?

Discussion in 'General Electronics Chat' started by DexterMccoy, Feb 21, 2014.

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  1. DexterMccoy

    Thread Starter New Member

    Feb 19, 2014
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    What is the O-scope External Input trigger used for?

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

    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
     
  2. BillB3857

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    Feb 28, 2009
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    Trigger off of any event that is the cause of other events you want to look at.
     
  3. DexterMccoy

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    But what events? can you think of any events that i would trigger off of? so i can learn from please?
     
  4. spinnaker

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    Oct 29, 2009
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    A clock pulse for one.
     
  5. MrChips

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    External Trigger Input has over 101 uses and applications I really wouldn't know where to begin to explain.

    How and when to use External Trigger?

    Use your scope as you would normally. Use both Channel 1 and Channel 2.
    If you cannot find a use for External Trigger then you don't need it.
    You will eventually figure out when you need it.
    Then you will be glad that the scope has it.
     
  6. BillB3857

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    Suppose you want to look at a composite video signal. Sync off of the horizontal sync pulse to see a line of video and sync off of the vertical sync pulse to see a frame of video,
     
  7. tindel

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    Another reason I use the external trigger is to measure the noise on the output of a switching regulator. Say you only have two inputs on your scope. Channel A is the output of your power supply with 50mV of switching noise. Channel B - the same output with an additional LC filter (with 20dB of attenuation) has 5mV of switching noise. It's VERY hard to get a good trigger on a 50mV signal.

    I will use the the switch voltage or an EMI 'sniffer' to trigger the external trigger so that I can see the unfiltered and filtered output of the power supply at the same time to verify the attenuation of the LC filter.

    This setup allows you to trigger on a large change in voltage so that you can see very small voltages without fighting with your scope to trigger on a 50mV signal.
     
  8. bountyhunter

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    It was used a lot in television service back in the old CRT days. You would trigger off the horizontal or vertical pulse and look at waveforms that track those .
     
  9. ErnieM

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    Let's assume you have a basic two channel analog scope, the mainstay of the industry for decades.

    Now you want to see some interesting signals that occur after some standard (and hence boring) signal. After you've seen and understood the standard boring signal you can use that as the external trigger signal to sych to other, perhaps more interesting signals.
     
  10. DexterMccoy

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    Feb 19, 2014
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    But what signals would you use as a sync signal to inject into the external trigger on the O-scope

    Can you please list them, so I can learn from them?

    Anybody can just list what they have used the external trigger for in their experience as in testing boards , circuits or troubleshooting
     
  11. BillB3857

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    It is really hard to describe the use of an external trigger until you want to sync a two channel display with a signal that is not going to be displayed on either channel. Need and experience will provide the answers.
     
  12. JoeJester

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    My simple rules of thumb are ...

    Synchronous to line ... use the line trigger

    Synchronous to a clock .... use the external trigger. + if the event occurs on the positive going edge, - if the event is on the negative going edge.

    looking at a single signal, internal (channel a if using ch A) + or - edge.

    Looking at phase delays on line frequency ... input on channel A, trigger channel A. Phase delayed signal on channel B

    You really need to have an idea of what you are looking at and when it occurs. Then set the oscilloscope channels and trigger appropriately. Came in handy when I was looking for a 150 nS pulse that occurred on a negative edge of a clock pulse at 5.02008 Hz. I triggered the clock on channel A positive edge and viewed channel B, using B delayed sweep, viewed the pulse right where the negative transition took place.
     
  13. ErnieM

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    Again you think there is some hard and fast set of rules to follow.

    Don't over think these things, the external trigger is another technique to put into your bag of tricks for the day when you want to observe something about 3 signals when you only have a 2 channel scope (or 5 signals on a 4 channel scope).
     
  14. MrChips

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    or a signal that is too low in amplitude to see,
    or buried in noise,
    or is early in time,
    or is late in time.

    As I said already, if you don't see a need for external trigger then you don't need it.
     
  15. DexterMccoy

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    Feb 19, 2014
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    How did u trigger the clock?

    So you have a clock signal on O scope channel#1? or just one clock pulse positive edge?

    Why do you want to view the pulse of the negative transition? what are you checking or looking for?

    So how would you use the O-scopes external trigger input for these tests please?

    When you have an RC early in time or late in time? is this what you mean? but how would you use the O-scopes external trigger for this?

    How to use an O-scope external trigger when signals are low in amplitude or buried in noise?
     
  16. tindel

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    strantor likes this.
  17. davebee

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    There is no one answer to this question, because the external trigger can be used on millions of different types of signals. That is why scopes often offer to trigger on either the rising transition or the falling transition, to trigger at different amplitudes of the trigger signal, or to hold off triggering until some delay passes after the trigger signal is detected; to give lots of choices for how and when to trigger.

    Using the trigger takes some thought. You have a signal you want to display, so you try to figure out what sort of other signal can be used to start the scope trace moving at the best time to display the interesting signal.

    It is not always even possible to find a part of the signal to trigger from to capture some kinds of events. Sometimes the only solution is to continuously record the incoming signal into some kind of storage, then stop recording after the event that you're interested in has arrived.
     
  18. DexterMccoy

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    Feb 19, 2014
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    Thank you for the video, I think i understand it somewhat

    When using the external trigger input to test timing circuits that occur earlier in time or occur later in time, which is a problem with timing circuit

    How do you use the external trigger input for this type of problems?
     
  19. MrChips

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    I suggest you wait until you have more experience using an oscilloscope.
    When you need external triggering you will realize it.
     
  20. tindel

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    Now your talking about something completely different than the external trigger.

    Now you're talking about a sweep delay. A sweep delay is where a trigger is received by the scope, but the scope waits (or delays) some amount of time before sweeping the beam across the screen.

    Not all analog scopes have a delay... but many do. All digital scopes have a delay, that I'm aware of. Generally speaking, things that happen prior to the trigger are unimportant. Digital scope do let you have a bit of a 'pretrigger' though to make sure that you get the entire edge of the triggering event, something that is difficult with a analog scope without knowing something about the circuit being measured.

    I do have a circuit (based on a 556) that I use occasionally to pretrigger on my analog scopes, but again, it requires some knowledge of the circuit being tested to use properly.
     
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