Oscilloscope Negative Trigger used for?

Discussion in 'General Electronics Chat' started by Danwalker, Oct 2, 2014.

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  1. Danwalker

    Thread Starter New Member

    Oct 2, 2014
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    What is the Negative trigger used for on the Oscilloscope?

    When I'm measuring pulse width, time period of different waveforms the time duration is the same when I switch from the positive trigger to the negative trigger

    So when is the negative trigger used for?
     
  2. Danwalker

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    Oct 2, 2014
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    When would a tech or what kind of application/circuit would you measure A+ to B- or A- to B- time duration?
    look at the time interval chart please
     
  3. alfacliff

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    Dec 13, 2013
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    the negative trigger is used when you want to trigger off the negative edge of a pulse signal. like to measure delay.
     
  4. Danwalker

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    Oct 2, 2014
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    But I get the same result when I use the positive trigger to trigger off the positive edge, it will measure the same time duration

    So when does it make a difference when triggering off the negative edge

    When I measure a TTL or CMOS logic waveforms time period or pulse width, triggering off the positive or negative edge will give me the same result , i don't get anything different when i measure it

    When does it make it a difference? what type of measurement?
     
  5. MrChips

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    If you are measuring the frequency or period of a periodic signal you will get the same result with positive or negative triggering.

    I cannot off the top of my head explain when one is better than the other. Put it aside for now. One of these days it will hit you in the head.
     
  6. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    Suppose you particularly want to look at that part of a waveform where the signal has a negative slope. There may be a "glitch" or something else of interest you wish to investigate around the negative going edge or immediately after the negative transition. Triggering off the negative edge would allow you to expand the time-base [i.e. less time (msec, usec, etc) per division] at and after the trigger point, thereby potentially allowing an even closer inspection of what's happening at the point of interest.
     
  7. crutschow

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    Another example. Suppose you have a slow rep-rate short negative going pulse (say 1us negative width with a 1s period) and you want to measure its width with good accuracy. If you only have a positive edge trigger the negative pulse would just be a little spike at the end of a 1 second sweep, since the sweep starts at the end (positive edge) of the pulse, requiring that the sweep speed be slow to see the next pulse. With a negative edge trigger you can speed up the sweep so its 1us width occupies most of the screen.
     
  8. alfacliff

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    what if the periodic signal yo want to look at after the pulse is close to the pulse and the pulse is long, positive triggering might not see the signal after the pulse. such as looking for a 1 ns pulse after a 10 sec pulse.
     
  9. nsaspook

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    Here is a negative edge trigger (at 2.8 vdc) from the transmit pulse CH1 and the receiver light-link signal CH2 delay (that changes as the connector is moved from the locked position) on a scope.
    https://www.flickr.com/video_download.gne?id=6793934950
     
    Last edited: Oct 3, 2014
  10. KL7AJ

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  11. KL7AJ

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    We used negative triggering all the time in TV, as sync pulses are negative going.
     
  12. Danwalker

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    Oct 2, 2014
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    How can you tell if a waveform is negative going?
     
  13. MrChips

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    When the waveform goes south?
     
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  14. Danwalker

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    Do you mean below the zero crossing , zero volts?

    All waveforms goes south when they swing or cycle
     
  15. Danwalker

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    Oct 2, 2014
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    Some Logic Ic chips are Negative EDGE trigger inputs and others use Positive EDGE trigger inputs

    This is the only time i have used the Negative trigger on the O-scope to measure the Time duration of when the Negative EDGE triggers the IC logic chip

    Logic IC chips Negative Edge Trigger input:
    Channel#1 on input of IC logic chip
    Channel#2 on output of IC logic chip
    O scope external trigger on Negative Trigger to the Negative Edge trigger input on IC logic chip

    Is there any other way of do this or am i doing this wrong to measure the time duration
     
  16. vk6zgo

    Active Member

    Jul 21, 2012
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    Not really,dc offsets are very common.
    A real square wave returns to zero volts between cycles,as does an unfiltered halfwave rectified
    sine wave.
    Of course,it depends on if the square wave is "+ve or -ve going",whether zero occurs in the positive or negative half cycle.(and the polarity of the rectifier in the second case)

    Some waveforms,such as those of analog Composite Video signals,are extremely complex,& have a large dc
    component.
    "Blanking level" is normally "clamped" to zero volts,hence,the actual "video" signal is mostly offset in the positive direction with respect to zero volts,whilst the synchronising pulses are negative.w.r.t. the same reference.
    It is not easy to trigger on a constantly varying signal,so negative triggering can be used to trigger on the syncs.
     
  17. Danwalker

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    Oct 2, 2014
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    What you mean by it's not easy to trigger on a constantly varying signal? Negative trigger is better compared to positive trigger?

    Clamped how so? what is clamping it to zero volts?
     
  18. vk6zgo

    Active Member

    Jul 21, 2012
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    The "picture information" part of the video signal changes constantly,following the variations of the scene being
    televised.
    For this reason,the signal may not reach a particular trigger point on each,say, line interval,or it may reach that point at a different time.
    In both cases the display becomes unstable.

    Not always--sometimes,a Composite video signal may be inverted in polarity within a device,while still retaining a dc offset.
    In that case,you would probably use positive triggering,& trigger on the now positive going syncs.

    In analog TV,a special circuit called a "clamp" circuit is used.
    The most effective method is known as a "keyed clamper."

    This uses the sync pulses to generate a narrower pulse which is delayed until the'back porch" (that's the bit where the colour burst is) of the video line signal.
    It is then used to operate an Electronic switch which connects the back porch to zero volts.
    In B&W TV that's all there is to it,but with colour,you need to clamp the dc component without destroying the colour burst,so it gets a bit messier.

    That said,at times you will find video signals which are ac coupled,losing their dc component.
    In that case,you use whichever form of triggering is most effective.
     
  19. ian field

    Distinguished Member

    Oct 27, 2012
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    A good demonstration is to sync a squarewave signal and flick the positive/negative switch.

    As the scope changes from triggering on a positive transition to a negative one, the trace will lead with a low level rather than a high one.
     
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