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Negative Trigger mode on O-scope used for?

Davewalker5

Sep 20, 2014
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When do you use the Negative trigger mode on the O-scope? and what is the negative triggering used for?

When I'm measuring TTL or CMOS Logic signals, the positive or negative trigger gives me the same result, doesn't change anything.

The pulse width, time period, time duration's all have the same result when triggering either positive or negative triggering
 

KrisBlueNZ

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Positive or negative triggering affects whether the scope triggers on the rising (low-voltage to high-voltage) or falling (high-voltage to low-voltage) transitions on the input signal. If you measure a sinewave, from, say, a power transformer, set the trigger level half way between the top and bottom peaks, and change the positive/negative switch, you'll see the difference.
 

Davewalker5

Sep 20, 2014
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Positive or negative triggering affects whether the scope triggers on the rising (low-voltage to high-voltage) or falling (high-voltage to low-voltage) transitions on the input signal.

True, but when do you use the negative trigger , for what kind of measurement?

you measure a sinewave, from, say, a power transformer, set the trigger level half way between the top and bottom peaks, and change the positive/negative switch, you'll see the difference.

But the time period will be the same result in either positive or negative trigger, what measurement will make it different?
 

KrisBlueNZ

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You use negative triggering when the event you want to see on your scope occurs just after the falling edge of the waveform you're triggering from.
 

Davewalker5

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You use negative triggering when the event you want to see on your scope occurs just after the falling edge of the waveform you're triggering from.

Can you please give some examples of when you have an event like this? what kind of measurements or waveforms that has this type of event

is this called a negative event?
 

KrisBlueNZ

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No. I've never heard of "negative event". I imagine it would be a party for depressives.

There would be lots of cases where you would want to trigger on the falling part of a waveform. For example if you want to view zero-crossings on a sinewave, such as the mains voltage, you would set the trigger at a few volts positive with negative direction and the scope would trigger just before the point where the sinewave crosses from positive to negative.
 

Davewalker5

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There would be lots of cases where you would want to trigger on the falling part of a waveform.

can you please give more examples, so i can learn from
 

KrisBlueNZ

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You'll know when you need to use it. Whenever the part of the waveform you want to look at closely is near to a falling edge on the signal you're triggering from.

You can vary the timebase to zoom in on a particular part of the trace, but the scope will always capture the waveform relative to the trigger time. So if the part of the waveform you want to zoom in on is near a falling edge on the signal you're triggering from, you use falling edge triggering. If the part of the waveform you want to zoom in on is near a rising edge on the signal you're triggering from, you use rising edge triggering.
 

Davewalker5

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zooming in is called window zones on digital O-scopes

but the scope will always capture the waveform relative to the trigger time

yes on Digital O-scope, they have a trigger event frequency read out

But the trigger event frequency read out gives the same result in either positive or negative trigger modes

The Only time the Trigger event frequency changes is when using the external trigger input and have it sync to a different frequency
 

KrisBlueNZ

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That's not what I mean by zooming in. I'm talking about changing the timebase.

Yes, the trigger event frequency display will give you the same number for both positive and negative triggers. This is because the signal must cross the trigger voltage threshold in the falling direction the same number of times as it crosses the trigger voltage threshold in the rising direction. The difference is the point in the waveform where the scope triggers.

Try this. Connect a transformer to the AC mains and connect the scope across the secondary. Set the trigger voltage to 0V. Adjust the timebase until half a cycle is displayed on the screen. Now change between rising and falling edge triggering. You will see the difference that the setting makes.
 

Colin Mitchell

Aug 31, 2014
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Basically the trigger control allows you to adjust the horizontal position of one waveform on the screen so you can match it up or compare it with another waveform. (dual trace situation)
 
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