Oscope Negative & Positive trigger used for?

[stanleystan]

When measuring waveforms or signals , when do you use or want to use the Negative or Positive trigger on the Oscope?

If you have are measuring logic TTL or Cmos waveform/signals, using the negative or positive trigger will give you the same result?

The Negative Triggering is used when you want to measure a negative transition or when a voltage is switching from a positive voltage to a negative state

The Positive triggering is used when you want to measure a positive transition or when a negative voltage is switching to a positive state

Is there any other examples to use the negative triggering to do other kinds of measurements?

When would you want to use the negative triggering? for what kind of measurements?

How do you know which triggering polarity to use to do what kind of measurements? how do you know which polarity to choose?

When Magnifying a small waveform , the triggering level and triggering polarity has to be fine adjusted?

 

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When measuring waveforms or signals , when do you use or want to use the Negative or Positive trigger on the Oscope?

depends on the signal you're looking at. e.g. looking at an active low enable that causes a design to crash and burn as soon as the enable is applied You probably want to check the falling edge of the enable in relation to other signals.

If you have are measuring logic TTL or Cmos waveform/signals, using the negative or positive trigger will give you the same result?

No, depends on what you are trying to look at. For a repetitive signal like a clock positive or negative triggering looks pretty much the same. Instead think of what happens if you are looking at a non-repetitive pulse of 1 ms and you need to look at another signal that has serial data on it that starts 100 ns after the rising edge of the pulse but end 1 us later. If you are triggering on the falling edge unless you have a DSO with with a huge buffer you won't have any resolution to look at the the leading edge of the pulse and the serial data.

The Negative Triggering is used when you want to measure a negative transition or when a voltage is switching from a positive voltage to a negative state
The Positive triggering is used when you want to measure a positive transition or when a negative voltage is switching to a positive state

Yes and yes

Is there any other examples to use the negative triggering to do other kinds of measurements?
When would you want to use the negative triggering? for what kind of measurements?

I've already given a few cases.

How do you know which triggering polarity to use to do what kind of measurements? how do you know which polarity to choose?

Look at the signal and what you are trying to capture. If the signal(s) of interest occurs around the falling edge of the triggering event use a negative edge trigger. If the signal(s) of interest occur around the rising edge of the triggering event use a positive edge trigger.

When Magnifying a small waveform , the triggering level and triggering polarity has to be fine adjusted?

Not sure what you mean by this. You normally adjust the waveform to some V/div setting and set the trigger to somewhere around the logic levels trigger level. If you were looking at undershoot problems you would likely set the trigger to less than 0V and the V/div to something really small like 10mV/div. Depends on what you are trying to look at.

I think you need to go into a lab and use a scope on some circuit that you understand and just take a look at what you can see and how changing trigger position and polarity affect your measurements.

Regards

 

[stanleystan]

I do need more examples of when you would want to use positive or negative triggering and which polarity to choose

Mostly when I choose the polarity of the triggering it seems to give the same result, why is that? when does the result change when choosing the polarity of the triggering?

What logic signals do you use the negative triggering when do what kind of measurements?

I know if the IC Logic chip is Negative triggered or needs a negative input , you use the negative triggering on the Oscope

But I'm not sure when to use the negative or positive triggering for signals, because they mostly give me the same result, so I'm confused when it changes the result or out come

 

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I do need more examples of when you would want to use positive or negative triggering and which polarity to choose

How about looking at the ramp up and ramp down of a power supply. You certainly don't use a rising edge trigger to monitor the voltage supply ramp down, unless you are trying to trigger on a non-monotonically decreasing supply rail to look at the problem.

Mostly when I choose the polarity of the triggering it seems to give the same result, why is that? when does the result change when choosing the polarity of the triggering?

What are you looking at, clocks, sine/cosine waveforms? Anythng periodic isn't very interesting on a scope.

What logic signals do you use the negative triggering when do what kind of measurements?

If you can't figure this out you're in for a world of hurt when you need to debug a board with a scope. I've already give some scenarios of why you would use different triggering.

I know if the IC Logic chip is Negative triggered or needs a negative input , you use the negative triggering on the Oscope

Not true you may want to look at the active edge of the active low signal or the inactive edge of the active low signal. Depends on what else you are looking at on the OTHER scope probe(s).

But I'm not sure when to use the negative or positive triggering for signals, because they mostly give me the same result, so I'm confused when it changes the result or out come

Because you're probably looking at only one signal and it's probably a clock or some other non random periodic signal.

 

[dick_freebird]

Maybe you're grasping for the concept of the "reference edge".

The choice of this, depends.

Sometimes is is simple, like you have a positive edge triggered
DFF and you just want to verify that data transitions safely
away from it. So set trigger to positive-going and the 'scope
channel for the clock. Done.

But maybe you really want to quantify timing jitter, and get
something like statistics or peak-peak. Then, you might
make the data input the trigger, and look at both positive
and negative edges in succession, with the signal of interest
being the clock. Accumulating a large number of clock traces
from a data trigger, will build you a nice eye diagram. But you
need to be sure that LH vs HL timing discrepancy does not
push you out of the eye.

Sometimes you want to trigger on the event that "makes it
happen". Sometimes you want to know how "what makes it
happen" relates to something else. Triggering off the result
sometimes tells you things that triggering off the input won't,
especially if the "result" is very sparse - you'd capture a lot
of useless records triggering off the clock and maybe not even
the one you were after.

 

[stanleystan]

Re: Oscope Negative & Positive trigger used for?

You probably want to check the falling edge of the enable in relation to other signals.

So the Triggering Polarity only works when it's compared or in "RELATION" to other signals?

The Triggering Polarity only works when you use the External trigger input on the Oscope?

If you don't compared other signals or use the External trigger input on the Oscope, the triggering polarity of the oscope won't give you a different result?

- - - Updated - - -

What are you looking at, clocks, sine/cosine waveforms? Anythng periodic isn't very interesting on a scope.

Yes Clocks signals, TTL or CMOS logic signals that are Periodic

So the Trigger Polarity will give the same result to periodic signals?

The Trigger Polarity will only change the result with Non-Periodic signals?

- - - Updated - - -

Not true you may want to look at the active edge of the active low signal or the inactive edge of the active low signal. Depends on what else you are looking at on the OTHER scope probe(s).

Active edge and the Inactive edge will give you the same result , what is the difference?

 

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Re: Oscope Negative & Positive trigger used for?

So the Triggering Polarity only works when it's compared or in "RELATION" to other signals?
The Triggering Polarity only works when you use the External trigger input on the Oscope?
If you don't compared other signals or use the External trigger input on the Oscope, the triggering polarity of the oscope won't give you a different result?

This isn't what I mean. I'm saying using various trigger polarities allows you to look at other signals on different scope probes (other than the one you are using as the trigger) in relation to the triggering scope probe. Say the trigger is on the scope probe with the clock showing and a data bit is on a second scope probe and you want to see if the data arriving at the pins of a device can be captured by the device input register. If you turn on persistence in the scope you'll get a screen full of all the transitions in relation to the triggering edge of the clock. If the capture of data is on the falling edge in the device it makes no sense to use a rising edge trigger as you wouldn't be synchronizing to the falling edge, which is what the device is looking at to capture data.

Yes Clocks signals, TTL or CMOS logic signals that are Periodic
So the Trigger Polarity will give the same result to periodic signals?
The Trigger Polarity will only change the result with Non-Periodic signals?
Active edge and the Inactive edge will give you the same result , what is the difference?

I don't think you've actually looked at the signal when you change the trigger polarity and absolutely nothing else on the scope's settings. Do this and observe carefully. You will witness the signal moving left/right by half a clock cycle. That is the difference. You need to look at signals that are NOT clocks, then you might have a better understanding. Look at the output of a PWM or something else that isn't a clock.

Regards

 

[stanleystan]

You will witness the signal moving left/right by half a clock cycle. That is the difference.

Yes it does moves left/right by some microseconds or time

But the Pulse width and Time Period of the waveform stays the same, in either triggered polarity

So I still don't get what the triggered polarity does, when it doesn't change the pulse width or time period of the waveform you're looking at

It only shifts left to right when you're viewing multiple Logic TTL or CMOS signals on multiple channels

But why would you care to know if the Logic signal is triggered to a Positive or negative? it's just a different reference or starting point

The Pulse width and period stay the same either way what ever the trigger polarity is

 

[Tunelabguy]

Yes it does moves left/right by some microseconds or time

But the Pulse width and Time Period of the waveform stays the same, in either triggered polarity

So I still don't get what the triggered polarity does, when it doesn't change the pulse width or time period of the waveform you're looking at

It only shifts left to right when you're viewing multiple Logic TTL or CMOS signals on multiple channels

But why would you care to know if the Logic signal is triggered to a Positive or negative? it's just a different reference or starting point..

There are several reasons for wanting the o-scope display to start at a specific polarity. One is that you may be interested in measuring the rise time or fall time of a signal. Often the thing that affects the rise time is not the same thing that affects the fall time, so being able to look at one edge or the other is crutial.

Another reason is that you may want to measure the delay between one signal and another signal. The delay starting at the positive edge may be very different from the delay starting from the negative edge, so again you would want to be able to control which edge is synchronized with the beginning of the o-scope display.

A third reason for wanting to control the triggering polarity is that the signal may not be strictly periodic. The positive edges may be precisely periodic, but the negative edges may be more irregular. In that case you might want to ensure that you were triggering on the positive edge.

You are assuming a very limited use for an o-scope: to simply measure the on-time and off-time of a strictly period signal. An o-scope can do so much more, and that is why it has that flexibility in triggering.

 

[stanleystan]

One is that you may be interested in measuring the rise time or fall time of a signal.

Measuring Rise time, set the trigger polarity to Positive edge?
Measuring Fall Time, set the trigger polarity to Negative edge?

Another reason is that you may want to measure the delay between one signal and another signal. The delay starting at the positive edge may be very different from the delay starting from the negative edge, so again you would want to be able to control which edge is synchronized with the beginning of the o-scope display.

But I have seen test procedures that want to measure the time delay between other signals on O scope channels 2,3,4 that start at the positive edge and measure again the time delay with starting on the negative edge

So I'm confused as to why and when you want to measure the time delay and time period with starting on the negative edge?

Or what have you guys used to measure that used the Negative triggering? for what kinds of measurements and when to use it?

Mostly when I measure the time period, time delay, duty cycle, pulse width,rise/fall time i use the trigger set to Positive

I'm not sure when to use the Negative trigger or when to use it

 

[Tunelabguy]

I gave the generic reasons for needing the option for both polarities, but if you still want more, I have to get into specific applications. OK, here is one. RS-232 serial communication is based on a signal that is negative in its resting state and goes positive when a character starts to be transmitted. The RS-232 signal is normally processed through a level-shifting inverter, such as the MAX232 chip, which outputs an inverted logic level signal to a microprocessor. Since this output is inverted, the resting state is logic high (say, +5v) and when a character starts, the level drop to 0v for the start pulse. Suppose you wanted to measure the pulse width of the start pulse at this point. (Of course to do that you have to assume the LSB of the character being transmitted is the opposite of the start pulse, but let's suppose the data being transmitted makes that possible). The only way to measure this pulse width is to trigger on the negative edge and observe where the positive edge is with respect to it. If you trigger on the positive edge, it is too late. The start pulse has already passed you by (unless you are using a digital scope with a pre-trigger memory, but let's assume an old-school analog CRT o-scope).

 

[stanleystan]

Since this output is inverted, the resting state is logic high (say, +5v)

So you use always the negative polarity trigger when you're measuring Inverted Logic or Inverted signals?

Do you ever use the Negative Polarity trigger when you're measuring pulse width, rise time, time delay, time period?

When would you want to use the Negative polarity trigger when measuring the Time period of a logic signal or analog waveform?

 

[Tunelabguy]

So you use always the negative polarity trigger when you're measuring Inverted Logic or Inverted signals?

No. Don't try to extract an "always" from an example. Each application is different. When you know what you want to measure, then you will know which polarity to use. Many times it doesn't make any difference. But sometimes it does. Only experience and knowledge of the application will tell you when that is.

 

[stanleystan]

Many times it doesn't make any difference.

So you have noticed that too, because I don't get a difference mostly , it gives me the same result with either positive or negative polarity

That is why I'm confused as to when to use the negative polarity

 

[Tunelabguy]

So you have noticed that too, because I don't get a difference mostly , it gives me the same result with either positive or negative polarity..

Of course. Many times I am just interested in measuring the period of a strictly periodic waveform. Then I don't even bother looking to see if my scope is set for positive or negative triggering because I will get my answer more quickly by just looking at the waveform regardless of how it is triggered. But I also gave you one example of when it does matter. So it is good to know how to do it when you need to.

 

[PrescottDan]

The Negative Trigger is used when you want to measure or view what DC voltage is PULLING down below the ground reference zero crossing

Channel1 is an AC waveform before above and below the ground reference

Set the Oscope trigger to negative edge

Set the Oscope input coupling to DC , on channel#2
Set the Oscope to Add mode

You can see the DC voltage Pulling down when the AC waveform is crossing below the zero crossing only when the AC waveform is in the negative cycle