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Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?

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sofakng:
I'm a very confused beginning and purchased my first oscilloscope a few months ago.  (Rigol DS1054Z)

Can somebody explain how a signal is drawn on an oscilloscope?  (normal display mode, not Free Running or Continuous, etc)

Does the signal draw from left to right and then start again at the left when it reaches the right side edge?  For example, if a wave form started towards the right side, would it continue from the left side?  It's confusing to me because I think of a signal as a continuous drawing like a really, really long piece of paper starting at the left and drawing forever towards the right.

How does a trigger stabilize the display?  For example, if you have a repeating signal/wave form and trigger on a rising edge, etc, then it looks like the display is static/frozen.  Am I correct in assuming the image appears the same, but every triggered edge is actually replacing the image but it happens so quickly you can't notice it?

Let me explain in a different way... if you have a continuous square wave and the trigger on the rising edge then you will have a signal with an infinite number of square waves.  Will the scope display the first sine wave (trigger) and then fill the rest of the screen, and then move the 2nd square wave to the left and display that information?

Sorry for such basic questions...

james_s:
This is why I usually suggest learning how to use an old fashioned analog scope first because the way it works quickly becomes obvious.

A DSO doesn't "sweep", it captures a series of samples and then displays a picture of the waveform on the screen all at once. The display represents time from left to right and voltage on the vertical axis. A trigger starts the sequence of captures (or starts the sweep on an analog scope) based on some condition such as rising or falling voltage or a specific voltage level. If you have a repetitive signal this will result in a stable display. With a DSO you can display a capture run for as long as you want so you only need to trigger once and then the display will be stable indefinitely until you trigger another capture.

There are lots of videos on how to use an oscilloscope that will illustrate all this if it still doesn't make sense.

rstofer:

--- Quote from: sofakng on January 09, 2019, 06:32:39 pm ---I'm a very confused beginning and purchased my first oscilloscope a few months ago.  (Rigol DS1054Z)

Can somebody explain how a signal is drawn on an oscilloscope?  (normal display mode, not Free Running or Continuous, etc)

Does the signal draw from left to right and then start again at the left when it reaches the right side edge?  For example, if a wave form started towards the right side, would it continue from the left side?  It's confusing to me because I think of a signal as a continuous drawing like a really, really long piece of paper starting at the left and drawing forever towards the right.


--- End quote ---
The trigger point on a DSO is usually the center of the screen.  Things to the left of center happened before the trigger and things right of center happened after the trigger.  Of course, this trigger point can be move left or right on the screen.  It can even be moved off the screen in some cases.


--- Quote ---How does a trigger stabilize the display?  For example, if you have a repeating signal/wave form and trigger on a rising edge, etc, then it looks like the display is static/frozen.  Am I correct in assuming the image appears the same, but every triggered edge is actually replacing the image but it happens so quickly you can't notice it?

--- End quote ---

You got it right, there is a waveform update rate and it's usually a very large number.  And that's the power of the DSO versus analog scope!  I can set the DSO for single shot and get exactly one capture.  This is important if the signal is some kind of digital stream and I want to look at a specific trace.  Yes, there are analog storage scopes and, no, I'm not going to follow that idea down a rathole.


--- Quote ---Let me explain in a different way... if you have a continuous square wave and the trigger on the rising edge then you will have a signal with an infinite number of square waves.  Will the scope display the first sine wave (trigger) and then fill the rest of the screen, and then move the 2nd square wave to the left and display that information?

Sorry for such basic questions...

--- End quote ---

Triggering on a truly infinite trace of a repetitive signal is pretty useless.  All you have done is trigger on some random rising or falling edge.  As I said above, trace to the right will be after the trigger, trace to the left will be before the trigger.  For a truly periodic waveform, this is useless information because every blip is the same, for all time.

Mostly I want to deal with aperiodic signals; signals that transfer information.  Something like a UART, SPI or I2C protocol.  In the case of SPI, I trigger on the falling edge of CS' and shift the trigger point over to the far left of the screen.  This gives me the maximum screen for transitions that occur after the trigger.

IDEngineer:

--- Quote from: james_s on January 09, 2019, 06:50:33 pm ---This is why I usually suggest learning how to use an old fashioned analog scope first because the way it works quickly becomes obvious.
--- End quote ---
+(1E+06) for your very astute comment. I totally share your opinion. Sadly, it's a topic of much and repeated debate on this and other forums. But as evidence for our position, I will say that you tend to see this question (or some variation of it) from DSO users who don't have experience with analog scopes. The opposite isn't true... folks familiar with analog scopes generally "get" the basics of DSO's and their questions are almost always related to the unique features that DSO's bring to the party. That's the proper sequence of progression IMHO.

Other's mileage may vary, along with their opinions! {grin}

rstofer:
And if the new user were dealing with an analog scope they would have the very same questions or variations thereof.  What's delayed sweep, how do I display more/less of the trace, how do I trigger on some event, when do I use External Trigger, etc.

It isn't the DSO versus analog that's the problem.  It's new versus old in terms of experience.  The new user is, well, a new user.  Maybe they are even new to electronics!

I will concede that the DSO has a lot more features and is therefore a more complex tool but it also has an Auto feature.  I paid for that feature and I use it fairly often even though I have been using scopes for nearly 60 years.  The feature is just another tool.  Like the "Beam Finder" on an analog scope, this will at least get a trace on the screen.

There are about 36 knobs, switches and pushbuttons on my Tek 485.  That's a lot of stuff to learn (even though the channel controls are identical for both channels).

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