# EEVblog Electronics Community Forum

## Electronics => Beginners => Topic started by: sofakng on January 09, 2019, 06:32:39 pm

Title: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: 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.

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...
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: 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.

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.
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: rstofer on January 09, 2019, 07:01:34 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.

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?

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...

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.
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: IDEngineer on January 10, 2019, 12:42:43 am
This is why I usually suggest learning how to use an old fashioned analog scope first because the way it works quickly becomes obvious.
+(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}
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: rstofer on January 10, 2019, 01:24:39 am
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).
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: IDEngineer on January 10, 2019, 01:37:29 am
And if the new user were dealing with an analog scope they would have the very same questions or variations thereof.
I doubt they'd be asking "Does the signal draw from left to right and then start again at the left when it reaches the right side edge?" The first time they turned the horizonal sweep down a bit, how it works and what it's doing becomes immediately and intuitively obvious.

Questions like "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" are the sort of question you ask AFTER you understand the basics of what is happening on the screen. I seriously doubt anyone asking "Does the signal draw from left to right and then start again at the left when it reaches the right side edge?" can even CONCEIVE of questions like "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".

Hence the observation that an analog scope is more naturally intuitive and rapidly teaches newbies the basics. Spend a couple of minutes twiddling the horizontal and vertical knobs, alone, and most astute people will "get it". DSO's, which I absolutely love and use daily, nevertheless don't convey that inherent understanding. Again, just my opinion.
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: james_s on January 10, 2019, 01:45:13 am
Whatever the case, I find my past experience with analog scopes to be beneficial in grasping exactly what is going on. I made the transition to DSO very easily, as was mentioned it was largely getting a handle on all the new abilities it had. At a low sweep rate it's immediately obvious what the scope is displaying, you can watch it all in real time and even generate a waveform by twirling a pot with your fingers.

One doesn't need to spend years using an analog scope, but it is certainly useful to play around with one, especially when they can be had for so little money.
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: Brumby on January 10, 2019, 02:43:25 am
I would encourage all first time scope users to be sat down in front of an analogue scope as a starting point, with the physical scanning process described and demonstrated.  Include basic triggering and chopped/alternate for multiple trace displays - and why that is important.

There are several reasons for starting out like this.

The first is that all scopes follow the same basic controls and presentation.  With the understanding as to why certain things were done in the analogue system, there will be no confusion when moving to the digital as to why something was done the way it was.

Next is probably the most important factor in my book: That all the controls are visible at all times and there are (generally) no hidden options, features or settings.  Trying to teach someone the significance of a parameter that could be buried under one level of menu is always going to be harder than pointing them to the switch on the front panel that is always going to do the exact same thing.  Put it further down the menu levels and by the time they've found the parameter, they've forgotten why they were looking for it.

Third is that changes on an analogue scope occur immediately.  Change the timebase and you see the spot respond instantly.  None of the digital dance delays.  This immediate feedback allows a newcomer to associate the change in setting with the change in display.  Once they "get" that, then delays from digital processing are easier to deal with.

Lastly, concepts like bandwidth are easier to deal with when there is only a 3dB point to worry about.  Nyquist is not a topic you want to include until you have to.  When it comes to scopes, analogue is the only game in town.

Now I'm not suggest you stick a Tek 465 in front of a complete newbie unless you want to freak them out - just a basic CRO.  Along the way, feel free to show the difficulties that analogue scopes are faced with and use those points to advance their understanding and introduce the solutions to those problems that digital scopes can provide.

JMHO
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: rstofer on January 10, 2019, 03:38:58 am
Whatever the case, I find my past experience with analog scopes to be beneficial in grasping exactly what is going on. I made the transition to DSO very easily, as was mentioned it was largely getting a handle on all the new abilities it had. At a low sweep rate it's immediately obvious what the scope is displaying, you can watch it all in real time and even generate a waveform by twirling a pot with your fingers.

One doesn't need to spend years using an analog scope, but it is certainly useful to play around with one, especially when they can be had for so little money.

We're not talking about new users if they have past experience.  Of course the transition is easy if you have a lot of analog scope experience.  But why spend the money to buy a POS analog scope likely to fail sooner rather than later and then turn around and spend more money on a new DSO.  Buy the new DSO and be done with it!

Just start!  Use Auto to get a trace on the screen, play with Volts/division and Time/division and see what happens.  Move the trigger level up and down and the trigger point left and right.  Move the traces up and down.  Stack them in different orders.  Watch videos, they're all over the Internet.

There is a long course available as a sticky to this forum  W2AEW's videos are always excellent.  Of the videos I have watched, he seems to just use analog scopes.  Old school, I suspect.

Lacking a signal source, even the compensation output on the front panel will give the new user something to look at.

If anybody has a problem sleeping, RTFM.  Guaranteed to put you right out!

I see no reason for anybody to go through the gyrations of buying and using an analog scope just to turn around and buy a DSO.  It's a waste of time and money.

You know, we let people start with calculators these days, we don't force them to learn how to use a slide rule and then upgrade.  Times change!

Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: Brumby on January 10, 2019, 05:02:50 am
You know, we let people start with calculators these days, we don't force them to learn how to use a slide rule and then upgrade.  Times change!

Yeah - but we still like people to learn how to drive competently before we throw them into Nascar or F1.

... and they might be a little less freaked about it too.
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: james_s on January 10, 2019, 05:55:25 am
Who said anything about going out and buying one? Spending an hour playing around with one is fine, or pick up and old one for little or nothing. Why do you think an analog scope is likely to fail? I still have a few of them around that work fine, I still use my 465b now and then. I've given away two different analog scopes over the past few years, it's not that hard for a noob to find one for little or nothing and when they finish they can give it to some other beginner. A good usable CRO is literally cheaper than a lot of books and likely at least as educational.
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: hamster_nz on January 10, 2019, 07:34:42 am
I would caution taking some of the earlier advice to get an analog scope to heart - they are honest opinions and all, but a car analogy is in order.

Qn: Hi! I want to buy a car, but not too sure if I need one with Electronic ignition and Traction control or not. Actually I don't understand traction control. Does it apply brakes to each wheel, or does it control the throttle?

Answer: I suggest you buy a older second hand model, before they had all these computers aids. You need to learn how to drive a real car before you get one into one of these modern ones. You will understand so much more about how they work, and how drive them properly. You can even pick up a really nice second hand one, even some real classics, dirt cheap!.

Or maybe you like biking:

Qn: I'm thinking of buying a full suspension mountain bike. I really don't understand about four-pivot rear suspensions - what should I look for?

Answer: If you don't know what you want in a full suspension bike, then you are not ready to own one yet. You will appreciate it so much more if you use a fully rigid bike on the trails first. Once you are able to ride that properly you will know what to look for in your next bike. You can't buy this sort of bike any more, but you can even pick up what was a really nice one dirt cheap on eBay!.

I say:

If you have the budget to buy new, see if you can pick up a low-end DSO that stretches your budget a little higher than you would like to go. The budget models from reputable brands are exactly the same as budget models of cars and full suspension mountain bikes - not the nicest to use, they have modest specifications, but perfectly usable within their limitations. Premium brands are nice, but usually have premium prices - you get more for your \$ from a second-tier brand.

Old analog scopes on auction sites are dirt cheap on auction sites for one reason -  because they are of little value. It hurts, but the market has speaks the truth - people who tell you otherwise have very different appreciation for the worth of these things from most, usually through nostalgia, passion,  or a "I learned so much using one of these, you can too!" kind of way.

If budget is a problem, buy a second-hand DSO. If you can't find one you can afford second hand it is because they have real utility and value above what you are willing to pay.

Sorry if I am typing cranky - it has been a really hot & trying day here!
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: Brumby on January 10, 2019, 10:11:22 am
I would caution taking some of the earlier advice to get an analog scope to heart - they are honest opinions and all, but a car analogy is in order.

Yeah - but we still like people to learn how to drive competently before we throw them into Nascar or F1.

... and they might be a little less freaked about it too.

... which was far more appropriate than "traction control".

Sorry if I am typing cranky - it has been a really hot & trying day here!

Yeah, I can understand that.  A few days ago I'd have been the same.
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: fsr on January 10, 2019, 01:16:40 pm
Well, a DSO has more features, but shouldn't be a problem to learn to use one, without previous analog scope experience.
The DSO is a computer displaying a waveform in it's screen. So, the signal isn't drawn in the screen directly like in an analog scope, it's sampled to memory and then drawn in the screen. If the signal is really slow, it uses roll display (at least the gw-instek does that).
The signal is in memory, so that the trigger point is in the middle, you can move it, you can trigger once or manually and still explore the full captured signal on memory, etc (trigger has to be in manual mode for that, auto mode will trigger itself periodically after a timeout even if the signal doesn't trigger the scope).
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: rstofer on January 10, 2019, 07:14:03 pm
You know, we let people start with calculators these days, we don't force them to learn how to use a slide rule and then upgrade.  Times change!

Yeah - but we still like people to learn how to drive competently before we throw them into Nascar or F1.

... and they might be a little less freaked about it too.
NASCAR (at some levels) will allow drivers to be as young as 14 so there is no chance they have a driver's license.  Some leagues go as low as 12!

If the new driver has competence, it's because they got it on a track, not on the freeway.

Here's a young girl hitting 100 MPH on the straights and 80 MPH through the corners:
https://abcnews.go.com/WN/katie-brice-young-drive-nascar/story?id=10468298

The top 3 series for NASCAR (Trucks, Xfinity and Cup) have a minimum age requirement of 18 so presumably they have freeway experience.  Not that I think it would be useful sliding around Daytona at nearly 200 MPH.  There just isn't a lot of drift experience driving through town.

When I taught my grandson to drive, one of the very early lessons was entering a freeway at speed.  I taught him to use the "Punch It and Fry 'Em School of Driving" approach.  Just "Stand On It!"  The first entrance we ever made (unplanned) had us enter into the left lane.  Punch it!  Now!

I may be old but I still enjoy having the rear end step out from time to time.  My passengers?  Not so much!
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: IDEngineer on January 10, 2019, 11:19:46 pm
Qn: Hi! I want to buy a car, but not too sure if I need one with Electronic ignition and Traction control or not. Actually I don't understand traction control. Does it apply brakes to each wheel, or does it control the throttle?
That analogy is inaccurate. It's like a newbie asking about advanced triggering and delayed timebases, as cited earlier in this thread. Actually, even worse... since electronic ignition and traction control operate autonomously and require zero driver input whether the driver is a novice or experienced. Perhaps a similar analogy would be "Should I buy a scope with a linear or switching power supply?", again a behind-the-scenes "feature" that requires zero involvement from the operator.

Quote
people who tell you otherwise have very different appreciation for the worth of these things from most, usually through nostalgia, passion,  or a "I learned so much using one of these, you can too!" kind of way.
Or perhaps we've actually trained people using both types of scopes and have seen how the learning curve is improved by starting with an analog scope. But hey, that's just me and my actual personal experiences teaching other people how to use scopes. The opinions of others who have trained lots of actual people to use actual scopes is just as valid as mine. YMMV, and I'm seriously not trying to start or perpetuate an argument - just trying to help someone who is asking how to use scopes.

Next up: Tabs or Spaces in source code?  >:D
Title: Re: Does a DSO scope sweep from left to right and how a trigger stabilizes a signal?
Post by: Brumby on January 11, 2019, 01:49:48 am
Analogy fails:
NASCAR (at some levels) will allow drivers to be as young as 14 so there is no chance they have a driver's license.  Some leagues go as low as 12!
OK.  I didn't know that.

Qn: Hi! I want to buy a car, but not too sure if I need one with Electronic ignition and Traction control or not. Actually I don't understand traction control. Does it apply brakes to each wheel, or does it control the throttle?
That analogy is inaccurate. It's like a newbie asking about advanced triggering and delayed timebases, as cited earlier in this thread. Actually, even worse... since electronic ignition and traction control operate autonomously and require zero driver input whether the driver is a novice or experienced. Perhaps a similar analogy would be "Should I buy a scope with a linear or switching power supply?", again a behind-the-scenes "feature" that requires zero involvement from the operator.
Yes.  That's the sort of flaw I saw as well, but couldn't get the words out.