Author Topic: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy  (Read 26981 times)

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Offline David Hess

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #50 on: April 12, 2014, 04:55:57 am »
Runts and display persistence don't enter into what i'm observing.  It's a distinct delay between my hand turning a tuning core and what is displayed on the DSO screen.  The analogue scope appears to be much more real time than the DSO.

As i mentioned earlier - i'm happy to attribute this to my DSO being peasant specification but i would be interested in seeing how it has evolved though the generations of DSO.

There is no storage on analogue scopes, that's why they are more responsive to inputs. Analogs have to do computations before showing the results. But some high end ones can update that pretty quick, but since it's not analogue they have an inherit delay from input to output.

And by storage, even if they do have storage, they display the output instantly, the storage happens later.

My Tektronix 7834 would like a word with you.
 
There *are* (or were) analog CRT based storage oscilloscopes and they had the same responsiveness as the familiar analog CRT oscilloscopes.  They coexisted with the early DSOs up to about 1991 but DSOs were more economical and displaced them quickly.
 

Offline miguelvp

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #51 on: April 12, 2014, 05:09:51 am »
Oh, and to answer your other question, I've been using a second hand tektronix 7603 for the last 15+ years, I still have it but I recently purchased a Rigol DS2072, yeah nothing fancy, but that doesn't mean that I don't know what is out there (that I can afford but my wife won't let me get it).

My dad was an electronic engineer and I used to read all his manuals (he used to work for GTE) as a kid, I did turn to software because in the mid 80's programming was king (and still is) that doesn't mean I didn't keep up with my other side. I'm a strong believer that if you don't understand the electronics behind of what you are programming you are not a good programmer at all.
 

Offline miguelvp

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #52 on: April 12, 2014, 05:12:04 am »
Runts and display persistence don't enter into what i'm observing.  It's a distinct delay between my hand turning a tuning core and what is displayed on the DSO screen.  The analogue scope appears to be much more real time than the DSO.

As i mentioned earlier - i'm happy to attribute this to my DSO being peasant specification but i would be interested in seeing how it has evolved though the generations of DSO.

There is no storage on analogue scopes, that's why they are more responsive to inputs. Analogs have to do computations before showing the results. But some high end ones can update that pretty quick, but since it's not analogue they have an inherit delay from input to output.

And by storage, even if they do have storage, they display the output instantly, the storage happens later.

My Tektronix 7834 would like a word with you.
 
There *are* (or were) analog CRT based storage oscilloscopes and they had the same responsiveness as the familiar analog CRT oscilloscopes.  They coexisted with the early DSOs up to about 1991 but DSOs were more economical and displaced them quickly.

And that's why i said, and you quoted as well:

Quote
And by storage, even if they do have storage, they display the output instantly, the storage happens later.

meaning that they do store it without altering their analogue response, maybe I wasn't clear on that, sorry.

Edit: but yeah, I did say that analoges have to do computations, but I really meant digital scopes have to do computations, sorry again maybe the wine is affecting my typing :)

« Last Edit: April 12, 2014, 05:29:31 am by miguelvp »
 

Offline David Hess

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #53 on: April 12, 2014, 05:39:32 am »
Runts and display persistence don't enter into what i'm observing.  It's a distinct delay between my hand turning a tuning core and what is displayed on the DSO screen.  The analogue scope appears to be much more real time than the DSO.

As i mentioned earlier - i'm happy to attribute this to my DSO being peasant specification but i would be interested in seeing how it has evolved though the generations of DSO.

This problem definitely exists even on some high end DSOs.  Waveform acquisition rates have gone up which is good but the latency from acquisition to display and from user interface to display can be very noticeable.  On the interface side, the modern low and high end DSOs I have evaluated are little or no better than my ancient real time DSOs except in acquisition rate.

DSOs will never achieve the display and interface latency that analog vector CRT oscilloscopes had but for all practical purposes they should get close enough and they can certainly perform better than they do.  Latency includes:

1. Waveform record fill time - use a short record length to minimize this.  Early DPO style DSOs wrote directly to the display memory and effectively processed acquisitions in real time so they would not suffer from this.  I do not know if any modern ones operate this way.
2. Trigger rearm time - some DSOs take a long time to rearm their trigger.
3. Processing time - the waveform record has to be transferred and processed for display.
4. User interface latency - changing the operating parameters may have considerable latency.  This seems to have gotten a lot worse with oscilloscopes that use embedded desktop operating systems.
5. The display refresh time - Not much can be done about this but 50 or 60 Hz should be plenty fast.  Some LCD displays have a latency of more than 1 frame increasing their latency beyond what the refresh rate implies but I do not know that they are used in embedded systems.
 

Offline miguelvp

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #54 on: April 12, 2014, 05:54:40 am »
But at the end of the day what do you prefer?

Acquisition rate or Display rate.
I will go for the former.

I rather capture the waveform and see all the detail than see it in (almost) real time.

Also, can you blink an led fast enough that you can't see it? a high speed camera will see it. heck, there are cameras that can track light moving in short distances nowadays, actually pretty cool tech that you can see in non real time how light travels (yeah, just like you expect as a wave more than as a particle)

but speaking of dualities:

Programmers we are way behind of what is possible hardware wise. Let me rephrase that: programming languages are way behind of what is possible, I know that. There have been so many tech advances and the only thing holding back is really the programming paradigms side of things. But I do blame "us" programmers for that because most of us are creatures of comfort and don't want to learn a new way of thinking.

But I think it's getting better even if we are still lagging by at least 9 years behind what is achievable. I also blame managers that want to keep things "safe" so don't rock the boat.

I know the whole disrupting technology terminology has been used way too much, but what it really means is that someone finally had the time to mix together common sense technologies that no one wanted to spend the effort doing before. Even if the amount of effort is minimal nowadays.

And it's disruptive because they wake us (programmers) up from our nap.
 

Offline David Hess

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #55 on: April 12, 2014, 05:55:49 am »
Runts and display persistence don't enter into what i'm observing.  It's a distinct delay between my hand turning a tuning core and what is displayed on the DSO screen.  The analogue scope appears to be much more real time than the DSO.

As i mentioned earlier - i'm happy to attribute this to my DSO being peasant specification but i would be interested in seeing how it has evolved though the generations of DSO.

There is no storage on analogue scopes, that's why they are more responsive to inputs. Analogs have to do computations before showing the results. But some high end ones can update that pretty quick, but since it's not analogue they have an inherit delay from input to output.

And by storage, even if they do have storage, they display the output instantly, the storage happens later.

My Tektronix 7834 would like a word with you.
 
There *are* (or were) analog CRT based storage oscilloscopes and they had the same responsiveness as the familiar analog CRT oscilloscopes.  They coexisted with the early DSOs up to about 1991 but DSOs were more economical and displaced them quickly.

And that's why i said, and you quoted as well:

Quote
And by storage, even if they do have storage, they display the output instantly, the storage happens later.

meaning that they do store it without altering their analogue response, maybe I wasn't clear on that, sorry.

Edit: but yeah, I did say that analoges have to do computations, but I really meant digital scopes have to do computations, sorry again maybe the wine is affecting my typing :)

Wine will do that. :)  I have run across people who had no idea analog storage oscilloscopes ever existed though.

Where it gets weird are the fast transfer mesh storage oscilloscopes (the 7834 supports this) where storage occurs before display.  This is done to raise the writing rate but in practice, variable persistence mode is usually fast enough and a lot easier to use.  The persistence is typically adjustable over a range of 10s of milliseconds to minutes.  For oscilloscopes that support bistable storage mode which is much slower, persistence is hours to days and the display may even be preserved when the oscilloscope is turned off but bistable storage does not have graded intensity like variable persistence mode.

For myself, I elected to refurbish a 7834 for my variable persistence needs and get an old but maintainable DSO for everything else instead of paying for a modern DSO which supports both.  Just duplicating the capability of a single old differential comparator input would cost more than a new DSO.
 

Offline David Hess

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #56 on: April 12, 2014, 06:02:17 am »
But at the end of the day what do you prefer?

Acquisition rate or Display rate.
I will go for the former.

I rather capture the waveform and see all the detail than see it in (almost) real time.
Why not both?  There is no conflict between the two unless you use segmented memory which trades one for the other.  Latency only needs to be "video" speed anyway and most DSOs do not even achieve that.

Quote
Also, can you blink an led fast enough that you can't see it? a high speed camera will see it. heck, there are cameras that can track light moving in short distances nowadays, actually pretty cool tech that you can see in non real time how light travels (yeah, just like you expect as a wave more than as a particle)
The Tektronix 7104 oscilloscope can sweep its screen comfortably faster than the speed of light:

http://www.oregonlive.com/silicon-forest/index.ssf/2011/09/a_tektronix_oscilloscope_that.html
 

Offline miguelvp

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #57 on: April 12, 2014, 06:06:39 am »
maybe it's the same tech they use to capture light faster than the speed of light.



But my point is, that you want the image to persist so you can detect it with your eye. That doesn't contradict anything, you can capture faster than light, but at least display slower that you can see it.


Edit: let me rephrase what I said, not capturing faster than light, but capturing light at 1 millimeter travel per frame, or at a femtosecond interval. I do want a scope that can do femtoseconds, nah, it will take a lot of memory to get anything useful.

« Last Edit: April 12, 2014, 06:14:25 am by miguelvp »
 

Offline David Hess

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #58 on: April 12, 2014, 06:22:39 am »
Edit: let me rephrase what I said, not capturing faster than light, but capturing light at 1 millimeter travel per frame, or at a femtosecond interval. I do want a scope that can do femtoseconds, nah, it will take a lot of memory to get anything useful.

My fastest oscilloscope only gets down to 10 picoseconds per division.  I am so behind the times.
 

Offline miguelvp

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #59 on: April 12, 2014, 06:49:08 am »
Edit: let me rephrase what I said, not capturing faster than light, but capturing light at 1 millimeter travel per frame, or at a femtosecond interval. I do want a scope that can do femtoseconds, nah, it will take a lot of memory to get anything useful.

My fastest oscilloscope only gets down to 10 picoseconds per division.  I am so behind the times.

a 200 MHz scope is 5 nanoseconds per division so your's is 2,000 times faster than that.
They are talking about 10,000 times faster than yours, or 20 million times faster than a 200MHz scope. In binary it's 24 orders of magnitude better than a 200MHz scope. And they are doing a whole 2D image not just one pixel.

 

Offline hikariuk

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #60 on: April 12, 2014, 08:38:40 am »
My Tektronix 7834 would like a word with you.
 
There *are* (or were) analog CRT based storage oscilloscopes and they had the same responsiveness as the familiar analog CRT oscilloscopes.  They coexisted with the early DSOs up to about 1991 but DSOs were more economical and displaced them quickly.

As would my Tektronix 5115 :)
I write software.  I'd far rather be doing something else.
 

Offline R_G_B_

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #61 on: April 12, 2014, 10:09:47 am »
Use the FFT with a good adc resolution if you looking for hidden anomalies.
Scopes are good for time varying signals e.g checking if a waveform is there has the correct amplitude, time, phase, frequency.

Use the right tool for the job.
R_G_B
 

Offline waspinator

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #62 on: April 12, 2014, 11:17:23 am »
the closeup of the $10K Tektronix 3000 screen is really disappointing. Looks just as bad as the cheap Rigol. An HDPI+ screen is probably less than $100, why haven't they bothered upgrading that?

they optimize for display speed over display quality, and rightly so.  Oscilloscopes are not meant for precise measurement, they're meant to be used to observe the shape of a waveform.

The display included lets one easily see the shape of the waveform.

at 10K they shouldn't need to compromise. not many would care if it was 10.1K, but I'm sure they would appreciate not looking at pixels.
 

Offline David Hess

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #63 on: April 12, 2014, 04:29:12 pm »
Edit: let me rephrase what I said, not capturing faster than light, but capturing light at 1 millimeter travel per frame, or at a femtosecond interval. I do want a scope that can do femtoseconds, nah, it will take a lot of memory to get anything useful.

My fastest oscilloscope only gets down to 10 picoseconds per division.  I am so behind the times.

a 200 MHz scope is 5 nanoseconds per division so your's is 2,000 times faster than that.
They are talking about 10,000 times faster than yours, or 20 million times faster than a 200MHz scope. In binary it's 24 orders of magnitude better than a 200MHz scope. And they are doing a whole 2D image not just one pixel.

To borrow a phrase from Jim Williams - I'm sorry, but 14GHz is the fastest scope in my house.

The first time I used my sampling scope as a TDR, I was amazed at what could be seen.  It literally can display propagation of light, albeit RF photons in a transmission line, down to millimeters limited in my case only by my signal source.
 

Offline David Hess

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #64 on: April 12, 2014, 04:35:03 pm »
the closeup of the $10K Tektronix 3000 screen is really disappointing. Looks just as bad as the cheap Rigol. An HDPI+ screen is probably less than $100, why haven't they bothered upgrading that?

they optimize for display speed over display quality, and rightly so.  Oscilloscopes are not meant for precise measurement, they're meant to be used to observe the shape of a waveform.

Doesn't the TDS3000 series, not to be confused with the THS3000 series which is completely different, use an anti-aliased and index graded display?

I remember a comment over on TekScopes@yahoogroups.com that the TDS3000 series was the lowest end of oscilloscopes from Tektronix worth getting in connection with its DPO functionality but I wonder if the less expensive DSO2000 series would be better.  I have considered both at one time or another.
 

Offline rf-design

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #65 on: April 17, 2014, 01:57:49 pm »
Very good demonstration Dave but I missed some background explaination on the board.

To my understanding the best demonstration of the visibility of the noise distribution is to show a gaussian distribution on the board and how the eye makes a threshold on the visibility of the more rare events that the voltage or trace is 2x times the noise RMS value away from the mean. This threshold depend a little bit on the persistance of the analog scope phospor.

There are analog scopes built in the past which use analog physical backprocessing of the electron trace with a microchannel plate. This plate have much higher vertical resolution than most DSOs and amplifies the electrons from the trace with a logarithmic function. Then the amplified electrons are displayed with phospor.

Do you know a DSOs which have a logarithmic grading function?

It will be fair to show the best analog art in terms of a MCP scope in comparison to DSOs.

To me DSOs manufactures have still a way to go because most of them does not give numbers to there products. I did not see a SNR over sensitivity and only some give me a ENOB at some point. Also the ADCs resolution seem to contribute to SNR more than the frontend. True matured ADCs are noise vs. power limited, not by resolution.

The scope vendors offer +3 bit resolution with averaging, but did not tell if it is true resolution or done with additional, not specified, dither noise.

The scopes did not lie but the vendors hide the critical informations about there product because they better know how fare they are away from matured products for professionals.
 

Offline David Hess

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #66 on: April 17, 2014, 03:35:11 pm »
To my understanding the best demonstration of the visibility of the noise distribution is to show a gaussian distribution on the board and how the eye makes a threshold on the visibility of the more rare events that the voltage or trace is 2x times the noise RMS value away from the mean. This threshold depend a little bit on the persistance of the analog scope phospor.

I have linked an example in this forum of doing this a couple of times.  This is one of those measurements which is esoteric but trivial on an analog oscilloscope while being difficult or impossible on a DSO unless it can calculate it for you.  In theory calculating the RMS value will work and that is how sampling voltmeters do it but somehow the DSOs I have tested return the wrong results.  A DSO which can generate a histogram and return the standard deviation should work better but that is one of those features used for market segmentation.

This is also one of those measurements where record length and aliasing are irrelevant.  Neither alters the standard distribution (or RMS value) of the signal so if it changes, the oscilloscope has failed by lying.  This includes oscilloscopes which have an anti-aliasing filter linked to their sample rate and is a good example of why anti-alias filtering has no place in a DSO.

Quote
Do you know a DSOs which have a logarithmic grading function?

It would be interesting to know which produce an index graded display accurate enough (or at least not flawed enough) to make a dual trace RMS noise measurement by mark I eyeball.  My informal test of a high end Tektronix oscilloscope failed this spectacularly although it should be an easy thing to get right unless the display sacrifices functionality to look pretty.

Quote
The scope vendors offer +3 bit resolution with averaging, but did not tell if it is true resolution or done with additional, not specified, dither noise.

The input referred noise is so high that dithering should not be needed.

Quote
The scopes did not lie but the vendors hide the critical information about there product because they better know how fare they are away from matured products for professionals.

The most common might be conflating vertical sensitivity settings with digital magnification which just throws away bits.  Even my ancient DSOs can do this but at least they do not lie about it.
 

Offline rf-design

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #67 on: April 17, 2014, 05:56:44 pm »
To my understanding the best demonstration of the visibility of the noise distribution is to show a gaussian distribution on the board and how the eye makes a threshold on the visibility of the more rare events that the voltage or trace is 2x times the noise RMS value away from the mean. This threshold depend a little bit on the persistance of the analog scope phospor.

I have linked an example in this forum of doing this a couple of times.  This is one of those measurements which is esoteric but trivial on an analog oscilloscope while being difficult or impossible on a DSO unless it can calculate it for you.  In theory calculating the RMS value will work and that is how sampling voltmeters do it but somehow the DSOs I have tested return the wrong results.  A DSO which can generate a histogram and return the standard deviation should work better but that is one of those features used for market segmentation.

This is also one of those measurements where record length and aliasing are irrelevant.  Neither alters the standard distribution (or RMS value) of the signal so if it changes, the oscilloscope has failed by lying.  This includes oscilloscopes which have an anti-aliasing filter linked to their sample rate and is a good example of why anti-alias filtering has no place in a DSO.


I imagine a gray level grading where a logarithmic function is applied to the distribution and the positive and negative single sigma values get a red line. I think I simply recall what I have seen in some user plotting matlab code over the years.
 

Offline Wim_L

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #68 on: April 17, 2014, 06:54:56 pm »
It would be interesting to know which produce an index graded display accurate enough (or at least not flawed enough) to make a dual trace RMS noise measurement by mark I eyeball.  My informal test of a high end Tektronix oscilloscope failed this spectacularly although it should be an easy thing to get right unless the display sacrifices functionality to look pretty.

I suspect almost all, perhaps all, will fail at that test.

In an analog scope, the two beams share a screen. In a DSO, each channel has its own memory where points are accumulated. The data is then added up into the display buffer in a separate colour for each trace. When traces overlap, the usual result is that the trace drawn last casts a shadow over the previous traces.
 

Offline miguelvp

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #69 on: April 18, 2014, 02:27:17 am »
One thing no one mentioned is that the Rigol DS1052E that Dave used in the video is probably the noisier (because of the fan that is).
 

Offline David Hess

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #70 on: April 18, 2014, 02:56:30 am »
It would be interesting to know which produce an index graded display accurate enough (or at least not flawed enough) to make a dual trace RMS noise measurement by mark I eyeball.  My informal test of a high end Tektronix oscilloscope failed this spectacularly although it should be an easy thing to get right unless the display sacrifices functionality to look pretty.

I suspect almost all, perhaps all, will fail at that test.

In an analog scope, the two beams share a screen. In a DSO, each channel has its own memory where points are accumulated. The data is then added up into the display buffer in a separate colour for each trace. When traces overlap, the usual result is that the trace drawn last casts a shadow over the previous traces.

I think the older monochrome CRT DSOs that I have used could have done this with or without variable persistence since not all of them supported it.

It is not as accurate but this test works on analog sampling oscilloscopes where the display relies on dot density instead of grading.
 

Offline David Hess

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #71 on: April 18, 2014, 02:59:43 am »
To my understanding the best demonstration of the visibility of the noise distribution is to show a gaussian distribution on the board and how the eye makes a threshold on the visibility of the more rare events that the voltage or trace is 2x times the noise RMS value away from the mean. This threshold depend a little bit on the persistance of the analog scope phospor.

I have linked an example in this forum of doing this a couple of times.  This is one of those measurements which is esoteric but trivial on an analog oscilloscope while being difficult or impossible on a DSO unless it can calculate it for you.  In theory calculating the RMS value will work and that is how sampling voltmeters do it but somehow the DSOs I have tested return the wrong results.  A DSO which can generate a histogram and return the standard deviation should work better but that is one of those features used for market segmentation.

This is also one of those measurements where record length and aliasing are irrelevant.  Neither alters the standard distribution (or RMS value) of the signal so if it changes, the oscilloscope has failed by lying.  This includes oscilloscopes which have an anti-aliasing filter linked to their sample rate and is a good example of why anti-alias filtering has no place in a DSO.


I imagine a gray level grading where a logarithmic function is applied to the distribution and the positive and negative single sigma values get a red line. I think I simply recall what I have seen in some user plotting matlab code over the years.

Just adding a indicator for the standard distribution of the vertical signal at each horizontal point would be enough.  I am going to add that to the feature list of my imaginary DSO design.  At least then the false color display could be informative.
 

Offline katzohki

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #72 on: April 25, 2014, 03:07:13 pm »
I really appreciated this video, as I was one of the ones questioning how well my digital scope really was. Thank you for explaining it very well.

My Instek has other weird idiosyncrasies where if you zoom in really close on the noise (when paused) it looks like sin(x)/x pattern. Pretty sure it's a software glitch because it disappears when you change the timebase. Yikes!
 

Offline David Hess

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #73 on: April 25, 2014, 05:30:24 pm »
My Instek has other weird idiosyncrasies where if you zoom in really close on the noise (when paused) it looks like sin(x)/x pattern. Pretty sure it's a software glitch because it disappears when you change the timebase. Yikes!

Could it be aliasing with the horizontal resolution on the display side?
 

Offline Zbig

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Re: EEVblog #601 - Why Digital Oscilloscopes Appear Noisy
« Reply #74 on: April 25, 2014, 05:32:43 pm »
My Instek has other weird idiosyncrasies where if you zoom in really close on the noise (when paused) it looks like sin(x)/x pattern. Pretty sure it's a software glitch because it disappears when you change the timebase. Yikes!

Well, no, it's intentional. It is using sine interpolation in order to present you a nice smooth waveform no matter how far apart the actual consecutive samples are. Most scopes also offer an option to connect the dots linearly or not to connect them at all.
 


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