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Old 27th April 2014, 09:34 PM   #1
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Default Oscilloscopes and noise: Digital Versus Analog

There is a common myth that has been propagated around here that the input stages on digital scopes are inherently noisier than analog scopes. While I have known that this isn't true, I have not found a great comprehensive source on why this is so. Dave Jones over at EEVBlog just did a two part series(actually three if you count the episode that started this) on this topic. It's a must watch for anyone who works with circuits. He explains how sampling technology picks up and reveals noise/signal that an analog scope will hide. The effect of memory depth on the illusion of noise. The effect of scope bandwidth on noise. He also discusses the advantages to the analog method as well as the digital method. Explains the logic behind technologies like Digital Phosphor (aka Intensity Gradient, Variable Intensity, etc.). It's a really good comprehensive primer into noise and oscilloscopes; a must watch for anyone working with circuits.

Part 1:
https://www.youtube.com/watch?v=Znwp0pK8Tzk

Part 2:
EEVblog #610 - Why Digital Scopes Appear Noisy - Part 2 - Page 1
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Old 27th April 2014, 10:10 PM   #2
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I don't know, but suspect that a) for most people the residual noise of a good analog scope is a non-issue, and b) that for the most part a top quality used analog scope is as good or better than a cheap digital scope in that department.

I suspect that the high quality new Agilent, Tek and other top brand digital scopes are tremendously good instruments.

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Old 27th April 2014, 10:43 PM   #3
DUG is offline DUG  Canada
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What might be happening is an analog 20MHz scope (that has (and can use) a 2mV/div scale ) is being compared to a 500MHz digital scope ( that usually has a 5mV/div scale).

The wider bandwidth may contain LESS noise per root Hz but cannot demonstrate that.

Just a thought.

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Old 28th April 2014, 05:32 PM   #4
PChi is offline PChi  United Kingdom
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At least one of the early digital oscilloscopes had terrible inter channel cross-talk. I can't remember which model. With all the switch mode power supplies and processors in the digital Oscilloscopes they may have been circumstances with some models when the noise was worse than an analogue one. So I think that the myth had a grain of truth. Not generally true today though.
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Old 28th April 2014, 07:04 PM   #5
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I don't think I made myself clear enough earlier. I am not attempting to say that analog scopes are bad. Just trying to clear up some misinformation I commonly see spouted about digital scopes being noisy.

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Originally Posted by bear View Post
I don't know, but suspect that a) for most people the residual noise of a good analog scope is a non-issue, and b) that for the most part a top quality used analog scope is as good or better than a cheap digital scope in that department.

I suspect that the high quality new Agilent, Tek and other top brand digital scopes are tremendously good instruments.

_-_-
The point that Dave makes is that the noise you can clearly see in a digital scope is HIDDEN by the analog scope because of our persistence of vision. He's saying that what looks like a crystal clear sharp trace on an analog scope can actually hide a lot of signal if its very high frequency because of persistence of vision.

I think each has its benefits. An analog helps you see the "real" signal better without the noise. Pure digital scopes show you the "true" signal with the real noise thrown in which can sometimes obscure the "real" signal. DPO type scopes try to bridge both giving you both types of information at once. That being said, if you get a low-end scope from one of the more reputable manufacturers (Rigol, some Siglents, etc) then you will have a scope on par or significantly better than old top-end analog scopes. I often cite the Rigol DS1074Z, it has DPO functionality, ability to decode digital data, plus lots of other stuff. In my opinion putting it at least on par with the old Tek 24xx series scopes and for $500-600 new. If you don't go with a reputable manufacturer then I would agree (Owon for example). The market has changed dramatically in the past few years. 4-5 years ago I would have agreed whole-heartedly that one of the old Tek scopes is a better value. Now the bottom line is really putting some downwards pressure on the old analog scopes.

As for a top-end new scope, no doubt they are AMAZING instruments. It's really a golden era of scopes we have entered. There is true market pressure across the board which have made available to hobbyists stuff they could only dream of a decade ago. Agilent recently had a deal one their DSO-X scopes where if you purchased a single option they gave you ALL of the other options for free. This is in direct response to encroachment in performance by companies like Rigol (and Tek to a much lesser degree).

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Originally Posted by DUG View Post
What might be happening is an analog 20MHz scope (that has (and can use) a 2mV/div scale ) is being compared to a 500MHz digital scope ( that usually has a 5mV/div scale).

The wider bandwidth may contain LESS noise per root Hz but cannot demonstrate that.

Just a thought.

I think that is one of the points her tries to illustrate so you hit the nose on the head. About the dependence on bandwidth and noise.

My digital scope (LeCroy WaveAce 214) has a 2mV scale. Some go even lower. The Rigol DS1074Z/DS1104Z (which should not be confused with the DS1052/DS1102 series as they are 100% different) has resolution all the way down to 500uV/div.

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Originally Posted by PChi View Post
At least one of the early digital oscilloscopes had terrible inter channel cross-talk. I can't remember which model. With all the switch mode power supplies and processors in the digital Oscilloscopes they may have been circumstances with some models when the noise was worse than an analogue one. So I think that the myth had a grain of truth. Not generally true today though.
Yeah that might have been, the early era of digital scopes was an interesting place filled with loads of compromises and strange choices. Plus people not quite sure what they were doing. Just look at the old Nicolet digital scopes. THose are interesting beasts. Some are high precision scopes, the likes you barely see even today. Just they had really low frequency ranges, I'm guessing they were intended more for physics labs than electronics labs.
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Old 30th April 2014, 01:32 PM   #6
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The first digital Tex scopes used CCD (charge coupled delay lines) which were very noisy.
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Old 5th May 2014, 08:12 PM   #7
Elias is offline Elias  Finland
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Digital scopes have averaging function to remove the noise from the display

The biggest benefit of a digital scope over analog is you can take screen shots more easily Other than that I can hardly find another benefit.


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Old 6th May 2014, 12:08 PM   #8
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Quote:
Originally Posted by Elias View Post
Digital scopes have averaging function to remove the noise from the display

The biggest benefit of a digital scope over analog is you can take screen shots more easily Other than that I can hardly find another benefit.


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Old 6th May 2014, 12:21 PM   #9
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The digital scope's case size/depth is the third.
Colored traces the fourth.
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Old 6th May 2014, 06:11 PM   #10
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FFT the fifth.
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