How important is Vertical Resolution on Rigol and other scopes? - Page 1
Forum section: electronics / general

author: Trinity
author_rank: Contributor
timestamp: November 17, 2010, 01:56:00 am
content: I watched the video blogs regarding the Rigol oscilloscope that Dave recommends. And I'm suitably impressed, with one exception.From information that can be found here:http://www.picotech.com/applications/oscilloscope_tutorial.htmlhttp://www.picotech.com/applications/resolution.html...it would appear that digital scopes with an 8 bit resolution are not a good choice for any serious work. From the information and examples given here, it would appear that 12 bit resolution should be the minimum desired, and 16 bit being the preferred specification.Tektronix doesn't get above 8 bit resolution until their 3000 series, and Rigol doesn't make a single scope above 8 bits.How does this fit with the recommendation of Rigol as a quality scope? Is the information on those two pages exagerating the problems? It doesn't appear to be, but I figured I would ask and get people's take on it.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 17, 2010, 02:34:18 am
content: "unsuitable for many applications ranging from monitoring sensors and transducers (temperature, current, pressure) to detecting noise and vibration. Precision oscilloscopes require more than 8 bit resolution."hmm. quite a concern for me... since thats the things i usually measure,those fancy sensors

author: chscholz
author_rank: Regular Contributor
timestamp: November 17, 2010, 03:09:19 am
content: As far as I know all current real time scopes (that consist of acquisition board, various knobs, display, etc.) are 8-bit.The better scopes have an enhanced resolution feature, that mathematically increases the number of bits up to 11 bits (using boxcar filtering).The drawback is that you loose bandwidth.I have been told that both LeCroy and Tektronix used to make scopes with higher resolution, but that there was essentially no market for higher resolution scopes.The marketing webpages you quote are correct in the sense that scopes do not make the greatest spectrum analyzers. If you need a "real" spectrum analyzer you will not be happy with a scope. Also note that the resolution numbers these webpages quote are theoretical values. The actual performanceof a scope depends on a lot more than the sampling depth.I can not evaluate if the scope you quote is adequate or if even a 12 bit scope module is good enough for you. Keep in mind that for serious audio work engineers tend to use 24-bit audio analyzers. I'd say that the best thing would be to test-drive the scope you are interested in and see if it works for your specific need.ChrisQuote from: Trinity on November 17, 2010, 01:56:00 am

author: EEVblog
author_rank: Unknown
timestamp: November 17, 2010, 03:32:19 am
content: Quote from: Trinity on November 17, 2010, 01:56:00 amThat is completely incorrect.Most digital scopes on the market are only 8bit vertical resolution.QuoteFor anything that requires a high dynamic range, yes, of course.But that is generally not the realm of the general purpose oscilloscope.QuoteIncorrect, they still only use an 8bit converter.They achieve a higher resolution only through averaging and filtering.Even insanely expensive oscilloscopes are still 8bit vertical resolution.The market simply has not demanded that general purpose scope need more than 8 bits.Look at the 7000 series scopes for example.And their 70000 series that cost more than a house, still only 8bit.See:http://www.cnam.umd.edu/anlage/Microwave%20Measurements%20for%20Personal%20Web%20Site/55W-17443-2.pdfQuoteIt doesn't, not at all, unless you have very specific needs that warrant a high dynamic range scope.QuoteYes, it is marketing that helps sell Pico and other high dynamic range scopes.It is not bullshit of course, high dynamic range scopes can be very useful and in many cases essential for some work.But the point it is a niche requirement, and that is why almost all general purpose digital scopes use 8bit converters, because it's "good enough" for most general purpose work.With sensors for example, you might typically use more specific instruments like a Dynamic Signal Analyser for example.Same for frequency analysis, you might be better off using a proper spectrum analyser.Audio? most will use a proper audio analyser like an Audio Precision.etc...Those 12 and 16 bit oscilloscope can be useful for some applications, but I would not think of buying one over a proper general purpose oscilloscope.As always, YMMVDave.

author: Mechatrommer
author_rank: Super Contributor
timestamp: November 17, 2010, 04:52:11 am
content: so is this means... my report on256 (vertical) resolution (8 bit)is all we need/have to do any wave analysis? is that the max resolution the scope got (8bit x V/div)? i doubt it, since we can get more detailed resolution values out of the scope... i believe. maybe i'm confused here.

author: Trinity
author_rank: Contributor
timestamp: November 17, 2010, 04:59:46 am
content: Quote from: EEVblog on November 17, 2010, 03:32:19 amThank you for explaining, Dave.However, as technology progresses over time, the idea is that we are improving and enhancing what we're capable of. I find it very odd that a brand new digital scope has, effectively, a lower resolution than most ancient analog scopes. I'm dumbfounded that the market would not have demanded at least equivalent performance before shifting from one to the other.

author: EEVblog
author_rank: Unknown
timestamp: November 17, 2010, 05:13:35 am
content: Quote from: Trinity on November 17, 2010, 04:59:46 amThe problem is trying to get decent performance out of a high speed converter above 8 bits.Once the manufacturers moved to "real time" scopes with sample rates 10 times the analog bandwidth, then you are talking seriously high speed converters. They can't even get a genuine 8 bits out of them across the board in many cases.Dave.

author: Trinity
author_rank: Contributor
timestamp: November 17, 2010, 05:59:15 am
content: Quote from: EEVblog on November 17, 2010, 05:13:35 amDoes that mean that it's recommended to own an analog scope in addition to a digital one? Or would you never actually need that?

author: jahonen
author_rank: Super Contributor
timestamp: November 17, 2010, 06:35:03 am
content: Quote from: Trinity on November 17, 2010, 05:59:15 amAnalog scope does show the "z-axis", i.e. modulated signals somewhat better than common digital scopes (no aliasing), although there are some digital scopes which do the same thing quite well also, if you can accept the price tag. Also, waveform capture rate is usually higher in analog scopes, limited only by time it takes to deflect the electron beam back to left edge and rearm the triggering system.Most people don't ever need to measure something likethisorthis, just for example, so it is not end-of-the world if there are no digital mimic of analog phosphor behaviour, although it gives somewhat better understanding of frequency of some random event.Regards,Janne

author: jahonen
author_rank: Super Contributor
timestamp: November 17, 2010, 06:39:15 am
content: BTW, at least National has dual channel1.8 GSa/s 12-bit ADC, with SNR of 58.5 dB and 9.4 ENOBs (Effective Number of Bits). I think the chip probably costs more than complete Rigol scope, so it comes with a price tag.Regards,Janne

author: GeoffS
author_rank: Unknown
timestamp: November 17, 2010, 06:44:36 am
content: Quote from: jahonen on November 17, 2010, 06:39:15 amPrice per chip $3258 but I'm sure they would work out cheaper if you bought a thousand or more

author: Psi
author_rank: Super Contributor
timestamp: November 17, 2010, 07:06:24 am
content: i find it funny that they list 'set-top box' as a possible application for that IC.

author: chscholz
author_rank: Regular Contributor
timestamp: May 13, 2011, 10:29:08 pm
content: This just changed Dave!LeCroy now makes 12-bit general purpose oscilloscopes!http://www.lecroy.com/Oscilloscope/OscilloscopeSeries.aspx?mseries=352&capid=102&mid=504ChrisQuote from: EEVblog on November 17, 2010, 03:32:19 am

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 14, 2011, 04:47:20 am
content: Quote from: chscholz on May 13, 2011, 10:29:08 pmwhat is changed? the price?

author: Zero999
author_rank: Super Contributor
timestamp: May 14, 2011, 11:00:47 am
content: I don't own a propper digital 'scope and was thinking about this a few days ago.I would've thought there would be a cheap 'scope with a high vertical at low frequencies, i.e. below 500kHz, even if it's only 8-bit at higher resolutions but then I concluded there's obviously not much demand for one.Quote from: Trinity on November 17, 2010, 05:59:15 amJust another thought, does a cheap analogue 'scope even have equivalent to 8-bit precision?Whilst it may be true an analogue signal theoretically has an infinite number of bits per sample, in practise it doesn't always mean it carries more information than a certain bit rate. For example, if the vertical amplifier in the analogue 'scope has > 0.39% error then it can't be any better than an 8-bit analogue to digital converter.

author: saturation
author_rank: Super Contributor
timestamp: May 14, 2011, 12:55:50 pm
content: Scopes, analog or digital, lack vertical precision, and thus too its accuracy, when compared to a DMM. But as a tool, scopes main value are to trend changes over time, not to measure absolute values like a DMM.Most transducers I work with output DC, so even a DMM can do the task but a scope is a nicer way to see fast glitches than a typical DMM [ but you can do it with a minmax, peak or data logging capable DMM.] Many automotive signals are low frequency variable DC, and can be resolved with both a DMM or a scope, however, again its not the absolute value that is vital but the waveform and changes over time.As far as low voltage readings, tranducers aren't going to output say, a steady 10V and transition 1mV to represent the sensor value; usually the entire range of output is proportional to the sensor value and thus, the selected scope scale is more than adequate to resolve changes.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 14, 2011, 01:00:15 pm
content: Quote from: Hero999 on May 14, 2011, 11:00:47 amThe avarage analogue scope has a screen height around 100mm, so 1 LSB at 8 bits corresponds to a deflection of 0.4mm, i.e. less than the beam width, so the effective resolution of most analogue scopes will be of the order of 7 bits. Unlike digital scopes, zooming in isn't an option. Even the Lecroy can only show about 10 bits of vertical resolution without zooming.

author: Richard W.
author_rank: Regular Contributor
timestamp: May 14, 2011, 02:42:20 pm
content: 8 bits resolution means 256 different values.A standard DSO-screen is about 320*240 pixels.Effective screen-size for the "beam" is about 250px wide and 200px high.256 values for 200px are enough, at least if you don't zoom in.Take also in account that the vertical accuracy is mostly worse than +/-1,5%Compared with the vertical resolution of an analog scope (screen dimensions and beam diameter) the values are almost the same.and by the way: its called oscilloSCOPE (it shows you something) and not oscilloMETER (wich does accurate measurements)Compared with the vertical resolution of an analog scope (screen dimensions and beam diameter) the values are almost the same.and by the way: its called oscilloSCOPE (it shows you something) and not oscilloMETER (wich does accurate measurements)

author: vk6zgo
author_rank: Super Contributor
timestamp: May 14, 2011, 03:41:39 pm
content: Quote from: saturation on May 14, 2011, 12:55:50 pmBravo!!It is good to see someone make this point.So many people on this forum get all tied in knots about absolute accuracy,when:-(1) The measuring device is not capable of such accuracy.(2)The degree of accuracy is not necessary to carry out the job.(3) In many cases there are other unconsidered effects,(for instance stray capacitance) which can cause errors of a similar,or greatermagnitude to the instrument's inbuilt error.VK6ZGO

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 14, 2011, 04:10:02 pm
content: ...searching the word oscillometer, its in medical. o well, the more accurate the vertical reading, the better the oscilloSCOPE should be. you never experience how useless my rigol was for analog design when its calibration data got corrupted, ie 5V square signal read as 3V signal. but it sure can show the trend of an oscillation. if you dont know what i mean, try corrupting your calibration data

author: Zero999
author_rank: Super Contributor
timestamp: May 14, 2011, 06:20:31 pm
content: Quote from: mikeselectricstuff on May 14, 2011, 01:00:15 pmThat's not true, I have an analogue 'scope with a zoom function.The accuracy of the vertical amplifier and CRT are more likely to be sources of error than reading the screen.

author: FreeThinker
author_rank: Frequent Contributor
timestamp: May 14, 2011, 09:06:54 pm
content: Quote from: Hero999 on May 14, 2011, 06:20:31 pmYep! I had a very nice tek scope (475a i think) and it had horizontal zoom.Wish I still had it

author: qno
author_rank: Frequent Contributor
timestamp: May 14, 2011, 09:13:17 pm
content: When I was starting in electronics the moving coil galvanometer of the d'Arsonval type where the only one available.The good ones had an accuracy of 1% and a resolution of 1% depending on the width of the needle.They send people to the moon with electronics developed with those kind of meters.The first digital oscilloscopes had a 7 bit converter. This was considered more than sufficient when the overall scope accuracy was 2 to 3 %.An 8 bit oscilloscope has a 2 times higher resolution and accuracy. I think that is good enough for general scope measurements.Usually one trades in speed for vertical resolution.I would go for the longer waveform memory rather then vertical resolution.

author: Zero999
author_rank: Super Contributor
timestamp: May 14, 2011, 09:21:43 pm
content: It should be easy to develop a 'scope with 9-bit/sample, all that needs to be done is a high speed fullwave precision rectifier be connected before the DAC and a sign bit taken from a comparator, giving a dynamic range of +/-255.

author: qno
author_rank: Frequent Contributor
timestamp: May 14, 2011, 09:27:25 pm
content: Why bother, that's exactly what the MSB does.

author: Wim_L
author_rank: Regular Contributor
timestamp: May 14, 2011, 09:47:57 pm
content: Well, the Tektronix TDS 3000 series has 9 bits. Effective number of bits tends to be lower though, they get pretty noisy.And yes, those folks at Picoscope are right. For some applications you need more than 8 bits. But that doesn't mean you should get a high resolution oscilloscope. Because high resolution fast ADCs are a rarity (and very expensive), high resolution usually means slow. Too slow for a general purpose oscilloscope. Most of Picoscope's scopes are 8-bits too, just check their list. Some claim more in high resolution mode, but that's averaging multiple samples together, and lowers the bandwidth. The ones that really do have higher resolution are also slower products. So the real solution is to get a normal scope, and get additional tools for lower speed high resolution work.

author: Zero999
author_rank: Super Contributor
timestamp: May 14, 2011, 09:58:35 pm
content: Quote from: qno on May 14, 2011, 09:27:25 pmI thought ADCs use twos complement.I'm talking about using an 8-bit ADC and adding a sign bit.

author: Wim_L
author_rank: Regular Contributor
timestamp: May 15, 2011, 12:00:13 am
content: ADCs are mostly unsigned (especially pure flash ADCs by themselves --- Integrated devices that contain more than just the ADC might have options for other output formats). You just apply an offset to the signal or ADC reference voltages and adjust the values appropriately afterwards if you want a sign.

author: tekfan
author_rank: Frequent Contributor
timestamp: May 15, 2011, 02:03:26 am
content: I think Keithley makes bench multimeters that sample at 1MHz. Not as fast as a scope but definetly more precise.Quote from: qno on May 14, 2011, 09:13:17 pmFor a scope, memory really is far more useful than vertical res

author: sacherjj
author_rank: Frequent Contributor
timestamp: May 15, 2011, 02:24:00 am
content: Quote from: Hero999 on May 14, 2011, 11:00:47 amThe cheapest way to get upto 196 kHz at 24bit is with a high end audio card.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 15, 2011, 05:55:30 am
content: Quote from: FreeThinker on May 14, 2011, 09:06:54 pmThe cheaper analogue oscilloscopes don't have the dual/delayed timebases needed for this.This very useful facility is usually confined to more expensve ones,such as,HP,Tektronix,Iwatsu,etc.The term "zoom" is not commonly used with analogue units,but it does the same job.Of course,an analogue oscilloscope cannot save a waveform,then"zoom" in on the saved waveform,like a DSO,as it's only available in real-time.VK6ZGO

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2011, 08:44:56 am
content: Quote from: Wim_L on May 15, 2011, 12:00:13 amYes that makes sense to me.Twos complement can be generated by biasing the input signal at half the supply voltage but it seems just as easy to use a precision rectifier in the manner I described earlier and get an extra sign bit.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 15, 2011, 12:16:49 pm
content: sign is just a concept which needs/take up 1 bit from the system. we remove that concept, then we will have extra 1 bit for double magnitude unsigned value. my 2cnts.

author: joelby
author_rank: Frequent Contributor
timestamp: May 15, 2011, 01:27:41 pm
content: Quote from: Mechatrommer on May 15, 2011, 12:16:49 pmIf the signed values are stored in 2's complement representation, it doesn't make any difference whatsoever.In 8 bits:2's complement: -128 to 128 = 256 valuesUnsigned: 0 to 255 = 256 valuesIf you're using a sign bit, then you can represent:-127 to -0, +0 to+127 = 255 values (zero is repeated)I don't think there are many ADCs that would use this representation, though.

author: chscholz
author_rank: Regular Contributor
timestamp: May 15, 2011, 05:46:58 pm
content: You are right about slow, the LeCroy 12 bit scopes go only up to 600 MHz (as opposed to 45 GHz for 8 bit scopes), processing speed is the same.Yes the 12 bit scopes a more expensive than the equivalent 8 bot scopes.Quote from: Wim_L on May 14, 2011, 09:47:57 pm

author: chscholz
author_rank: Regular Contributor
timestamp: May 15, 2011, 05:51:08 pm
content: How about a scope with WXGA resolution and a screen that can be flipped into "portrait mode"?They call it "High Resolution Oscilloscope" (HRO), not scopeMETER, though.Quote from: Richard W. on May 14, 2011, 02:42:20 pm

author: Wim_L
author_rank: Regular Contributor
timestamp: May 15, 2011, 07:44:49 pm
content: Quote from: Hero999 on May 15, 2011, 08:44:56 amWell, try making a 1 GHz precision rectifier, then come back to tell us how easy it was

author: Zero999
author_rank: Super Contributor
timestamp: May 15, 2011, 07:57:52 pm
content: Quote from: Wim_L on May 15, 2011, 07:44:49 pmIt's much more easy to make than a 1GHz ADC.

author: Drirr
author_rank: Regular Contributor
timestamp: May 15, 2011, 08:34:51 pm
content: I think that vertical resolution of Rigol is enough. But sometimes some details of signal disappears >Rigol:https://picasaweb.google.com/lh/photo/T79gwN4hyidXu2mCnPgGVA?feat=directlinkAnalog oscilloscopehttps://picasaweb.google.com/lh/photo/tXaV9DXK_VJwS_4MDOiL6Q?feat=directlinkLook carefully at the top, same sine.

author: colinbeeforth
author_rank: Contributor
timestamp: May 16, 2011, 08:47:27 am
content: Nicolet used to make slow sample rate, higher resolution digital storage oscilloscopes (10-12 bit). It was a very niche market, and hardly any were sold in Australia. I think they are no longer made. Most purchasers were researchers in physiology labs, in particular people studying brain and nerve waveforms.There have been a few exceptions, I remember an older LeCroy 10 bit scope. One of the first HP digital scopes was only 6 bits!I don't think most people have a need for more than 8 bits for general scope purposes, which is fortunate since flash ADCs have 2xcomparators, where 10 bits = 1024 comparators, it gets out of hand quickly past 8its. Anything other than flash ADCs are much slower. So 8 bits is a sweet spot.8 bits is useless for audio measurement, but then don't most audio enthusiasts want more bits than can be had on a planet with an oxygen atmosphere?

author: abuabacus
author_rank: Newbie
timestamp: May 21, 2011, 04:51:58 pm
content: Hi there,I think few people think a little bit too "digital"Let assume that the beam width is 0.4 mm. Does it mean that the trace must run on a 0.4 mm raster ?No, definitely not. An average human eye can recognize 0.1 mm difference easily (from a reasonable distance).So on a 10 cm high display we can recognize 1000 lines. I have only 8 cm high display, so for me it is "only" 800values to use. Even if I could recognize only 0.15 mm on my 8 cm display I will have more than 9 bits resolution.On a high quality scope with 10 cm high display it is closer to 10 bits than 7 bitsAnyway. I've just received my Rigol DS1052E yesterday. I like it very much. This will be mine main 'scope for everyday work.What I don't like: the input noise... and of course I'll keep my old analog ones: a 100 MHz Kikusui and a 60 MHz C1-64.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 22, 2011, 02:15:17 am
content: once digitized, then its become digital. if someone explained that a few mm'th on 8bit scope is equivalent to 10bits value, then thats just a plain bullshit. no offenseor maybe u explaining something else.and as i recalled someone explaning about noise, i translated it as the noise is so dense that the analog scope's phosphor cannot keep up and only showed up as a nice thick faded line on the analog that is pleasing to the eye, unlike the finite'd sampling rate of digital scope.

author: abuabacus
author_rank: Newbie
timestamp: May 22, 2011, 07:09:27 am
content: "no offense"none taken"or maybe u explaining something else."I've just reflected on:"...so the effective resolution of most analogue scopes will be of the order of 7 bits." in #16.

author: Bored@Work
author_rank: Super Contributor
timestamp: May 22, 2011, 07:09:53 am
content: Quote from: Mechatrommer on May 22, 2011, 02:15:17 amIt is even worse. You have the ADC doing one digitalization, including a quantization. Then the software needs to map the data to the screen. The screen also only has discrete positions. When doing it, the software needs to take things like the current vertical offset and vertical V/div into account. Chances are slim that the calculated position of a sampled value on the screen would exactly hit one of the discrete pixel positions. The software needs to round to the nearest position. This is a second quantization, which you can model as noise introduced.

author: Zero999
author_rank: Super Contributor
timestamp: May 22, 2011, 09:42:05 am
content: Quote from: BoredAtWork on May 22, 2011, 07:09:53 amBut surely that's not a problem, if you use the cursors to read the voltage of a given sample?

author: Bored@Work
author_rank: Super Contributor
timestamp: May 22, 2011, 12:23:17 pm
content: Quote from: Hero999 on May 22, 2011, 09:42:05 amIt is. You can only position the cursor on pixels that exist on the screen, not between pixels. And you therefore can only get discrete cursor positions on an LCD. If you e.g. use a horizontal cursor to measure the amplitude of a signal, you can only position the cursor at fixed increment heights. The horizontal cursor is suffering from the same quantization as the amplitude.If you have a more advanced cursor system and use a vertical cursor to measure the amplitude of a signal at a certain position in time, it depends from where the cursor software fetches the amplitude value. If it is the screen buffer, you again have the same problem.And again you can only move the cursor in fixed horizontal increments. If the horizontal (time) axis for whatever reasons isn't representing one sampling interval per pixel (e.g. you zoomed in or out of an already acquired waveform, or some interpolation is used to connect samples), you again have a quantization problem.

author: Unknown
author_rank: Unknown
timestamp: May 22, 2011, 02:42:01 pm
content: I believe the automated measurements are also based on the displayed signal, so even they might suffer from the double quantization errors.

author: Leo Bodnar
author_rank: Frequent Contributor
timestamp: May 22, 2011, 02:51:03 pm
content: Quote from: abuabacus on May 21, 2011, 04:51:58 pmVisual acuity of most humans is 1-2 arcminutes which makes 10 cm display from 30 cm distance an equivalent of ~10 bit resolution grid.Of course you need at least 11 bits of resolution to make it look "analogue."Brain is a complicated parallel processor (think 2D FFT analysis) so maybe things like slanted lines need even better quality to look really natural.

author: Zero999
author_rank: Super Contributor
timestamp: May 22, 2011, 02:52:50 pm
content: Quote from: BoredAtWork on May 22, 2011, 12:23:17 pmQuote from: alm on May 22, 2011, 02:42:01 pmThat's poor.The question is what 'scopes use the raw data for cursors and how can one tell the difference? Is it normally given in the specification?The horizontal cursor problem could be overcome by using the zoom as long as the raw data is used of course.

author: Trinity
author_rank: Contributor
timestamp: May 22, 2011, 04:00:01 pm
content: Quote from: abuabacus on May 21, 2011, 04:51:58 pmCan you elaborate a bit on this? Is it worse than spec, or is it that the specs aren't good enough? How much of a problem does it cause?

author: Wim_L
author_rank: Regular Contributor
timestamp: May 22, 2011, 10:37:37 pm
content: Quote from: Hero999 on May 22, 2011, 02:52:50 pmIt doesn't make sense, as the cursors are moved by the scope user, who only can see their position on the pixel grid on the screen.Now, for effective resolution of an analog scope, I have a Hameg combiscope (analog scope plus digital storage mode). In analog mode, speaking of a display resolution doesn't make much sense. However, cursor resolutions are based on a 2000 horizontal by 1000 vertical grid. So the manufacturer does consider that a useful resolution, even though in digital mode, the 8-bit converters limit vertical waveform resolution to 256.

author: Unknown
author_rank: Unknown
timestamp: May 22, 2011, 10:53:34 pm
content: Quote from: Wim_L on May 22, 2011, 10:37:37 pmBut many scopes do indicate the intercept of the current cursor with the signal (eg. voltage of the signal at the current time cursor), which could use the same resolution as the raw signal. Same for automated measurements.

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 23, 2011, 07:10:26 am
content: i think it should be like this... whats digitized (ie saved in memory) thats supposed to be displayed on proper pixel location (or double pixelated if necessary depending on screen size). when the cursor moved, it should be reading that pixel location and hence the same value in the memory. otherwise, the manufacturer should find another business. even so there is discrepancy between screen pixel position and memory value, it should be somewhere just between ±1 pixel. so far i cannot tell the difference in any reading i've done, it doesnt matter, there's alot of noise there if you really peek into a single particular pixel. and as someone said, dso got more error % in reading accuracy, then i will simply admit that with salt and grain.

author: Zero999
author_rank: Super Contributor
timestamp: May 23, 2011, 04:32:39 pm
content: I don't even see why a low X resolution should cripple the measurement capability. Suppose the 'scope is set to 1ms per screen and the X resolution is 500 which is 2us per pixel but the memory buffer holds a total of 2000 samples, one every 500ns. All that needs to be done is have a fine tune mode which causes the x measurement to be incremented sample by sample, even if the cursor is only moved every four samples.

author: seattle
author_rank: Contributor
timestamp: May 23, 2011, 08:46:10 pm
content: QuoteThe raw data is all you have.Say a scope is at 2V/div, and there are 10 gridlines. In order to not the show the user a clipped signal, you'd want the input range to be +20V to -20V (not including any offset voltage the scope might be able to apply). That means 40V covered by an 8-bit ADC. That gives you 156 mV resolution.On a good scope such as Rigol and beyond, you'd have a VGA after the input and as you reduced the input sensitivity, you'd increase the gain, so again your entire 8 bit could be used to digitize a +/- 10V signal, which would give 20/256=78mV resolution.As you move down to 5mV/div, you'd again adjust the gain so that 8 bits was digitizing +/-50mV or 100/256 = 390uV resolution.And of course, the analog offset voltage allows you still get that full 8 bit resolution on a signal riding atop a large DC component. For example, if I have a 30V DC signal, I can offset that with -30V and then set to 5mV/div and get 390uV resolution. Or switch to AC coupling.Now, on a cheap scope, like a Link Instruments USB scope (MSO19), there isn't a variable gain amp after the input. So, as you get to lower voltage levels you can actually see the ADC quantiziation. But these are fortunately rare.Thus, given a proper scope with a VGA following the input stage, you can practically measure just about any resolution signal you might want to know with great accuracy, regardless of scope ADC.THE PRIMARY CASE where the an 8-bit ADC might bite you is if you are looking at noise and using your scopes FFT function.An 8-bit scope will BEST CASE give you a noise floor of around -56 dB full scale at the highest sample rate. A 10-bit converter will do about 12 dB better. A 12-bit converter will be about 24 dB better than an 8 bit scope.Note that fast sample rates can be traded off for resolution. A 10-bit scope running at 100MSPS that is sampling a signal at an effective 100KSPS can pick up almost 6 more bits of effective resolution, meaning a 10-bit converter can look (almost) like a 16-bit converter at the lower sample rates.

author: chscholz
author_rank: Regular Contributor
timestamp: May 24, 2011, 04:02:15 am
content: In my world cursors are estimates and measurements are, well measurements.Here is an admittedly extreme example of what happens when not using the full voltage swing. We are measuring risetime, falltime, width and period of a 1-0 pattern with 6.4 Vpp, once using 5 V/div the other time using 0.65 V/div.Measurement uncertainty (as measured by standard deviation over 1,000 edges) for the risetime at 5 V/div is 41 ps, for 5 V/div it is 214 ps, etc.ChrisQuote from: seattle on May 23, 2011, 08:46:10 pm

author: vk6zgo
author_rank: Super Contributor
timestamp: May 24, 2011, 05:49:45 am
content: Of course,there is seldom any reason not to use as large a display height as you can with a non-CRT display.With Analog CRT 'scopes,you can get some deflection distortion at the extremes of vertical display size,so for best accuracy,the full range of the screen is not usually used.The error from trying to read rise time,etc with insufficient display height happens with analog too,but here it is due tothe fact that we humans have a hard time discerning the 10% to 90% interval on the waveform edge,with a very small displayed height on the screen.VK6ZGO

author: Mechatrommer
author_rank: Super Contributor
timestamp: May 24, 2011, 09:51:28 am
content: @chscholz: thats normal, thats the world of digital (binary) quantisation, there is only integers, not real numbers, in this case of screen pixel mapping and limited bits domain. if you can amplify the pixels into large visible blocks and analyze the x,y distance and dx/dy, then you'll know why using max swing will give you better result. for properly design scope esp the analog front end, full swing will translate to max resolution.

author: Wim_L
author_rank: Regular Contributor
timestamp: May 24, 2011, 11:24:24 am
content: Quote from: vk6zgo on May 24, 2011, 05:49:45 amTo help with this, many analog oscilloscopes have markers etched on the inside of the tube to align the waveform on for these measurements, so you don't really need to guess that much. Some even have double cursors designed specifically for it (horizontal cursors that align to the starting and ending voltages, followed by crosshairs at the 10 and 90% positions relative to the horizontal cursors that you can then place on the waveform).