Understanding Oscilloscope specs - Page 1
Forum section: electronics / testgear

author: mcgee
author_rank: Newbie
timestamp: May 11, 2016, 03:04:55 pm
content: Hi EEVBlog Community,I've a question concerning an oscilloscope and if it fits my needs. I'm not a professional in electronics (and light years away from Dave's skills) but I "experiment" with AVR µc's and can build basic electronic circuits but mostly digital stuff (and on an Arduino level if you know what I mean). So I think that I'm an advanced beginner. For my newest project I need some kind of oscilloscope since I need to visualize/inspect/debug data on a data bus. The data bus has a clock with 10 MHz and runs also with almost 25 MHz (24-odd-MHz).Therefore I searched for a reasonable scope and found the "Analog Discovery 2" in a video of Dave (actually v1 is mentioned in the video) and I quite like it: it's cheap (for me, as a student even more) and the software seems to be quite nice.Here you'll find the reference manual.The important specs are:Two-channel oscilloscope (1M?, ±25V, differential, 14-bit, 100Msample/sec, 30MHz+ bandwidth - with the Analog Discovery BNC Adapter Board)16-channel digital logic analyzer (3.3V CMOS, 100Msample/sec)My questions are:(1) Can I calculate this way:If I have a clock at 25 MHz the signal is toggled every 40ns. 100Msample/sec means 100,000,000 sample points per second meaning that every 10ns a sample is acquired. Therefore we get 4 samples per "signal" state. For the 10 MHz signal I'll get 10 samples. But from this I can't really tell if this is good or not.(2) So can I measure the desired signals?(3) For the analog channels: I assume from Dave's video that this is a reasonable spec for a beginner. What does 30 MHz mean?(4) The digital LA is 3.3V CMOS. Does this mean that I can't feed in 5V signals or does it mean that the digital threshold is 3.3V?Greetings from Germany!P.S.: It would be quite nice if Dave could make a Fundamental Friday's Video: The specs of a scope and their meanings explained!

author: ECEdesign
author_rank: Regular Contributor
timestamp: May 11, 2016, 03:38:01 pm
content: Heres a quick video from Keysight about basic oscilloscope specs. This channel also has some other good videos about oscilloscopes.

author: rstofer
author_rank: Super Contributor
timestamp: May 11, 2016, 04:14:48 pm
content: Quote from: mcgee on May 11, 2016, 03:04:55 pmA good experiment is far better than a bunch of numbers. I just used my AD (original) to sample a 4 signal bus at 25 MHz, 12.5 MHz, 6.25 MHz and 3.125 MHz. The display shows 4000 samples at 100 MHz which would seem to indicate 1000 samples per channel at 100 MHz. But that's the display, the signals are sampled 16 bits wide at 100 MHz. As to whether 10 nS sampling is sufficient, it depends on how fast the signal is changing and, in my case, that's 25 MHz. My FPGA has sloppy rise/fall time so there is a definite slope to these edgesQuoteYes, but you're getting a little thin up at 25 MHz. Generally, logic signals are synchronous so we kind of know the timing relationships. If you're trying to look at setup or hold time, at higher frequencies, this probably isn't the right tool.QuoteThe bandwidth is the frequency at which a signal is 3 dB down from where it should be. As you approach this point, the displayed level will fall off.QuoteIt may turn out that the inputs are protected from 5V signals but that's about it. The device is intended for 3.3V inputs so use the appropriate Arduino. This device is too expensive to expose it to extraneous voltages beyond its capability.QuoteI was stationed in Mannheim back in '66-'67 - a very nice tour considering the alternatives.I think you'll like the Analog Discovery - I certainly like mine. But it isn't a high frequency anything. The first attachment shows those 4 signals I posted above and the second shows the scope image of the 25 MHz signal. Note the '+' marks at each 10 nS grid. Those are the actual samples and, as you can see, it isn't much of a square wave and the voltage levels (0..3.3V) aren't correct. It looks a good deal better on my DS1054ZI prefer to think of the AD as a device for 1 MHz and below. Not that it doesn't work at higher frequencies, it's just that there are better tools for those frequencies.

author: rstofer
author_rank: Super Contributor
timestamp: May 11, 2016, 04:20:16 pm
content: In Dave's video re: the AD, he mentioned something about cursors. The cursors are added to the screen by clicking on the little X in the lower left corner. The cursors are smart enough to do 'delta' arithmetic.

author: mcgee
author_rank: Newbie
timestamp: May 11, 2016, 06:52:22 pm
content: Hi,and thank you for your response "rstofer".Quote from: rstofer on May 11, 2016, 04:14:48 pmNice. Okay, Mannheim isn't that great for representing Germany but hey Americans might like it since the city of Mannheim has a district which has "American style" layout (rectangular streets) and house numbers :-) I'm a great fanboy of you Americans over there ...Quote from: rstofer on May 11, 2016, 04:14:48 pmI had the opportunity to play around with a Rigol DS1074Z with 1 GSample/sec, 70MHz and 12 Mpts memory depth. I attached a screenshot of a 7.57 MHz square wave. And for me this also does not look very nice.The point is that I'm a hobbyist and I'll not buy such a expensive tool since I've only little time for this hobby - so it's not worth. And most of the time I'm doing digital stuff with Arduino and AVR and there I don't need a scope most of the time (or up to now I hadn't a situation where I would have needed a scope). But I've this one pending project where I'm dealing with a parallel data bus where I really need to peek into the lines to debug. But I could run it during development on a lower "bus clock speed" like 5 MHz debug everything and finally ramp up the clock speed. This plan might work.Besides that I've to decide if I should buy it ...Greetings!

author: nidlaX
author_rank: Frequent Contributor
timestamp: May 11, 2016, 07:05:50 pm
content: Quote from: mcgee on May 11, 2016, 06:52:22 pmHow are you generating that square wave, and what are your probe / input attenuation settings?

author: rstofer
author_rank: Super Contributor
timestamp: May 11, 2016, 08:17:08 pm
content: Slowing things down a bit, I have attached another pair of waveformsThe single channel scope trace has a frequency of 390 kHzThe logic analyzer traces has a max frequency of 3.125 MHz and the others are binary sub-multiples.I am much more comfortable dealing with these frequencies simply because there are more samples.The signal generator is an FPGA with slow slew rate and dangling wires. Nothing is terminated, the ringing is just what it shows. The waveforms are the same on my DS1054Z.Sometimes it is necessary to adjust the scope probe compensation to get rid of rounded edges.I was pretty happy using a 200 MHz logic analyzer on a 25 MHz project so I don't know why I wouldn't feel the same about a 100 MHz logic analyzer on a 10 MHz project. Something around 10x seems like a good place to start.The Hobby Components Logic Analyzer I order a couple of weeks ago showed up last week. Tomorrow I will see what it does. Since sample rate is a function of USB bus speed, I don't know what to expect. I do have some USB 3.0 ports so I'll have to see how they work.http://hobbycomponents.com/test/243-hobby-components-usb-8ch-24mhz-8-channel-logic-analyserMaximum sample rate is 24 MHz so I suppose I would be happy using this gadget at 2 MHz or less. The thing is, if the USB bus is slow, this device won't sample more than 2 Msps and I would probably want to use it at 200 kHz or less. We'll see...

author: tautech
author_rank: Super Contributor
timestamp: May 11, 2016, 08:55:52 pm
content: Quote from: mcgee on May 11, 2016, 03:04:55 pmOne must remember that the sample rate is a collection of datapointsthat are then joined with aninterpolationprocess to form the displayed waveform.Interpolationcanbring inaccuracies to the displayed waveform if there's a low # of data points from which to reconstruct the waveform.Memory depth also has a bearing on how deep one can investigate a waveform as a nice little .gif from member rf-loop in this thread demonstrates.From this thread:https://www.eevblog.com/forum/testgear/siglent-sds1000x-series-oscilloscopes/

author: tggzzz
author_rank: Super Contributor
timestamp: May 11, 2016, 10:09:04 pm
content: It is good to see someonethinkingabout the theory and practice of measurements.The sample rate and bandwidth are two independent characteristics. You can get 1GS/s 20MHz scopes, and 25MS/s 1GHz scopes. The first is useful for non-repetitive signals with frequency components less than 20MHz and risetimes longer than 17.5ns. The second is useful for repetitive signals with frequencies up to 1GHz and risetimes longer than 350ps; an important use with pseudo-repetitive signals are "eye diagrams".

author: rstofer
author_rank: Super Contributor
timestamp: May 11, 2016, 10:15:08 pm
content: You can see that sampling phenomenon quite clearly in Waveforms1.png in Reply #2 above. The scope takes periodic samples and simply draws a straight line between them. It doesn't have enough information to realize the signal was a square wave. Given a select sample frequency, a square wave could look like a straight line.Analog scopes don't have sampling issues. Of course, they also don't have storage, measurement or math capabilities (beyond Ch1 +- Ch2) unless you get into very high end scopes.It comes down to knowing what you expect to see and adjusting the scope to get as close as you can. Then realize that it's pretty much a figment of your imagination!

author: rstofer
author_rank: Super Contributor
timestamp: May 12, 2016, 06:15:03 pm
content: So I bought that Hobby Components 8 channel logic analyzer and I have attached a screen shot.I'm sampling a 3.125 MHz (and binary sub-multiples) signal that is perfectly symmetrical. With a million samples at 24 MHz, you can still see where sampling error has produced odd widths. I am sampling at just 8 times the frequency.These odd results happen quite often simply because the sampling clock is not synchronous with the incoming data. Actually, it doesn't matter how fast you sample, there can always be a 1 clock difference of opinion on signal edges.I'm assuming I am getting 24 MHz sampling because I am using a USB 3.0 port but I don't see a way to prove it. In any event, there are no error messages.The PulseView software has a ton of decoding options, none of which I have had time to use.Some logic analyzers also have a feature call 'state' display where the sampling clock is an input, synchronous with the data stream. This is a much more useful feature than it first seems because your display will always be related to the actual clock.You can experiment with the sample rate and the number of samples. I get much better results with 100k samples and a 6 MHz sample clock.