First Oscilloscope Questions - Page 1
Forum section: electronics / beginners

author: Fantome
author_rank: Newbie
timestamp: October 22, 2014, 09:45:42 pm
content: Hi, I'm Fantome, and yes, I am a beginner.I just purchased my first ever oscilloscope, a Rigol DS1074Z, and I am slowly feeling my way around. I have watched a number of video OS tutorials and have started to experiment using an Arduino Uno. I have a few questions, mostly regarding some print-screens that I have hopefully attached properly.The program I am using is the blink program with the delays removed. So the code turns the LED on then immediately turns the LED off, and repeats forever, basically generating a square wave with a period defined by the capabilities of the Arduino Uno hardware.Here are my questions (and please feel free to correct my wrong assumptions and improper use of electronics terms/vocabulary:)In image 1, it would appear that it takes approximately 4 microseconds to "Turn the LED on", and 4.6 microseconds to "Turn the LED off". But if you look carefully at the image, there appears to be some ghosting which I am guessing occurs because sometimes the Arduino is interrupting my code with its own code which causes the LED to stay on or off longer than normal. Question: Is the brightness of the ghosting mostly proportional to how often it is occurring or might it only be occurring very rarely but something like phosphor persistence lets me see it?In image 2, I have zoomed in on the time scale and I notice that it is using only 60 points. So at 1G Samples per second, and 60 points per 1 plot, am I right in thinking that the OS is showing me approximately 20800 plots per second? By plot I mean one single occurrence of the observed portion of the waveform, is there better terminology for this?In image 3, I have pressed the “single” button which I assume captures and shows me just one plot. The screen is 800 pixels wide by 480 pixels high and it is still at 60 points, but the image looks way too smooth for just 60 plotted points, what is going on?Is it possible to remove the menus on the left and/or right to have more horizontal divisions?Is it difficult to replace the noisy fan with a quieter one?Thanks for your consideration,Aspiring Beginner.

author: tggzzz
author_rank: Super Contributor
timestamp: October 22, 2014, 10:14:43 pm
content: Those are thoughtful sensible questions from somebody that is taking the trouble to think about what they are and aren't seeing - so it will be my pleasure to give my interpretation. But be aware that my interpretation might well change once I saw all the details of what you are doing, all the scope settings, and fiddled around with the scope.Quote from: Fantome on October 22, 2014, 09:45:42 pmQuite probably, but there might be alternative explanations.If it was an analogue scope, the brightness of the ghosts would be proportional to how often it is occurring. But with a digital scope it is also dependent on how the scope "processes waveforms for display". Often scopes will attempt to mimic the effects of "conventional" phosphors, and will succeed to a greater or lesser extent.An alternative explanation is that there is some "mis-triggering" occurring, possible due to a noise or a poorly choosen trigger threshold voltage. It should be easy to avoid that with this waveform.Similarly sometimes "auto trigger" can give such a display, but I doubt that is happening in this case.You might also take note of the large overshoot on transitions, which can be clearly seen in image 2. This is an artefact of your probing technique, in particular using a ~20pF scope probe with a ~15cm ground lead (15cm => ~150nH, resonating at around 90MHz). The only way of avoiding that is to use a much shorter ground pin/bayonet/wire, or by using a lower capacitance probe. I particularly favour "low impedance" or "Z0" probes for this type of signal; commercial ones are excessively expensive, but for these frequencies they are easy to build yourself.QuoteThis is probably due to the scope "preparing" the sampled data for display by interpolating between fixed points - probably with a sinx/x filter.Personally I usually prefer to turn such interpolation off, and rely on my eye to connect the individual dots representing individual samples. Other people will disagree - your choice!

author: Jope
author_rank: Regular Contributor
timestamp: October 23, 2014, 12:35:05 pm
content: QuoteYou can switch betweenvectoranddotmode. According to the manual, in the menu DISPLAY->TYPE.

author: miguelvp
author_rank: Super Contributor
timestamp: October 24, 2014, 01:07:05 am
content: Quote from: Fantome on October 22, 2014, 09:45:42 pmPress the Menu buttons on top left and right to hide the menus.Also the ghosting is because it can't lock, you can put conditions to trigger setting it to pulse and stating to trigger when the off time is greater than the smaller times, then you'll be able to see the pattern better.Edit: by default you are triggering on rising edge, so the scope will trigger on the ones after a long or short valey, the predominant one will be more visible, the less predominant ones will be less visible and the intensity grading helps to notice which one is which.Edit again: On the old 50MHz Rigol without intensity grading you wouldn't be able to tell which pulses are more prevalent than the others, they will both show the same way, making it very confusing to determine how the signal really is.(Trigger menu) Type: pulse, When: Negative pulse width is greater than, Setting: 2.4us since your smaller negative pulse is around 2 us but no greater than 2.4 us it will only trigger on the longer one.Of course there are other settings like pulse positive/negative greater/less/greater&less last one with lower and upper limit.That trigger mode is handy for looking at NTSC or VGA signals since you can trigger specifically on the Vertical sync and then you can look at a particular video line by moving your horizontal position left/right.Edit: if you select positive pulse greater than 2.5us you might see longer positive pulses, and adjusting that value you can find out what the longer positive pulses are. (the ones ghosting seem to be larger than 8us)

author: miguelvp
author_rank: Super Contributor
timestamp: October 24, 2014, 01:30:31 am
content: Also, if you can afford it (and I know it cost as much as half of your scope) you could get yourself a demo board.http://www.tequipment.net/Rigol/DS6000-DK/Analog-Demo-Board/If you don't want to spend the money, at least look under Documents and download the Manual and Application Examples, they are for the DS Rigol series and your scope might not have some of that functionality but it will help you understand your scope better, even if you have to imagine measuring the signal instead of actually doing it if you don't have the $220 to spend.Edit: I so want one, but then I figure that for $220 I can get other things instead and I already know most of the functionality of the scope, but I still want one even if I might not ever get it.I can see it very handy for someone that is not really quite sure what they are doing and how to measure different things, or just to see if your scope falls within specs when you are unsure.