Sorry...still a bit confused with oscilloscopes...:( - Page 1
Forum section: electronics / beginners

author: 871auto
author_rank: Newbie
timestamp: July 14, 2021, 11:33:43 pm
content: I search everywhere and can never find an answer to the most basic question...I finally have a good table top scope and a nice handheld...both dso types. But for some reason, I can wrap my head around this ONE thing:How does an oscilloscope actually take the input voltages happening so fast, visible, and slowed down enough, to see on the screen? Am I lost? Lol. Like no one mentioned scopes record the data, buffer, store in memory automatically etc. So that's why I have to ask. Here's an example of what I mean:Using my original post a long time ago, let's stick with CAN bus, in other words digital logic voltage differential to create a binary output.Standard CAN has a speed of 500 kbps.So, to me, that means in just 1 second, a module or node on the CAN bus can send 500,000 bits of information. Ie. A differential voltage of 0.0 or 2.5 volts, aka a 1 or 0, a dominant or recessive bit.That means 1 bit ever 2000 nanoseconds.Let's say, for math sakes, a full CAN frame is 100 bits, start to finish. So 1 message is 100 bits, from any node.If these 100 bits are sent in only 0.0002 seconds, HOW am I able to see every bit!? Like how does the scope show me all these bits if it doesn't have automatic memory or someway of storing these 0.0002 seconds of data/wave form?This is where I'm lost and no one has answers online. I've ready every guide regarding oscilloscopes and how they operate etc. But all they do is tell you what they do, not HOW they do it.Anyone? It must be a dumb question that no ever asks. Lol.

author: ledtester
author_rank: Super Contributor
timestamp: July 15, 2021, 12:02:08 am
content: QuoteDigital scopes have a memory. They are constantly taking readings are storing those readings in that memory. What you see on the screen can differ depends on what trigger mode the scope is in.In single-shot mode the scope waits for a trigger event while it is recording the voltage on the probe into its memory. When the trigger event happens it transfers the stored readings to the screen. It then stops looking for additional trigger events so what you see on the screen doesn't change.In "normal" trigger mode the same thing happens except that after a trigger event happens and the display is updated the scope continues recording voltage readings and waits for another trigger event. When that occurs the screen display is updated with the latest readings from the memory, and the scope waits for the next trigger event, etc. What you see on the screen will remain static if there are no more trigger events.In "automatic" trigger mode the screen is updated if a trigger event occurs OR if a certain amount of time has passed.

author: radiolistener
author_rank: Super Contributor
timestamp: July 15, 2021, 12:12:31 am
content: Quote from: 871auto on July 14, 2021, 11:33:43 pmThe oscilloscope record your signal into memory at high speed and then content of the memory is processed and displayed on the screen. That's it.The speed which is used to record data into memory is shown as "Sample Rate" and has measurement units Samples Per Second, for example 1 = S/s, 1000 = kS/s, 1000000 = MS/s, 1000000000 = GS/s.When your oscilloscope shows that it works at 1 GS/s, it means that the signal is recorded into memory with a speed 1000000000 samples per second. It means that every sample in the memory represents 1 ns of the signal state.

author: 871auto
author_rank: Newbie
timestamp: July 15, 2021, 12:45:24 am
content: Omg thank you...finally someone explains exactly what's going on. That was always the hardest part for me to grasp. I am very grateful for your answers.So basically, nothing you see on the screen or display of an oscilloscope is really "current" or in "real time"...cause if that were the case, it would just scroll way too fast to ever be useful. Correct? Like when using my example, everything I see on the screen is technically "old", even if only a few seconds old?Trigger was actually the last thing I had studied and pretty sure I understand now. I think for most of what I do, Single mode is best. However, if I use single mode, the screen pauses once the criteria is met, which is normal, but how do I see the continuation of that same wave form without restarting all over? I mean, obviously if I'm trying to capture, say a 100 bit frame, I can't just rely on the first 10 or 20 bits captured and paused on the display when in Single Trigger Mode? Right?

author: james_s
author_rank: Super Contributor
timestamp: July 15, 2021, 01:14:32 am
content: On a DSO yes, what you see on the screen is a snapshot from a moment in time, the length of time represented depends on the horizontal timebase setting and when the snapshot is taken depends on the trigger settings.Analog scopes display everything in real time though, there is no memory, the beam sweeps across the screen repeatedly and the instantaneous voltage at the input determines the vertical position of the spot. Unless you use a scope camera to capture the trace they only work with repetitive signals. If you try to look at something that is constantly changing like a digital signal on an analog scope you will just see noise, every sweep will paint a different trace on the screen. If you look at something like a sine wave from an oscillator you will see a steady image because the waveform repeats, it is tracing over the same path on each sweep, assuming the triggering is set up properly so it can trigger at the same point each time.A DSO should have a "auto" trigger setting that will make it trigger repeatedly, the actual terminology varies from one scope to another but they all do approximately the same thing. When set to auto it will trigger over and over whenever the trigger condition is met, without requiring you to arm it for a single shot capture each time.

author: ledtester
author_rank: Super Contributor
timestamp: July 15, 2021, 02:16:33 am
content: QuoteOn scopes with a large amount of capture memory usually the scope captures a lot of readings before and after the trigger event. You can then scroll through the trace before and after the trigger event.For instance have a look at this pic:At the very top you see an orange "T" marker showing where you are in the entire capture history.Then in the top 40% of the screen you have a blown up view of the traces.And in the bottom 60% of the screen you have yet another zoomed-in view.You can move portion of the trace you are zooming in on with the horizontal position knob.So the scope is able to capture several screen-worth's of data.Consult your scope's operating manual to figure out how to get the scope to do that.

author: 871auto
author_rank: Newbie
timestamp: July 15, 2021, 06:15:29 pm
content: Thanks for all the help and input.I think I got single trigger mode working pretty good now. My only issue is how do I see the whole waveform and not just the part that freezes on the screen?I captured 30 bits of data, but the entire waveform will be more like 90 to 100. So once I decipher the first 30 bits on screen, how do I move it along or scroll right to see more? I can't figure that part out even with the user manual.I can post pics if it helps.

author: ledtester
author_rank: Super Contributor
timestamp: July 16, 2021, 02:53:58 am
content: What scope do you have?On the DS1054Z make sure you are going to capture enough data by hitting the "Acquire" button and setting up the Mem. Depth and Sample Rate.The capture duration = Mem. Depth / Sample Rate, so the DS1054Z can record 12ms before & after the trigger event even at 1 G samples/sec.If your Mem. Depth is set to "Auto" you might not capture enough samples for your transmission.Then start your single capture and use the Horizontal Position knob to scroll around the trace. The Horizontal Position knob is the smaller knob above the "MENU" button in the horizontal control area.

author: 871auto
author_rank: Newbie
timestamp: July 16, 2021, 02:07:20 pm
content: Hi ledtester, because most of what I do is mobile or automotive related, I have a bench and portable dso. The portable one is this:https://www.amazon.ca/gp/aw/d/B08W1Q2KNL?psc=1&ref=ppx_pop_mob_b_asin_titleThe bench one is this:https://www.amazon.ca/dp/B06XZML6RD/?coliid=I20PQDVAUA9DM3&colid=382MPRKQOLXM0&psc=1&ref_=lv_ov_lig_dp_itThey're both very good and very accurate. I test them all the time using different pwm, frequency and tone generators, and they're always bang on.Thanks again!

author: ledtester
author_rank: Super Contributor
timestamp: July 16, 2021, 04:16:44 pm
content: Your bench scope will operate like the DS1054Z.I don't think the portable records any off-screen data.For capturing long digital traces consider an inexpensive USB logic analyzer and the free PulseView application:- Getting Started with a $10 Logic Analyzer using Sigrok and PulseViewhttps://youtu.be/z8Tdz7eQ8n4

author: TimFox
author_rank: Super Contributor
timestamp: July 16, 2021, 04:31:24 pm
content: Before digital memory and ADCs were fast enough to digitize a fast waveform, analog storage in a CCD (analog shift register) was used as temporary storage of analog voltages, which could be read out at a slower rate suitable for the ADC to digitize. Plessey made such a CCD, which was famous for its resemblance to a railroad and a locomotive image placed in the open space around the CCD "track".