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Basic Electronics » Oscilloscope

December 14, 2011
by met_fredrik
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Hi, I've seen quite a few threads here about Oscilloscope buying. But I want to hear some opinions on the specific one I am looking at before I buy anything!

I read about Ralphxyz's ATTEN ADS1102C, and I think that one would be in the range of my needs. However I found a smaller model, the ATTEN ADS1042C(Ebay site) for 228$.

What I'm going to use it for: Measure AC voltage, watching the sinus curves from 240V@50Hz, TRIACs etc. And measure AC/DC down to the mV and signals like pwm, i2c etc from microcontrollers.

Would this scope fit for me? I see the Vpp is 400V and the 40Mhz frequenzy must be enough. Am i missing something or should I go for it? :)

Thanks for any help!

-Fredrik

December 14, 2011
by treymd
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Whenever I'm looking at O-Scopes on EBay I always go back and read THIS THREAD There is some good insight in there, particularly the post near the bottom from Bobamosfet.

December 15, 2011
by met_fredrik
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Thank you! I read through the whole thread and it was really informative!

Still I would like some direct advice on this one, I've done some more reading about measuring the 240VAC directly from the wall, and I understand it as if 240VAC has a Vpp of over 600 volts?(!)

As for the bandwidth I guess 40MHz would do the job for me, I can't imagine why I would need to measure more than 40/5MHz.

Any thoughts?

December 15, 2011
by BobaMosfet
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met_fredrik-

This is one of those areas, where the decision is very easy. Take the reciprocal of the maximum frequency of the scope (which is not a measure of the frequency waveform it can display, but rather just the frequency which it can chop 1 second of time into discrete slices) and that will give you the smallest time measure that the scope will display.

1/40,000,000 = 25nS. That's .000000025 Seconds.

Take the reciprocal of any frequency you might be dealing with (PWM, clock crystal, etc.) and compare that value to the one above.

I'm going to say straight up that my personal recommendation is that you do NOT get the scope you're describing; I think you'll end up regretting the purchase, finding it is only useful with AC, but not any micro-controller work.

BM

December 15, 2011
by met_fredrik
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Thanks alot! These forums are always of great help. Hope to be able to give something back after some time.

I could stretch my budget up to the 100MHz version that Ralphxyz got. That way even the 14~MHz crystal wouldn't be a problem, am I right? I'm just having a little trouble understanding what you are saying about the smallest time measure the scope can display.

But what about the voltage peak to peak? The specs says 400V, but 240VAC has a peak to peak of 670~ V. Am I missing something?

December 16, 2011
by BobaMosfet
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met_fredrik-

I would urge you to avoid AC until you actually understand it. 240VAC can easily kill you. So to answer your question, yes you're missing some things.

BM

December 16, 2011
by met_fredrik
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I find that answer kind of offensive(?). I am half way through my bachelor in electrical engineering, and I would like some explanation rather than some vague hints about me beeing to inexperienced. I was wondering about the specs for the specific oscilloscope, and this is the reason I am wondering.

December 17, 2011
by BobaMosfet
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met_fredrik-

Not meant to be. Your previous post made it sound like you were very uncertain about the AC aspect. I issued the caution on that note, and as I reflected mentally on it later, I realized it might be non-ideal; this was the first chance I have had to get to the terminal today). So I apologize, and I appreciate your patience.

Knowing that this is of more interest to you than a simple hobby, delights me. Let's put on our goggles and wade in!

AC is normally discussed in terms of RMS. You are aware then, that the actual (DC equivalent) would be more on the order of 1.414 times the RMS value (we'll only deal with USA distribution here, to keep discussion singular).

240VAC is the peak to peak value (RMS). That is because in the USA, the pole/pad-mount transformer drops neighborhood distribution (normally about 7.6KV from a substation) to discrete 240V secondary windings for each residence. This is delivered to the distribution panel on the side of the house. This 240VAC secondary winding is center-tapped to provide 120VAC on each side of the distribution panel.

So, we know that 240VAC (RMS) is actually: 240 x 1.414 = 339.36VDC Peak to Peak. So a 400V oscilloscope will probably do what you need (and is one of the reasons that so many DMMs and O-Scopes are in fact rated at not more than 400V.

Now, about the time slicing. Don't think about an oscilloscope as anything special. It's just a voltmeter. That's it. It's just that it takes a voltage reading every time-slice (at its maximum resolution), and displays that on the oscilloscope's screen*.

So, if you have a 40MHz scope, it will take a voltage reading 40 Million times/second. Each reading represents what the voltage level was at for 1/40,000,000 of a second (1 reading every 25nS). Each time it does this, it displays a new dot on the screen* to the right of the last dot, pushing the 'waveform' to the left.

That's a very crude but understandable description of how it works whether it's a DSO, Analog, or otherwise.

Let's say you have a 20MHz signal coming in- that means an entire (full) cycle will be displayed on your 40MHz scope in just two slices of time (50nS). -- 2 Dots on the screen -- You'll only see the peaks (or wherever the waveform is at the time it's scanned). Or said another way, an entire 1-cycle sine-wave will be represented by only two dots--- insufficient detail to even see the waveform at all.

Even at 100MHz... how many dots will represent a 100MHz waveform? Each slice is 10nS, so you could describe the entire 1-cycle waveform with 5 dots.

Does this help make sense?

The main thing is this- make sure you understand what the scope literature is actually telling you it can read. Some may mean the highest waveform they can scan, others mean highest frequency of timeslice- as I've described... but that is depended on the marketing team for the manufacturer.

Since this is not a hobby for you- you're putting educational dollars on this, I STRONGLY (Cannot overemphasize this) recommend you not settle for anything less than 2GHz scope. I bought mine on ebay for $450 with 3 probes, used. Most scopes come with a diagnostic which will also tell you how many hours are on the scope. Mine is not a digital display, its a tube-type (which is just fine). Mine is a TDS360, with low hours and no phosphor burn in (on the tube). Phosphor burn in occurs when someone leaves a tube-type scope on for long periods and the graticule gets burned into the phosphor leaving an 'image' when the scope is off. If you get a tube type, once you turn it on, leave it on for a while, and if you step away, simply turn the display down so you can't see an image-- when you return, turn it back up to where you like it). I run mine at a comfortable level so the info is visible, but without over-driving.

I'm serious. I know you're anxious to get one, but seriously, this one tool will be the most valuable tool you own in terms of electronics, and if you get the right one, likely you may only ever need to by one. You're already looking to put a signficant amount down- save your money, add to your kitty and then you have options!

I waited 2 years to find mine. It was worth the wait.

BM

  • based on your horizontal and vertical settings (which determine how much of the scopes full resolution capability you want to see- max res is 1:1 ratio)
December 17, 2011
by BobaMosfet
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The last 'bullet' is supposed to be an asterisk (*) footnote.

BM

December 18, 2011
by met_fredrik
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Thanks for taking it like you did, I realized I might have sounded a little harsh my self in my last post.

Now back to the 240VAC! I can't let this go until I got a firm understanding. Thank you for your patience so far! I would like to give a sample picture of where I have my basic understanding from(I live in Norway): 240VAC

I agree that the peak voltage here is 325(This sketch is made by the assumption that the rms voltage is 230). But how can the peak to peak value be anything other than the double of this?

To qoute you:

So, we know that 240VAC (RMS) is actually: 240 x 1.414 = 339.36VDC Peak to Peak

In my opinion this would be the peak value, not the peak to peak(I also understand that there is one right and one wrong answer and that my opinion wouldn't have much to say to the correct answer! :P )

Just to clarify my understanding of mains in the different countries:

Is it correct that you got(In the US) one live wire and one neutral in addition to earth. Which makes 110V(RMS) and 155VAC peak.

But here in Norway we got(Isolated Terra) two live wires 110VAC 180degrees off from each other which makes the rms 240V and the peak to peak (2 * sqrt2 * 240)=678V.

I am still VERY much open for the possibility that I am completely misunderstanding something, especially the different mains systems in the different countries makes it difficult to find information which is reliable based on the type of grid we have in Norway. Maybe you or anyone else know of some good reading on the subject?

Back to the frequency! I do think you have some very valid points about this being a big investment as it is, so why not put some extra money in it to never have to replace it. However I live in a small apartment(I will most likely be living in an even smaller one for the next three years). That's why I want a digital one, because of my space limitations. But to find a digital one with a bandwidth of 1GHz and above for under 1000$ seems impossible! So I guess I will end up with something in the middle, but I will take your advice and won't rush into buying one. If you hadn't enlightened me on the subject I think I would have ordered the 40MHz one already!

Thanks for the thorough feedback your giving!

December 18, 2011
by met_fredrik
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And yes, the explanation you gave about the frequency really did make sense :)

December 18, 2011
by BobaMosfet
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met_fredrik-

I don't honestly know why I get stuck on typing certain things sometimes. I meant 'peak' not 'peak to peak'. To answer the voltage question specifically with this in mind, we still work with the RMS value. Norway is 230VAC at 50Hz, so let's just think about that.

RMS = 230VAC
Actual:
230 * 1.414 = 325.22

Peak to Peak:
325.22 * 2 = 650.44.

But wait.... think about that. You can't be on both sides of ground at the same time with a waveform, so you aren't actually after a voltage range to handle peak to peak, just peak. So that would mean you can still use just 400V Max input on the scope to handle the 325.22VAC waveform.

Displaying the waveform on a 700V scale is a different problem, solved in logic. The scope knows which side of neutral you're on, based on polarity.

My main concern is, for the money spent, you get a tool that serves you well and that you are satisfied with. I think every guy here can particularly appreciate purchasing a tool to solve a problem that turned out to work less well than advertised- it's frustrating, and disappointing. Worse, guys tend to purchase tools to get things done- so when you need it, you need it to deliver.

The scope you want will show up. Gotta be patient. If you're a student, and have access to a lab; talk to your instructor. They may make a scope more readily available to you, or even know where you might find a good deal local. Check out your local electronic parts shops, or robot/gizmo user groups. Check out business liquidation. Check out manufacturers and see if they have refurbed units.

Please let us know what you end up with, we have lots of input on how to use them.

BM

December 18, 2011
by treymd
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We have 2 live wires and a neutral here in the U.S. Plus the ground. Depending on the voltage needed one or both of the live wires may run in a circuit. Most outlets will have one or the other phase, the neutral, and the ground. Special circuits for Electric Dryers, heat, and perhaps an electric stove will have both phases, neutral and the ground run to them.

December 20, 2011
by met_fredrik
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Boba, now I agree in everything your saying! Thanks again for the clarification. As for the scope I will put it on a hold until I find some more options! Actually I found a 500Mhz digital scope for around 650$ here in Norway, but it's a bit pricey!

Treymd, that's what I though. Thanks!

February 18, 2012
by Osarus
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I was in a similare position, don't get the cheap ones with tiny tiny screens, you will regret it :)

If your not 100% on spending some serious money on a full on scope and just want something to get your feet wet, find out what u want it to do then id consider getting Owon's SDS7102 dual 200mhz(yes it says 100mhz but its not, google it :P) 1gsa with 10m per channel memory for about $380. For the money its quite nice and the main reason im suggesting it is it has a battery compartment so can be used as a very nice portable scope when u do upgrade to something better once u understand what want and have more experiance with one.

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