17 replies to this topic

[mcinnes01]

Hi,

I am getting to the stage in my projet where I need to start testing the output from my hardware and I am told I need an oscilloscope.

I know very little about scopes and wondered if anyone could suggest a scope that will do the job.

I don't want to spend loads, but I don't want something that won't give me a reliable reading.

The types of things I want to check are PWM signals from TLC5940s and clock signals from oscillators etc.

I have seen various types on ebay, some are digital usb ones with no screen and you see the output on your pc.

Others are small hand held ones and the last type are the typical bench top ones.

Here are a few examples:

Nano

Usb

Bench Top

Your opinions would be greatly welcomed, and ideally as cheap as possible, but obviously not to the point that they are useless. I have no idea of the accuracy etc that I need, but I want to check what is being output from some TLC5940s, the oscillator clock circuit Nevyn made to control the TLCs and also to see what the 74hc595 shift registers are outputing, plus anything else netduino related I made need to check.

Many thanks,

Andy

[hanzibal]

Been there and bought the T-shirt. Let me tell you - you're not going to regret getting a good scope for the money. Basically, it all depends on the speed of the signal you want to measure. In your case that would be the frequency of the PWM signal.

However, if you want to look at a scope, why don't go for something that's will be more versatile for you along the way. So forget about nano (it's a toy) and probably USB too. The "Bench top" seems more like it even though pricing seems better than to expect in quality

As a rule of thumb you want a scope capable of measuring at least twice the frequency of the signal you want to measure (due to the Nyqvist folding theorem, according to which wheels appear to spin backwards on a 25Hz TV set, which they also tend to do).

Now, the "Bench top" has a bandwidth of "only" 25Mhz which means you can only measure signals slower than 12,5 Mhz at best. While 12,5Mhz can seem like much at this point, you will not be able to measure signals faster than that (.e.g. SPI or I2S).

I recently bought a RIGOL DS1052E 50Mhz scope at some 350$ which I am very pleased with. These scopes are on sale across the world right now and are great value for money. Highly recommended!

USB scopes are not much cheaper than regular ones and don't forget you'll have to boot that old PC up every time you want to take a look at a signal. Furthermore, the RIGOL mentioned has both USB device and host from which you can interface from a PC and save waveform screenshots directly onto a thumbdrive respectively.

Are you sure you want a scope, maybe you're looking for a logic analyzer? Then I would recommend something from Saleae, don't know if they have enough bandwidth though.

[nakchak]

As Hanzibal said above you cant go wrong with a bench top scope.

I too have been looking at that owon model of scope as my old trusty tektronics scope literally blew up on me last week (something went pop, probably a cap and a lot of smoke came out) but it was an ebay bargain and i have easily got my money's worth out of the £50 i paid for it 10 years ago.

With regards to that Owon scope CPC/Farnell sell them and also offer a calibration service which certainly can offer some peice of mind when it comes to accuracy, although they are a bit more expensive than the Ebay sellers, you get the peice of mind for returns and warrenty then.

Depending on your budget and amount of time you have to look for a scope, you can snag some great bargains on ebay, if a CRO (Cathode Ray Oscilloscope) will suffice and you dont need a DSO (Digital Storage Oscilloscope) then you can get some great bits of kit for < £100. But it really depends on your needs, if all you require is to visulise PWM and Oscillator output then a CRO could well be a bargain for you. However if you also want to analyse bus traffic (spi, uart etc.) then a DSO would be more applicable as the storage feature allows you to scan back and forth the trace. Also if you want to capture waveforms a DSO with USB output should allow you to save the output as an image (or at the very least take a screen grab from the program).

As an alternative to a DSO for bus analysis you may find some thing like the IKA Scanlogic 2 to be adequet for your needs. I have just received mine as i needed a stop gap replacement for my scope and needed to be able analyse uart and spi traffic for the day job and I am very impressed with my initial experiments with it. They can be purchased for about £55 here.

Really it all depends on how much you would like to spend and what your plans for usage are, ultimatly you will not regret forking out for a DSO as there versitility pays for them selves in the long run.

[Valkyrie-MT]

I just got a USB based scope for $50, but there is some assembly required... I don't know if it's any good though because I have yet to assemble it, but I just want to inspect SPI, I2c, 1-Wire, and RS232. This appears to do that... http://www.pdamusician.com/dpscope/ -Valkyrie-MT

[Chris Seto]

I have a Picoscope. Can't go wrong there. Forget the DSO nano thing, it's a toy and cannot be trusted as a real lab instrument. Rigol seems to be doing very well so far.

[Nevyn]

I am getting to the stage in my projet where I need to start testing the output from my hardware and I am told I need an oscilloscope.

I know very little about scopes and wondered if anyone could suggest a scope that will do the job.

I don't want to spend loads, but I don't want something that won't give me a reliable reading.

I second the comments on the Nano - forget it.

Regarding the bench scope you mentioned, remember that it is coming from Hong Kong and if the couriers work out it's value then you will get stung for an extra 20% VAT on import to the UK.


Being in a similar position last year (i.e. not a big budget and needing a scope) I went down the USB route and bought a Piscoscope and a Saleae logic analyser.

I'm not an expert on this subject so all I can say is so far they have both served me well.

Regards,
Mark

[h3mp]

i got a bitscope from ebay as don't have the spare space for a bench scope and the picoscopes were a bit too expensive for a newbie, the thing seems OK so far but im still learning

[mcinnes01]

Thanks for all the brilliant replys there is a lot of useful advice here! I have looked at all the suggestions, and it is clear that there is no point looking at the "toy scopes" so that is a start. In terms of logic analysers, these sound quite useful as I will later down the line as I get in to the bus side of my project more, what to look at SPI, I2C, UART and 1-Wire buses in more detail. Will the logic analysers allow me to read the output from an oscillator circuit as well or are the primarily for testing digital bus signals as oposed to clock and pwm signals? Hanzibal thanks for explaining the bandwidth, it is useful to know that a broader range is better to go for and so I guess you would suggest aim around the 50Mhz range and above for a reasonable scope right? I will say from looking at the scopes I do see the argument for get a desktop as you can do readings with or without the PC and it is a valid point, however me being me I think I am going to tend towards a USB DSO, I like things that plugin my PC and are compact! The scanlogic 2 looks very reasonable, if this type of product will meet all my needs I will definately get one although one of you will need to tell me in terms of measuring the PWM and clock signals as I am clueless? I'm guessing without someone having used both what would be the opinion in terms of logic analysers of the scanlogic versus the Saleae and also perhaps to throw in to the mix the bitlogic could also be part of that comparison? In terms of the bit logic if this does both and the pricing seems reasonable, what would everyones opinions be on this product? It seems to me like it could be the ticket 100Mhz analogue badwidth!?! In terms of picoscopes what are your opinions between the 2104 which seems good and what additional benefits may there be from the 2204 or does bitscope seem a good option. Thanks so much for all the advice, this has been a great help (hopefully when I get one I won't take me too long to figure out how to use it ) andy

[Mario Vernari]

Well, sometime I'll write a post about the scopes...

I'd to keep this shorter...
A periodic real waveform of any shape can be seen as composed of one or more sine waves, called harmonics.
See here: http://en.wikipedia..../Fourier_series

Analog bandwidth and sampling rate are not the same thing.
For example, you could see a scope targeted for 100MHz and 1GS/s =(samples/second). Upon the Nyqvist theorem, the maximum frequency should be 500MHz, but it's actually far away.

Any scope has an analog stage (input), and a displaying stage. While the analog stage is much the same for both digital and analog scopes, the displaying stage is completely different.

Analog bandwidth.
It's by far the most important, and typically expensive part of a good scope.
The analog stage *should be* an amplifier/attenuator that takes the signal on input (probe), and scales it for a certain factor. Theoretically, the shape of the input signal should be perfectly preserved. However, that's impossible.
Saying K the ideal scaling of this stage, the actual K' is mostly accurate up to a certain frequency. After that the decay is dramatic.
The frequency at which the K' = K / sqrt(2) is defined as the bandwidth.

When you are going to display the SPI running at 1 MHz (far away from the max), you could consider the shape of the waveform as a square wave. The steep is the edge, the higher are the frequencies needed for compose the waveform. Since a square wave should have "infinitely steep" edges, the frequencies involved are very high. For a 1MHz square wave of amplitude A, the next harmonic is the third (3 MHz) having amplitude A/3, the next is the fifth (5MHz) at A/5, and so away. The decay is not so fast.
Well, if you try to analyze the 1MHz SPI using a 10MHz bandwidth scope, then probably the wave form displayed will be pretty different from the actual one. That's because the analog stage of the scope "cuts" several important harmonics above 10MHz...and we've seen they're still important.

Sampling rate.
That refers to the digital scopes only.
Basically is the *maximum* sampling rate at which the ADC is able to sample the signal at the analog stage output. Of course, the higher it is, the better is the scope.
BTW, when the ADC is collecting the data, there should be some component able to store the samples. Bear in mind that this task has to be performed in *real-time*, thus is much faster than a normal PC. So, you might have a powerful scope capable of 1GS/s, but ...how deep is the cache?
If you cache offers some kBytes only, then clearly the period will be very short. For instance, collecting one sample each ns (1GS/s), a 4kB cache will be full in 4us. That could be a limitation, because you would collect many more samples: stop the refresh, and analyze them with ease.

Note: often you see something like "50GS/s equivalent". That's NOT meaning a super-ability to sample, nor any embedded wizard doing that.
Again, it's something related to the signal analysis. Have a look here: http://www.picotech....e-sampling.html

Hope it helps.

[mcinnes01]

Thanks as ever Mario for a fantastic detailed explination. I definately want a usb dso, for space requirements really, do you think the bitscope will be good for what I need or the picoscope 2204? I also found these based on the shop you suggested: http://www.ebay.co.u...=item256c233378 I'd really appreciate it if you could give me you opinion on my suggestion or if you've seen anything similar that may be better? Many thanks, Andy

[Mario Vernari]

Well, none of the usb model is able to excite me particularly, but they are cheap instruments...

Here is my suggestion, but I'd like that someone else would give its opinion.

The Picoscope 2204 is too limited (10MHz, many functions missing, etc) comparing to the price. The smallest decent is the 2207, but its cost is higher than a desktop, powerful scope. Of course you pay the miniaturization, but...money are money.

Same conclusion for the bitscope, other than I don't know what are you referring to.

The model of the link isn't bad, considered the price. Although is not highlighted, it's pretty easy to understand that's an Hantek.
So, I dug a bit into the official Hantek site, and I've see this one, that looks pretty nice.
http://www.hantek.ne...st.asp?unid=132
Here is an eBay.UK auction:
http://www.ebay.co.u...#ht_5956wt_1139

What I like is that it's a 60MHz (enough), it has 4 inputs, plus the external trigger (sometimes useful). Then a separate supply instead the USB, a function generator, and a deep enough cache for the samples. The case looks much robust, and they gives you a bag with all the kit. However, the price is about 270GBP...I'd say to feel better to choose a decent tool, instead a cheaper limited one.

Come one guys!...Anyone would add anything?

[Nevyn]

Just a thought, do you know anyone who works at one of the local Universities? Might be worth checking if they have any second hand kit for sale. Regards,Mark

[carb]

Just as an update, while looking for a recommendation on which Ocilloscope to buy I came across this thread, the RIGOL DS1052E was mentioned as a good scope (I am sure that it still is).

While this model can still be found for sale I found that it has been replaced by the RIGOL DS1102E for the same price range as the older model (was $795.00 now $399.00) and has a 100mhz bandwidth. RIGOL DS1102E

[nakchak]

Just as an update, while looking for a recommendation on which Ocilloscope to buy I came across this thread, the RIGOL DS1052E was mentioned as a good scope (I am sure that it still is).

While this model can still be found for sale I found that it has been replaced by the RIGOL DS1102E for the same price range as the older model (was $795.00 now $399.00) and has a 100mhz bandwidth. RIGOL DS1102E

AFAIK you can turn any DS1052E into the DS1102E through a simple firmware hack details here although it looks like Rigol have seen sense and changed prices on the DS1102E to be about the same as an un-modded DS1052E. They are very nice cheap scopes, which I can recommend.

[Dan Morphis]

Just as an update, while looking for a recommendation on which Ocilloscope to buy I came across this thread, the RIGOL DS1052E was mentioned as a good scope (I am sure that it still is).

While this model can still be found for sale I found that it has been replaced by the RIGOL DS1102E for the same price range as the older model (was $795.00 now $399.00) and has a 100mhz bandwidth. RIGOL DS1102E

The DS1102E doesn't replace the DS1052E. The DS1102E is a same board, chips, components, etc as the DS1052E, except it has a different label on it, and bit has been twiddled in the firmware to allow capture at 100 MHz. As @nakchak mentioned, with putty (or any other serial terminal program) and some very easy instructions on the net you can turn your DS1052E into a DS1102E.

-dan

[carb]

The DS1102E doesn't replace the DS1052E. The DS1102E is a same board, chips, components, etc as the DS1052E, except it has a different label on it, and bit has been twiddled in the firmware to allow capture at 100 MHz. As @nakchak mentioned, with putty (or any other serial terminal program) and some very easy instructions on the net you can turn your DS1052E into a DS1102E.

-dan

Thanks Dan & Nakchak,

The main reason for the post was to hopefully prevent someone from paying too much for the DS1102. I found the DS1052E on Amazon and was about to purchase it when I did some more looking and found the DS1102E. The problem is that a lot of paces are selling the DS1102E for > $600.00 (Amazon & Ebay). I hate for someone (particularly me) to pay too much for anything.

Chuck

[Mattster]

I went through this same step of the learning curve about 6 months ago. In the end, I settled for a "toy" scope, the Oscium iMSO. It was expensive for a toy but I bought it because I already owned the iPad and iPhone so the idea of a tiny portable scope with 4 channels of logic analyzer was attractive even at the price. And for a first scope it is fine. So my opinion is that if you've never owned a scope before and you're not currently working with 50+ MHz signals, a cheap toy can be fine for learning. Then with your experience in mind you can better shop for a higher-end scope. While shopping I found the Quant Asylum QA-100, a USB unit that has some pretty nice specs - 2 Analog inputs and 12 channels of 10-bit resolution Digital logic input. At $350 it's not cheap, but for a portable device it looks pretty good. I think it's going to be my next one. In the meantime, if you look at the QA-100 web page (http://www.quantasyl...ucts/QA100.aspx), there's an excellent feature comparison chart with most of the low-end to medium scopes compared.

[carb]

Thanks Mattster, I looked at several of the scopes that are on the chart, but decided to go with a stand alone oscilloscope and logic analyzer. FedEx just delivered the oscope about 2 hours ago, I have it setup and working, just need a project to try it on. I went with the Rigol DS1102E ($400) and the Saleae LA ($150), both seem to work very good and are for the most part easy to operate. I have connected them to a mini Maestro servo controller to look at and measure the pulses. I will start playing with I2C & SPI signals before too long (after I finish looking at Mario's post on the subject, a lot of good information ). It will be a lot easier with the right tools. Chuck