Which Digital Oscilloscope should I buy? - Page 1
Forum section: electronics / testgear

author: Nilres
author_rank: Contributor
timestamp: September 08, 2014, 10:33:14 am
content: Hey Guys,I know there is a similar topic going on right now but I have slightly different requirements. After three years with my analog HM203 (20MHz, 2 Channels) scope it's time for a digital scope. The old scope is great and I was able to pick it up cheaply on ebay. And for my small projects and tests it worked just fine. But now I want to face some bigger projects. I'm not professionally involved in electronics it's just a hobby. Also I'm a student so I don't have a huge amount of money. But I also want a scope that I can use over years from now (buying a second scope in a couple of years is costly then saving now for a few months and then buy directly a better scope).I'm working a lot on projects with Microcontroller and maybe later I want to test out some stuff with FPGAs a build in Logic Analyser would be quite handy I believe. A function generator could also be quite useful. Do you think buying these three components in one device is better? Also considering that I don't have unlimited space on the bench.Also I think a four channel scope is better for me then the two channel one. As I mentioned I only want one scope (or one digital I'm not sure if I want to sell my Hameg or keep it).I'm not sure what analog bandwith i really need. Is 70Mhz enough? Sounds a lot compared to my 20Mhz scope but on the other hand I currently working on a project with an MCU which runs at ~60Mhz I my not need to scope the main clock of the device but it could be good to have some reserves for future projects.So what I found that full fill all these requirements:Rigol: MSO1074Z-S 70Mhz - 965€or MSO1104Z-S 100Mhz - 1110€Something like the great MDO3000 series is simply to expensive.So what are your thoughts? What do you think is the best scope for a person like me? If you need additional information please ask.Best regradesNils

author: tggzzz
author_rank: Super Contributor
timestamp: September 08, 2014, 11:22:42 am
content: Given a scope with bandwidth BW, it will enable you to see risetimes trof tr=0.35/BW. Hence a 70MHz scope will enable you to see risetimes of 5ns. Note that is independent of the period: it is as true for a 10Hz signal as it is for a 10MHz signal.A dual channel scope allows you to see the time relationships between two signals, and to use one signal to trigger viewing of another. That is a definite advantage over a single channel scope. A four channel scope probably doesn't offer much more than a dual channel scope - unless your application has specific requirements.Don't forget the cost of four scope probes vs two scope probes. Do understand the tip capacitance you need for your applications (e.g. 20pF=>160ohms@50MHz). Do understand the effect of a 6" ground lead (150nH+20pF => oscillation at ~80MHz!), and get good probe accessories including a ground "spear" or similar. Consider getting 500ohm "Z0" probes.Once you have debugged theanalogueoutputs from your digital logic (i.e. got good signal integrity so theanaloguedigital signals are interpreted correctly as digital signals!), forget the scope and debug in the digital domain using a logic analyser. That LA can be either a conventional external LA, or could be extra debug circuits inside an FPGA, or simply software.

author: Rubi Han
author_rank: Contributor
timestamp: September 09, 2014, 08:29:39 am
content: Regarding the bandwidth, it depends on the Max. measured frequency. The bandwidth should up to 5 times of Max. measured requency at least.Actually, you can choose a handheld oscilloscope. Not only it including scope, muti-meter, recorder these function but also it is much more cost saving.Below is the brief info:1. Bandwidth: 70-200MHz2. Touchable screen3. 1GSa/s sampling rate4. 240KPts memory depth

author: tggzzz
author_rank: Super Contributor
timestamp: September 09, 2014, 09:08:35 am
content: Quote from: Rubi Han on September 09, 2014, 08:29:39 amI don't think you understand the conditions under which that rule of thumb is and is not valid. The devil is in what constitutes the "max measured frequency".As a simple example, suppose you have a TTL clock and TTL data going to a TTL flip-flop with a 5ns setup or hold time. If their period is 1s= 1Hz, having a scope with a 5Hz bandwidth isn't necessarily sufficient to ensure the flip-flop latches the data as expectedInstead the critical period is 5ns, which is closely related to the rise/fall times.The period (and hence frequency) of the signal is irrelevant.

author: Michael Weston
author_rank: Contributor
timestamp: September 09, 2014, 05:06:29 pm
content: Quote from: tggzzz on September 08, 2014, 11:22:42 amBravo to you, tggzzz. That was a very good, informative post.

author: nctnico
author_rank: Super Contributor
timestamp: September 09, 2014, 10:17:26 pm
content: Quote from: tggzzz on September 09, 2014, 09:08:35 amActually that is a poor assumption as well. Even on a scope with a risetime much higher than 5ns you'll still see a time difference of 5ns between 2 signals. That will also be affected by how well the channels are matched (skew between channels). Basically when looking at digital signals you need to have enough bandwidth to include the third harmonic. That is 3 times the frequency of the signal.

author: tggzzz
author_rank: Super Contributor
timestamp: September 09, 2014, 11:39:15 pm
content: Quote from: nctnico on September 09, 2014, 10:17:26 pmBy "higher than" I assume you mean "slower than".You are correct, but missing the main point I was trying to get over.In any case you will be in danger of inferring more than is justifiable given the instrument's capabilities - significant theoretical understanding and great care would be required. The OP wouldn't have that understanding.

author: nctnico
author_rank: Super Contributor
timestamp: September 09, 2014, 11:55:58 pm
content: As I wrote before: there are small lies, big lies and then there is what is on the screen of your oscilloscopeI'll make that my signature!

author: tggzzz
author_rank: Super Contributor
timestamp: September 09, 2014, 11:59:06 pm
content: Quote from: nctnico on September 09, 2014, 11:55:58 pmOld aphorism: "there are lies, damned lies, statistics - and adc/dac specifications".