Oscilloscope for exploring 500khz SMPS - Page 1
Forum section: electronics / testgear

author: blundar
author_rank: Contributor
timestamp: July 10, 2025, 03:58:38 pm
content: I'm going to be debugging/testing a multiphase boost converter running off a STM32G474 HRTIM peripheral. It's simulated to run at a switching frequency around 500khz.Isn't the rule of thumb that you need 3-5x and ideally 10x the bandwidth of the signal you're looking at?Would a SDS1104X-E that has been "upgraded" to 200Mhz be sufficient?Any other thoughts? I'm mostly concerned with looking at actual real world FET switching behavior in order to optimize deadtime phasing to maximize efficiency. This is going to mean significantly faster than 500khz signals, really. Probably closer to 50Mhz for edges?

author: jjoonathan
author_rank: Frequent Contributor
timestamp: July 10, 2025, 04:01:14 pm
content: Are you sure you don't mean 500kHz? 500MHz is about 1000x faster than the fastest SMPS I've ever come across.

author: blundar
author_rank: Contributor
timestamp: July 10, 2025, 04:09:14 pm
content: Thank you for catching my typo.(Fixing original post)

author: bdunham7
author_rank: Super Contributor
timestamp: July 10, 2025, 04:14:41 pm
content: Quote from: blundar on July 10, 2025, 03:58:38 pmRules of thumb really don't help here, you need to know specifically what you're looking at. As mentioned, a 500MHz SMPS would be truly extraordinary and you're likely off by three orders of magnitude or so, but the HRTIM has resolutions in the picosecond range and might require a 5GHz scope to see some things. IDK what specifically you want to look at with the scope.

author: mk_
author_rank: Regular Contributor
timestamp: July 10, 2025, 04:22:32 pm
content: Quote from: blundar on July 10, 2025, 03:58:38 pmScope - what ever you can get... with the highest bandwith you can get for your money. Even when the converter is running at 5MHz only. Glitches on FETs or diodes are difficult to catch but can have nasty effects on your design...More important then bandwith are more channels... 4ch are not always enough. And even more important and probes, a good understanding how the probes react and mechanicl robustnes of the probes.Normal probes can be useless when doing high current stuff, so low and high voltage diffprobes and the chance to desqew them are a must.. at least sometimes.good luck

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2025, 06:06:25 pm
content: Rise and fall time are more relevant than bandwidth. 100 MHz will be about 3.5 nanoseconds rise and fall time and good enough for a silicon MOSFET or bipolar transistor switching regulators. Even 50 MHz could be fast enough. But as others have and will point out, bandwidth is king; you can never have enough.

author: 2N3055
author_rank: Super Contributor
timestamp: July 10, 2025, 06:39:26 pm
content: 200 MHz SDS1104X-E is plenty enough for start.Your bigger problem is how you gonna probe and what.....

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2025, 06:44:46 pm
content: Quote from: 2N3055 on July 10, 2025, 06:39:26 pmI like 200 MHz as a good high limit for high impedance passive probes. Higher is possible, but then the use of high impedance passive probes becomes questionable.The fact that my Tektronix 7000 series of mainframe oscilloscopes are limited to 200 MHz with high impedance passive probes has nothing to do with it. That is my story and I am sticking to it.

author: 2N3055
author_rank: Super Contributor
timestamp: July 10, 2025, 06:48:18 pm
content: Quote from: David Hess on July 10, 2025, 06:44:46 pm

author: jjoonathan
author_rank: Frequent Contributor
timestamp: July 10, 2025, 07:36:30 pm
content: I have the same 200MHz rule even though I am free of ulterior motives as my scope and passive probes are both rated for 500MHz. Above 200MHz, the wheels are either rattling -- probe loading becomes real, 10pf hits 50 ohms at 300MHz -- or falling off entirely, as you can see with certain cheap passive probes "rated" for 500MHz that actually have a sharp resonant dip at 400MHz even when used correctly with a walking-stick ground.There are 20X passive probes that extend this a bit (and 20X passive probes disguised as 10X passive probes, LOL), but really you should aim for active or low-z probes above 100MHz and treat 200MHz as a limit beyond which you expect trouble. Even if your manufacturer gave you a sticker assuring that your "high impedance" passive probes were totally OK at 500MHz where they have a reactive impedance of 30 ohms

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2025, 10:59:23 pm
content: Quote from: blundar on July 10, 2025, 03:58:38 pmWhile the SDS1104X-E was a great DSO it has been outclassed by the SDS800X HD series especially for SMPS work due to its Power Analysis option especially suited to PSU work.However for full/complete efficiency analysis, current and differential probes are also required along with channel deskew tools like DF2001A.The rabbit hole can start to get deep.....Edit to add PA menu screenshot.

author: MathWizard
author_rank: Super Contributor
timestamp: July 10, 2025, 11:14:55 pm
content: What's your budget ? I own 3 Siglent scopes, and they have all been pretty easy to use, and hardly ever have a software issue.I think it hard to find used, modern scopes on ebay, most people just hang on to them I guess. The SDS1104X-E unlocked to 200MHz, is great. The screen seems small now compared to the newer scopes, but for me using it up close, it's fine, and fits easily on a desk.

author: kmoonwalker
author_rank: Frequent Contributor
timestamp: July 10, 2025, 11:20:07 pm
content: Quote from: blundar on July 10, 2025, 03:58:38 pmIt depends from your budget first of allwith more money you may more like 4-5 GS/s scope for some issues than 1 GS/s scope to have ability to look a bit deeper with one/two channels on certain thingsmax bandwidth is also important as it is determining how many harmonics scope will register and process to show you. the more you catch the better signal representation generally. even 10 times more freq is not something special if you see how many harmonics you have and would want (in case of complex signals)for most of ordinary things as Tautech said 800x hd is really a lot but and reasonably cheap (and also 2GS/s to share across channels not 1 GS/s -> twice more points of waveform really captured in every case generally speaking)the one you mentioned is also a good scope if you have such alreadybut there might be a moment when you may need a friend with better (faster/more GS/s, better probes, etc) scope to look more into matter at handbut for ordinary daily work 800x hd series will do nicesometimes a really good probe on even cheap scope can make a difference (but lets not loose a common sense chasing metrology rabbit hehe)also wfm/s parameter is very important and it can vary between models and with more money it is smarter to get a "faster" than "slower" one

author: MarkusAJ
author_rank: Contributor
timestamp: July 10, 2025, 11:38:48 pm
content: Quote from: blundar on July 10, 2025, 03:58:38 pmAny modern scope with 10HMz+ bandwidth will satisfy it. I'm assuming your project does include filtering (or?).