Scope to look at 125 MHz RGMII interface - Page 1
Forum section: electronics / testgear

author: Northy
author_rank: Regular Contributor
timestamp: October 18, 2023, 05:37:55 pm
content: Hi all,I'm soon going to have to verify an RGMII interface, what sort of scope should I be looking at for that? Or is there something more cost effective?Thanks,G

author: Northy
author_rank: Regular Contributor
timestamp: October 20, 2023, 10:48:12 am
content: Hi,Would anyone be able to offer some advice? This will be a large cost that I'd rather not get wrong.Thanks,G

author: Berni
author_rank: Super Contributor
timestamp: October 20, 2023, 11:09:19 am
content: What in particular you want to see on the interface?In general for checking signal integrity on such lines you need a scope with many times more bandwidth than the signal, so in this case something like a 500MHz scope. You will also need a active probe to avoid loading down the signal at these high frequencies.

author: Northy
author_rank: Regular Contributor
timestamp: October 20, 2023, 01:56:41 pm
content: Hi,Yes, just looking at signal integrity and making sure the interface is working with the most margin.Thanks,G

author: Berni
author_rank: Super Contributor
timestamp: October 20, 2023, 02:47:49 pm
content: You probably don't even need to verify it, just run a stress test on it in software while giving it some torture (like running really cold or really hot, try to jam it with RF interference etc) and if it keeps on working it is fine.These are not that terribly fast signals so as long as the usual high speed PCB routing rules are followed it should be fine.

author: nctnico
author_rank: Super Contributor
timestamp: October 20, 2023, 03:38:31 pm
content: If the circuit is build according to the manufacturer's guidelines where it comes to decoupling PLL power supplies and proper length matching, it should just work out of the box. RGMII is a synchronous parallel interface and there is a requirement for clock setup and hold times. IIRC some RGMII interfaces do allow to set / configure the active clock edge (rising or falling) so make sure that this is configured the same at both sides and the length matching is done accordingly.I've seen a design where somebody got the decoupling wrong which caused the RGMII clock to have excessive jitter.Checking the signals requires a scope with a bandwidth of >=500MHz and some Low-Z (or better) probes. A second hand DSO will do just fine for this purpose.

author: Northy
author_rank: Regular Contributor
timestamp: October 22, 2023, 06:25:42 pm
content: Thanks all.Are there any PC based systems that are >500MHz?G

author: jc101
author_rank: Frequent Contributor
timestamp: October 22, 2023, 06:41:12 pm
content: Quote from: Northy on October 22, 2023, 06:25:42 pmPicoscope 6000 series can do 500MHz+ depending on your budget...https://www.picotech.com/oscilloscope/6000/picoscope-6000-overview

author: AndyC_772
author_rank: Super Contributor
timestamp: October 22, 2023, 08:13:29 pm
content: For a single project, rent the proper equipment. You'll want a scope with a bandwidth of at least 1 GHz, and even more importantly, at least two active probes - one for the clock and one for data.

author: Berni
author_rank: Super Contributor
timestamp: October 23, 2023, 06:02:09 am
content: It is indeed a bad idea to buy a expensive tool like a scope for only 1 specific need in a project.When you go buy something like a scope, try to think into the future a bit. Look at your existing scope and think about what it is missing. Then find a scope that is really good at those things.The perfect scope for all uses does not exist unfortunately. So i find it is a good idea to have multiple scopes to cover all those cases. So for me personally i slowly accumulated a few scopes:- Agilent MSO6000: 4ch 300MHz, pretty average capability scope, nothing special, but it boots fast and is fast to set up for mesurements, gets most jobs done quickly and efficiently.- Agilent MSO9000: 4ch 4GHz behemoth running Win7. It is more cumbersome to use, but covers things my main scope can't do. It has 1GPts of memory, it has tons of bandwith, it supports active probes, can serial decode anything under the sun, can do fancy math..etc- Some old HP Sampling scope boat anchor: 2ch 20GHz, old, huge, loud, power hungry, clunky. Only reason to use it is when you need the bandwidth, horrible scope otherwise.- Owon HDS200: 2ch 200MHz nice little portable scope that is barely larger than a handheld DMM. A bit annoying to use due to lack of knobs, but allows me to take it anywhere while still being capable enough for most tasks.So in your case you likely want to buy a high bandwidth scope and go for the fastest thing you can get. Make sure to keep active probes in mind. Anythyng >100MHz is tricky without them.