1.5 GHz  scope useful for MIPI DSI / ARM AArch64 board layout? - Page 1
Forum section: electronics / testgear

author: loki42
author_rank: Frequent Contributor
timestamp: November 14, 2021, 11:57:37 pm
content: I've currently got a 100 MHz 4 channel scope that works fine. I have seen a few much higher end scopes second hand (lecroy / agilent etc) around the 1.5 GHz speed. The second hand ones lack probes which look like they cost a ton of cash but the scopes are around $2k. I mainly would think about needing something like that for looking at high speed digital signals (MIDI / DDR etc) but I've realised a protocol or logic analyser would probably suit better. I had to write MIPI DSI drivers a few years ago and seeing if I the kernel was sending the right physical stuff would have been handy. Is there a reason for needing a 1.5 GHz scope for this stuff? My current one does I2C / I2S / SPI fine, and I've realised a dirt cheap Cortex A53 can report these signals fine (in the digital, not signal integrity but if i'm not getting digital stuff I know there's a signal integrity issue).It looks to me that MIPI DSI with a 150ps rise time is way out of what any sensibly priced scope I can find could see anyway, if the -3db bandwidth is taken into account, unless I'm missing something. Will the old 1.5 GHz scope be useful or should I acquire other tools with the cash instead?

author: loki42
author_rank: Frequent Contributor
timestamp: November 15, 2021, 05:22:31 am
content: I was mostly thinking about one because there's one near by (Lecroy and an Agilent) that seems pretty cheap but I think I'll skip on it. Thanks for the confirmation! I'm buying more test automation gear instead.

author: Berni
author_rank: Super Contributor
timestamp: November 15, 2021, 06:53:48 am
content: I used a Agilent MSO9000 scope with 4GHz of bandwith to look at MIPI signals before(even does MIPI protocol decode)It can come in useful to have a fast scope around, even digital signals running fairly slow can have a good bit of junk and ringing on them that a slow scope won't even see. Even when the design still works with all that ringing you might have EMI issues due to the noise it makes, so its nice to be able to quantify it when testing various solutions. However you need active probes to actually make use of the extra bandwidth as the capacitive loading of a passive probe wrecks the signal too much. So once factor in buying a fast scope is also checking what active probes can you get for it on the used market (They can get pretty expensive).However in terms of board layout verification a scope won't be much use. Just do the board how the datasheet tells you to and it will be fine. I had MIPI running over 0.1in pin headers and floppy ribbon cable over to a FPGA dev board and even split it into a Y to connect two slave devices to the same master, didn't do proper termination...etc Yet it all kept working just fine. As long as you run the diff pair lines close to each other it is fine, the exact impedance doesn't even matter on very short runs. But one easy trick you can use to quickly check if your fast serial bus is on the edge of stability is to just press your finger on all the parts of it, this changes the parasitic capacitance and might make the bus glitch out when its right on the edge.