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LaPOD: Low cost Logic Analyzer probe for MSO5k, DHO900 and more!
ebastler:
--- Quote from: oliv3r on November 03, 2023, 09:52:16 pm ---All traces, from the 50 pin connector, to the output pin, should all be exactly the same length for each signal ;) I didn't length match pairs only, For obvious reasons, if I measure a 2 channel signal, across 2 pods, I don't to care which input is matched, but want to ensure the delay is identical everywhere. That's why some of the lines look a bit weird, while not being snakes, because they are length matched too.
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I did not express my thought well, I'm afraid. I realize that you have properly length-matched everything, and that it must have taken a lot of tweaking!
What I meant to say is that you can potentially simplify the length-matching. It looks to me like, on the pod PCB, you have (a) length-matched the 4 signal traces from the 2*4 connector to the LMH7324, and (b) also length-matched the 8 signal traces from the LMH7324 to the HDMI connector. But the requirement is only that the sum "length of trace before + length of trace after" the LMH7324 need to be matched between the four channels. So two of the channels may have shorter traces from the input connector to the near side of the LMH7324, and compensate by having longer traces from the LMH7324 to the HDMI.
This thought doesn't matter if you do not need to touch the layout any further; it is clearly good as it is. But if you needed to move things around, get the capacitors closer or such, it might help to work with that relaxed constraint, because it can probably reduce the overall trace lengths and wiggles somewhat.
--- Quote ---I got [the blocking caps] as close as I could, there's traces underneath for power already, so dropping them isn't an option really. The caps are 6mm away, which I admit is further then I'd prefer too. Also, I really wanted to avoid via's, as that introduces other problems of course. There's always the possibility to add extra caps at the bottom though, like what dren.dk did, but I didn't want to compromise on the heat dissipation ability, as the bottom plane is where the ground plane gives the heatsink, adding those capacitors reduce that by quite a bit.
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OK, I have not seen the inner layers of course. If they are too busy, my suggestion may be impractical -- never mind!
I would also prefer to keep the bottom side free of components -- better heatsinking, more mounting options, also easier to home-build.
oliv3r:
--- Quote from: ebastler on November 03, 2023, 10:26:35 pm ---
--- Quote from: oliv3r on November 03, 2023, 09:52:16 pm ---All traces, from the 50 pin connector, to the output pin, should all be exactly the same length for each signal ;) I didn't length match pairs only, For obvious reasons, if I measure a 2 channel signal, across 2 pods, I don't to care which input is matched, but want to ensure the delay is identical everywhere. That's why some of the lines look a bit weird, while not being snakes, because they are length matched too.
--- End quote ---
I did not express my thought well, I'm afraid. I realize that you have properly length-matched everything, and that it must have taken a lot of tweaking!
What I meant to say is that you can potentially simplify the length-matching. It looks to me like, on the pod PCB, you have (a) length-matched the 4 signal traces from the 2*4 connector to the LMH7324, and (b) also length-matched the 8 signal traces from the LMH7324 to the HDMI connector. But the requirement is only that the sum "length of trace before + length of trace after" the LMH7324 need to be matched between the four channels.
--- End quote ---
Yeah, I thought of that too, but that's actually harder, because then you need to do math a lot :) kicad can't relate and sum traces between components. Also the 'win' there is minimally anyway, but a descent idea anyway.
I'll try to route the traces around the capacitors, to get them closer. I realized, the extra 1 - 2 cm are insignificant, as a) they are differential digital signals, and b) look at the wiggles on the breakout board, I'm not carying about length there either ...
Regarding the HDMI vs mini/micro HDMI discussion, before the raspberry pi, mini-hdmi is something I always thought of, as those cheap tablets used to have them, so there always was some availability. But with the PI, getting cables might actually be quite feasable, so micro HDMI might not be that weird to aim for. I'll have to look at the pinout, but routing may not be a problem either, as the signal traces may be at the top row, and the bottom row be the grounds.
So then, mini-hdmi or micro-hdmi I wonder, they cost about the same, so that's not the issue.
edit: The signals are not all on the top, so routing would be much more painfull, so for now, i'll try mini-hdmi :)
ebastler:
--- Quote from: oliv3r on November 04, 2023, 08:26:19 am ---So then, mini-hdmi or micro-hdmi I wonder, they cost about the same, so that's not the issue.
--- End quote ---
Technically, mini-HDMI is probably the better choice: A bit sturdier if one bumps against the plugs by accident, and small enough to fit all four connectors onto a PCB not wider than the scope's port.
Future-proofing might speak in favor of micro-HDMI. As devices keep getting smaller, micro-HDMI is probably taking over and mini-HDMI cables will become harder to get in the long run. But I would not be overly concerned about this: You don't need to plan for a 10-year production lifecycle, and if push comes to shove someone could always add a micro-HDMI breakout board later.
oliv3r:
How do we feel about this? In terms of dimmensions, the PCB itself is now 68mm wide
ebastler:
--- Quote from: oliv3r on November 04, 2023, 09:27:27 am ---How do we feel about this?
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:-+
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