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[scopehal-apps] lain 8bbb0fe - Yet another macOS CI fix attempt
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<azonenberg>
ok so probe update
<azonenberg>
i ran some TDR measurements on a SMPM test board that had been literally sitting unpopulated on my bench since december
<azonenberg>
(yay for being crazy busy)
<azonenberg>
And it seems that a) oshpark is overetching my transmission lines and b) they were too small even if perfectly etched
<azonenberg>
The PT5 is currently designed with 0.4mm wide traces
<azonenberg>
the test board in question had 0.45mm traces (overetched to about 0.4 on the top and 0.46 at the bottom, i don't know the cross section profile yet) and TDR showed it in the 55-58 ohm range
<azonenberg>
Which means my 0.4mm nominal (probably overetched) traces are even higher than that
<azonenberg>
This doesn't quite match some of my simulations which makes me wonder if the dielectric config at oshpark isn't quite what i've been simulating
<azonenberg>
I'm probably going to run some cross sections later this week
<tnt>
azonenberg: couldn't see anything relevant in the validation layer.
<tnt>
So OTOH I think it couldn't run a bunch of the shader based validation because of this.
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<azonenberg>
yeah
<azonenberg>
so a lot of things around bad pointer detection etc dont work as a result
<d1b2>
<david.rysk> tnt: what intel gpu do you have?
<d1b2>
<david.rysk> if it's an older one, I can dig out my thinkpad x230t and do some testing
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<d1b2>
<tnt> @david.rysk Yeah, it's an older one but I think the x230t is even older. What CPU does it have ?
<d1b2>
<tnt> But it also happens on newer one apparently. (Else I wouldn't bother)
<d1b2>
<david.rysk> i7-3520M / Intel HD Graphics 4000
<d1b2>
<david.rysk> vulkan wouldn't work at all without updating mesa to bleeding edge (but this was before 23.0.0 was released)
<azonenberg>
Ok sooo let's see how this goes. About to submit a new AKL-PT5 board design for fab
<azonenberg>
first new probe design of 2023 i think since work has been so busy
<azonenberg>
it deletes the whole filter network and widens the trace from 400 to 450 μm
<azonenberg>
and i think i can also cut about 1mm off the long axis of the board
<azonenberg>
This means that the transmission length is now 5.9 mm, so if i have any more problems with impedance my null should be when that's a quarter wave long at a velocity factor of about 0.62. Which comes out to a period of 32 ps one way
<azonenberg>
so if we have a null from reflections it would be at a period of 128 ps or 7.81 GHz
<azonenberg>
actually no i can actually shorten it by 2mm
<azonenberg>
That gives about 4.8mm overall length
<azonenberg>
That gives 24 ps for one way, 96 ps for the whole wave, and a potential null at 10.42 GHz
<azonenberg>
So even if i didn't make the transmission line any better this tweak should be shifting badness significantly higher in frequency. I wonder why I didn't do this before?
<azonenberg>
I think there's potential to make it even smaller but this is a nice start
<d1b2>
<tnt> @david.rysk Yeah mine is i7-4600U, so Intel® HD Graphics 4400 , a generation after.