@tedyapo@fosstodon.org
@tedyapo@fosstodon.org avatar

tedyapo

@tedyapo@fosstodon.org

solderer of fortune

will work to fund passion projects

i live on great sacandaga lake in the adirondack park

author of obscure unreleased software

he/him/whatshisname

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tedyapo, to random
@tedyapo@fosstodon.org avatar

it's not perfect, but for a normal FR4 OSH Park 2-layer board with $0.20 SMA connectors and a slightly sketchy terminator on the end, i'll take it as a first ground-truth validation of Z₀CAD calculations

it also assumed an ideal square etch profile

screenshot from a VNA showing return loss. span is 3.2 GHz. the return loss is greater than 30 dB until about 2.5 GHz, and remains above 25 dB until 3.2 GHz

tedyapo,
@tedyapo@fosstodon.org avatar

i was able to coax (no pun intended) my SD-24 TDR head into a semi-calibrated state finally, so i can get a better look at these boards. the bottom red cursor is 50 Ohms; the top 54.26 Ohms.

the inductive bumps on either side are the mated SMA connectors

so, the trace isn't bad, a little high, which is probably due to assuming a square etch profile. including the actual profile would widen the as-designed trace, reducing the impedance

but there's something else interesting here

tedyapo,
@tedyapo@fosstodon.org avatar

first, those bumps really are the mated SMA connectors, not the launches. a few turns of wire solder on a bamboo skewer makes a convenient probe: touching the trace with the solder creates a capacitive marker you can see on the TDR return. one such test is highlighted in the red circle

with this, you can tell that the bumps are before/after the board itself, i.e. in the SMA connectors. $0.20 jacks aren't expected to be great

the launches themselves are indistinguishable from the trace

image/png

tedyapo,
@tedyapo@fosstodon.org avatar

but perhaps more interesting is this peak, right at the board edge. it only appears on the "input" side of the connector; if you flip the board, the peak remains on the left.

i had never considered that coax connectors might be "directional", i.e. having different reflection coefficients (other than possibly sign) in the input and output directions. from a purely impedance matching standpoint, they shouldn't

but if you consider field matching, it makes a lot of sense (cont.)

tedyapo, to random
@tedyapo@fosstodon.org avatar

tell me it's going to be OK if i use QFN packages on this board instead of the beloved TQFPs

azonenberg, to random
@azonenberg@ioc.exchange avatar

Trigger crossbar front panel board is back from fab.

It looks like I mostly pulled off the 0.5mm pitch BGA, but the Vbat via looks to be insufficiently tented so I might get it failing open there. Should have moved the via slightly further away from the ball to reduce the risk of such a failure.

Good news is, I don't think this will matter for two reasons.

  1. The actual Vbat ball is not the one next to the via, it's the one to the right. The outer-ring ball is an unused GPIO that I'm just using as a route-through. The same is true of the via in the northeast corner of the array where the annular ring is eating into the BGA land.

In both cases the outer-ring ball is a route-through and so some wicking resulting in the BGA failing open at that pin won't matter whatsoever.

  1. I'm not using the backup-memory feature anyway, and when Vdd is supplied the MCU runs off that instead.

Overview of BGA footprint showing fanout

tedyapo,
@tedyapo@fosstodon.org avatar

@azonenberg do you think you are measuring the top or the bottom of the -- let's call it trapezoidal for easy math -- etch profile?

MLE_online, to random
@MLE_online@social.afront.org avatar

I always wake up to a bunch of messages from friends who seem to stay up until 1 am or later every night of the week.

Sorry, friends, I go to bed at 10pm because I need my sleep.

tedyapo,
@tedyapo@fosstodon.org avatar

@MLE_online 10 pm aka 1 am

tedyapo, to random
@tedyapo@fosstodon.org avatar

last time i bought copper clad for prototyping, i got FR1 (i.e. phenolic substrate) figuring i could also mill it without the silicosis risk. it may turn out to be suitable for that, but for dead-bug prototyping, it's very bad

try to solder to it, and the board sizzles and crackles while bubbling up thru the copper. i can only imagine it's the adhesive holding the copper on?

tedyapo,
@tedyapo@fosstodon.org avatar

@jaseg i ruined my metal shear blades cutting FR4, lol. i use a wet tile saw for sizing it now

but i was going to mill traces on there, and that's the dust risk i'm concerned about

tedyapo, to random
@tedyapo@fosstodon.org avatar

trying my hand at measuring PCBs. cut them up with a wet tile saw -- easy, but needs a little fence mod, bc they can slide right under it.

i potted one in epoxy and did an initial polishing

tedyapo,
@tedyapo@fosstodon.org avatar

the result isn't bad. i have some finer grit paper (10000) on the way to see if i can smooth things out a bit more. all i had handy was 3000

i can probably measure the thicknesses fairly well from this view, but characterizing the etch profile might need a bit more zoom. these pixels are about 3 um across, so it's probably doable

tedyapo, to random
@tedyapo@fosstodon.org avatar

what would you do with a round LCD?

tedyapo, to random
@tedyapo@fosstodon.org avatar

finally a clear night to test a new distance prescription!

tedyapo,
@tedyapo@fosstodon.org avatar

@adamsdesk no, i am going to avoid optometrists

tedyapo, to random
@tedyapo@fosstodon.org avatar

thinking of a polyglot image file for plots that also contains the data and maybe the python/matplotlib code to generate itself

azonenberg, to random
@azonenberg@ioc.exchange avatar

Benchtop organizational idea - has anyone done this before?

Little 3D printed plastic feet with a threaded hex standoff press-fit into it. Attach the foot to your bench with command strips or double sided tape, then screw DUT into it.

When you need to move the board to solder to it etc, just remove the screws, lift it off, then put it back in when you're done.

Board is held firmly in place without needing to tape it down by the corners like I do now.

Good idea? Bad? Any practical experience?

Probably gonna try and throw something together in solvespace this afternoon and get a local friend to print it for me.

tedyapo,
@tedyapo@fosstodon.org avatar

@azonenberg you could glue a magnet in each base. idk about your tables, but mine are magnetic stainless, and a good neodymium magnet has enough stick to work through the 2mm mat

tedyapo, to random
@tedyapo@fosstodon.org avatar

can you hack a bi- or trinocular biological microscope to do front-illumination by making some sort of illuminator to insert instead of an eyepiece?

deshipu, to random
@deshipu@fosstodon.org avatar

deleted_by_author

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  • tedyapo,
    @tedyapo@fosstodon.org avatar

    @deshipu usually bandgap references, but occasionally buried zener diodes

    MLE_online, (edited ) to random
    @MLE_online@social.afront.org avatar

    The name trilobite (for the prehistoric marine arthropods) just means three-lobed (tri = three, lobite = having lobes).

    Thus, anything with three lobes could accurately be described as such.

    So, it would be correct to call the headwear favored by 18th-century pirates and America's founding fathers "trilobite hats"

    tedyapo,
    @tedyapo@fosstodon.org avatar

    @MLE_online yes, the very same founding fathers who created our trilobite system of government

    tedyapo, to random
    @tedyapo@fosstodon.org avatar

    you wouldn't download a microscope objective to c-mount camera adapter

    MLE_online, to random
    @MLE_online@social.afront.org avatar

    One of the dumbest things about being trans in Southern California is that people are always saying "dude" and then getting tripped up and apologizing. To me, personally.

    And then I have to be like, "Dude, it's fine to say dude. I don't care. It's a normal part of speech. You can always say dude."

    tedyapo,
    @tedyapo@fosstodon.org avatar

    @MLE_online having lived in each new england state at one point, i can tell you there are female yankees, from which the term may originally be derived

    tedyapo,
    @tedyapo@fosstodon.org avatar

    @MLE_online no, that's a chorus line

    tedyapo, to random
    @tedyapo@fosstodon.org avatar

    i can ignore actual etch profiles no longer

    you can just make out the etch profile on the edges of this nominally 40-mil-wide pad on an OSH Park 2-layer board. the sharp edges are the top of the trace, now some 38 mils wide, while the slightly gilded halo around them is the bottom of the trace, at around 39.8 mils

    i have some opencv code that finds the top trace edge reliably now, and hopefully can automate finding the bottom one

    tedyapo,
    @tedyapo@fosstodon.org avatar

    modeling the etch profile will be important for tuning Z₀CAD stackups

    assuming this were a micrstrip trace and not a pad, the difference between the nominal 40-mil trace and the 38-mil one with "feet" is 81.9 Ohms vs 83.6 Ohms

    things get worse for thinner traces, too, bc the etch profile is a larger fraction of the nominal trace width

    tedyapo,
    @tedyapo@fosstodon.org avatar

    it took me a while to see in the original image

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