<azonenberg>
so i've traced the ripples to an impedance mismatch at the SMA. Which, despite my ground plane cutout, is too low impedance
<azonenberg>
I confirmed the direction and location of the mismatch by adding a 400 fF lumped capacitor from signal to ground at the SMA
<azonenberg>
Which made things drastically worse
<azonenberg>
same ripples, same place, but muuuuch bigger
<azonenberg>
So I guess the next experiment will be trying to shave down the center conductor of the SMA a bunch
<azonenberg>
of the SMA footprint*
<azonenberg>
see if i can match it better
<azonenberg>
as much as it pains me to say this, I think i will need a respin
<azonenberg>
Unless i can figure out how to cost-effectively hand trim and vna test every single probe
<azonenberg>
which on one hand sounds awful, on the other hand it would let me salvage north of $1K of boards
<azonenberg>
and if they still meet spec...
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<azonenberg>
One takeaway from this is that the sma center pin is WAY too long in the footprint
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<azonenberg>
there is no way it needs to stick out that far
<azonenberg>
right now the wide area goes almost the whole way to the end of the connector
<azonenberg>
So altering the footprint will mean that even if there is a mismatch the magnitude is much smaller and it will be electrically short up to much higher frequencies
<azonenberg>
Red is stock AKL-PT2 pcb, no overmold
<azonenberg>
blue is after hacking it with a scalpel
<lain>
aha
<lain>
I see o.o
<azonenberg>
the ripples changed sign and the TDR shows a slight positive reflection, meaning i bumped the impedance up too far
<azonenberg>
but it's now a 6 GHz probe
<azonenberg>
vs a 4.3
<azonenberg>
for that alone it seems worth improving
<azonenberg>
So... selling the current boards as-is is out the window
<azonenberg>
I will definitely be respinning, and i hope to be able to use the mill to get the current rev boards to be within acceptable ripple tolerances
<azonenberg>
Soo the question is... what to do about the overmold? it seems like it causes a significant Eeff shift
<azonenberg>
Which messes up my 50 ohm line so it's not 50 ohms anymore
<azonenberg>
even if i match the connector to it, there will still be a mismatch when it jumps to the coax
<monochroma>
laser ablation systems are getting cheap
<azonenberg>
I can try to guesstimate Er of the rubber and tell the fab to control the long line to slightly not-50 ohms
<azonenberg>
Such that I can be matched to 50 once overmolded
<lain>
azonenberg: given measurements in free air vs. overmolded, you should be able to caluclate the Er
<lain>
I think
<azonenberg>
the other option is to give up on the whole molding and stick with bare flex pcb
<azonenberg>
which will be a lot easier
<azonenberg>
so the question is really i guess how much the rubber jacket adds to it
<azonenberg>
And the second question is, is it worth trying to rework and sell some of the current batch of 250 PCBs? Or am I better off just writing them off and respinning immediately
<azonenberg>
This batch of bare boards was $1020 plus import duty so probably $1250 or so (about $5 a pop)
<azonenberg>
Which is enough that just throwing them out is something I don't want to do casually
<monochroma>
could sell em as "early access prototypes", their specs are well known, and still useful for a lot of things
<azonenberg>
You mean as-is, no rework? Yes, that is another option
<azonenberg>
I mean, ~0.6 dB of ripple and 4 GHz bandwidth isn't THAT bad
<azonenberg>
I'm leaning towards killing the overmold though
<azonenberg>
it's a lot of work, i'd need at least 2 more revs of the mold, and there's a lot of unknowns around dielectric properties of the rubber
<azonenberg>
and i'm beginning to question the benefits
<monochroma>
well, how do you keep em from shorting against each other when you have multiple in a confined area
<monochroma>
that was the original reason
<azonenberg>
Yes
<azonenberg>
Except now they're under coverlay and not bare topped anymore
<azonenberg>
this is what the tip looks like with the overmold
<azonenberg>
without silicone it's coverlay flex all the way to the sam
<azonenberg>
sma*
<monochroma>
and it doesn't effect the signal much?
<azonenberg>
The coverlay or the rubber?
<monochroma>
coverlay
<azonenberg>
not that i can see
<azonenberg>
it's less lossy than enig
<monochroma>
that was my understanding of keeping it open, was that any mask covering was going to cause issues
<azonenberg>
yeah i found a reasonably low loss coverlay that should be fine for my purposes
<azonenberg>
it's not great but good enough
<monochroma>
oh? what's the downsidE?
<azonenberg>
i mean there's still dielectric loss
<azonenberg>
immersion silver + open air miiight work but i'd need to figure out how to fabricate an air gap inside the overmold
<azonenberg>
that was on my list of possibilities for future R&D
<monochroma>
yeah, probably worth coming back to later, but sounds like you have a basically complete product now with the coverlay and the SMA layout changes
<azonenberg>
Exactly
<azonenberg>
So i guess at this point the game plan is going to be selling off at least some of the current inventory as soon as i can write a manual
<azonenberg>
either as-is or with some quick hand bodging to thin out the SMA center pin and match impedance better
<azonenberg>
then if there's sufficient demand respin a new version that doesnt need any bodging
<monochroma>
hmmm maybe mask and chemical etch ?
<azonenberg>
not through enig
<azonenberg>
With a 250 um endmill it should be possible to do fairly well with hand trimming
<azonenberg>
My planned sale price of "not more than $100" estimated labor costs for the overmolding
<monochroma>
just need a little aqua regia :P
<azonenberg>
If i skip the overmold and hand bodge the sma, about the same time per probe should be doable
<azonenberg>
BOM is only $5 PCB + $6 resistors + $10 SMA = $21 ish
<azonenberg>
so i could sell them for $100 sustainably spending up to $79 of time assembling and reworking each board
<azonenberg>
So I guess i'm going to desolder the SMA from the board i just hack-bodged then hop on the mill with one of the other... 7? populated PCBs and see what i can do with milling it
<azonenberg>
Then decide best course of action based on results from testing that
<azonenberg>
Gut feeling is that if i made up a jig of some sort to hold the pcb, i could probably rework each one in a couple of minutes
<monochroma>
yeah
<azonenberg>
But at this point i'm pretty confident the overmold is going bye-bye at least for now
<azonenberg>
So I guess the plan is going to be writing up a datasheet based on the assumption that it's bare, include preliminary characterization data from one of the milled boards
<azonenberg>
then open up a public beta selling off as-is units
<azonenberg>
Get feedback from users then do a final v0.6 board spin fixing the impedance mismatch in gerbers
<azonenberg>
Blue is AKL-PT1 v1.3, cyan and pink are PicoConnect 921 with one and two ground needles making contact with the DUT
<azonenberg>
Fixture de-embedded from all measurements using the same .s2p
<azonenberg>
I think i may have been confused and looked at the version with the overmold on or something
<monochroma>
:o
<azonenberg>
This is good enough to sell as is. I need to characterize a few more and test consistency
<azonenberg>
but basically i think i don't need any rework or a respin or anything
<azonenberg>
I think i wrecked four boards barking up the wrong tree looking for an impedance mismatch that was probably just a bad solder job on that first prototype
<monochroma>
is the original board that kicked this off still intact?
<azonenberg>
Um, i can check
<azonenberg>
i think i salvaged the SMA off it to rework the other ones with
<monochroma>
ahhh
<azonenberg>
since i only had like three left and didnt want to wait for more to come in
<azonenberg>
and that solder joint is now suspect :p
<azonenberg>
So i destroyed the evidence
<monochroma>
yeah
<azonenberg>
Buut basically right now it looks like if i don't overmold, the PT2 as it stands is a 6 GHz probe that just needs a datasheet written and more units assembled and it's ready to offer to the public
<azonenberg>
note the vertical gridlines are only 0.5 dB spacing
<azonenberg>
-23.5 dB would be 3 dB down from the DC gain
<azonenberg>
and guesstimating a bit i think it's still a GHz or so from hitting that
<azonenberg>
i'd spec it at 6 GHz as i have no way to characterize further
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<bvernoux>
azonenberg, I just saw your latest video about probe testing up to 4GHz
<bvernoux>
very nice
<bvernoux>
You speak about you are waiting a new scope which works up to 16GHz correct ?
<azonenberg>
Yes
<azonenberg>
It's a LeCroy SDA 816Zi
<azonenberg>
Which I will be upgrading next year to the SDA 816Zi-B
<azonenberg>
I am removing the overmold from the AKL-PT2 though
<azonenberg>
I spent a lot of time on it, but it just caused too much of an impedance shift
<azonenberg>
I'm sure I could respin the board with a different trace geometry and make it work, but it seemed like a lot of effort for minimal benefit
<azonenberg>
So it's just going to be bare flex PCB
<bvernoux>
woo SDA 816Zi-B have impressive specs
<bvernoux>
80GS/s per chan
<azonenberg>
That's interleaved, so only on 2 channels
<azonenberg>
The original 8Zi does not have the interleaver so is 40 Gsps only
<azonenberg>
but on all channels simultaneously (my current scope is 40 Gsps interleaved, 20 Gsps on all channels)
<azonenberg>
The 8Zi can be upgraded to 8Zi-B which includes an improved frontend that adds interleaving capability and some other improvements
<azonenberg>
as well as a faster PC and more capable software
<azonenberg>
You can also do bandwidth upgrades, it's a modular design that lets you swap parts in and out without having to replace the whole scope
<bvernoux>
yes at least to make the jump to Windows10 instead of Win7 (as IIRC it was provided with Win7)
<azonenberg>
Lol
<azonenberg>
no, mine is a 2009 model year. It runs vista
<bvernoux>
the best will be native linux but it seems Lecroy does not do that ...
<azonenberg>
But AFAIK the chipset on that motherboard is too old (core2quad) to run win10
<azonenberg>
So I will be doing the win10 upgrade next year as part of the upgrade to the 8Zi-B
<bvernoux>
ha strange I have a old PC Core2Duo which work fine with win10
<bvernoux>
ok it is a bit slow ;)
<azonenberg>
It might be possible, but realistically it lives on an airgapped lab network anyway
<azonenberg>
every piece of gear on that subnet has an unauthenticated scpi interface
<azonenberg>
an old unpatched windows OS is hardly an additional security risk :p
<bvernoux>
yes it is not so critical when it is in lab with restricted access to internet ...
<azonenberg>
So I'm just going to keep vista on it until next year
<azonenberg>
Anyway, right now it's at lecroy's factory getting refurb'd
<azonenberg>
the dealer turned it on to test before shipping to me after it had sat in their warehouse for a while
<azonenberg>
and it wasn't working right
<azonenberg>
It's fixable, and I'm going to get a factory refurb'd scope at the dealer's expense, but will delay things by another week or so
<bvernoux>
Does they will provide high speed probe with it too ?
<bvernoux>
as they are crazy expensive ;)
<azonenberg>
The dealer is not. But I traded my 2 GHz scope back to LeCroy
<azonenberg>
In exchange for that I'm getting a rack kit for the SDA, a single 13 GHz active diff probe (D1330-PS)
<bvernoux>
ha ok nice
<azonenberg>
And upgrading the SDA's trigger board to the full 14.1 Gbps SERDES pattern trigger
<bvernoux>
13GHz actif diff probe is nice for reference probe
<bvernoux>
to check the diff with your own diff probe which are WIP IIRC
<azonenberg>
Which will let me trigger on an 80-bit NRZ pattern, a single 64/66b symbol, or up to eight 8b/10b symbols
<azonenberg>
So i can trigger on the start of a 10Gbase-R frame, a PCIe TLP, etc
<azonenberg>
That was my first time experimenting with live streaming
<azonenberg>
No editing, all real time
<bvernoux>
it is a real pain to solder those SMA highFreq I have the same it is awfull
<bvernoux>
each time it take lot of time to solder them correctly
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<noopwafel>
azonenberg_work: another vote for considering selling as 'early access prototypes' without the rework btw, if your alternative is writing them off
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