<Degi>
bvernoux, if you need help setting up nMigen, you can message me (though I mostly use arch linux, but ubuntu shouldn't be too different)
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<azonenberg>
Status update: AKL-AD1 v0.2 assembled and powered on
<azonenberg>
everything appears OK, no unusual power draws, MCU responds to SWD, and the amplifier is passing a plausible looking PCIe signal
<azonenberg>
But it's doing so with 6 dB of attenuation because that's the power-on minimum gain of the amplifier
<azonenberg>
Before I can properly characterize I need to write some firmware to bump the gain of the amp up to a more reasonable level
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<azonenberg>
gaaah
<azonenberg>
I'm getting bitten by the most annoying bug
<azonenberg>
i made the mistake of using an auto-sensing level shifter on the uart since the stm32 runs at 2.5V and i couldn't find any 2.5V uart dongles
<azonenberg>
aaaand apparently the pullup on my usb-uart dongle is enough to trip it
<azonenberg>
so it oscillates
<azonenberg>
Now to try and find a dongle that either has none, or a very weak, pullup. Or add some kind of external buffer
<azonenberg>
This isnt a fatal flaw, this uart is for debug only and worst case i can hard code the gain config or something
<azonenberg>
but i'm annoyed i didnt think of it
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<azonenberg>
Soooo this is interesting
<azonenberg>
it appears that it was not at minimum gain, it was in power-down mode :p
<azonenberg>
So it looks like the de-embed makes the PCIe waveform a little bit worse, because my current de-embed algorithm brickwall filters stuff past the end of the s-parameter dataset (6 GHz in this case)
<azonenberg>
but at lower speeds it definitely improves it
<azonenberg>
left=straight off the probe, note the overshoot on the rising edges
<azonenberg>
right=deembedded with approximated transfer function
<azonenberg>
approximate meaning I used the AKL-PT2 data for the tip since I haven't VNA'd an AKL-PD1 yet (although they're almost the same layout so should be similar)
<azonenberg>
and i used the P-to-out data instead of (inP-inN)-to-out
<azonenberg>
as i haven't yet figured out the right way to massage the s-parameters to get the differential-to-single-ended curve
<azonenberg>
I think I got pretty close though
<azonenberg>
actually no this wasn't using a tip, my bad
<azonenberg>
this was using the amplifier with 15 dB attenuators standing in for the tip
<azonenberg>
So i'm only de-embedding the amplifier here
<azonenberg>
I do wish that the P-to-N matching was better, because i believe that this this mismatch directly impacts CMRR (more precisely, the mismatch results in mode conversion)
<azonenberg>
There's a 2 dB P-to-N mismatch at 3 GHz
<azonenberg>
then it diverges more
<azonenberg>
Using inP as the reference point, gain is 1.6 dB below the nominal +20 at ~DC, then hits nominal at around 1.3 GHz. Peaks at +1.2 dB at 2.6 GHz, falls back to nominal at 3.4 GHz, then -3 dB attenuation at around 4.2 GHz
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<azonenberg>
david.lenfesty: So I'm happy enough with the current AKL-AD1 that I think I might actually table it until you've finished initial work on firmware etc
<azonenberg>
And then have the v0.3 AKL-AD1 use the usb-c power/data connection
<d1b2>
<david.lenfesty> The pressure's on 😛
<azonenberg>
In parallel with that, I'm going to start working on a handheld browser probe
<azonenberg>
which, based on the twitter poll and some thinking of my own, will either mate with the AKL-AD1 via cables, or incorporate substantially the same circuit internally
<azonenberg>
But in either case it's going to be a handheld differential browser probe with ~4 GHz bandwidth and adjustable tip spacing
<azonenberg>
Ideally targeting the oshpark flex stackup for a prototype, but doing the amplifier layout on a 2-layer board with good SI/PI might be a challenge though. i'd also need to have a hand applied fr4 stiffener to keep the amp from being damaged
<azonenberg>
So what i might do is design a passive tip to work out the mechanical side of things (even if that's not the long-term plan)
<azonenberg>
then go straight to a "real" fab to do a 4-layer flex-rigid design
<azonenberg>
with a flex signal+ground stackup in the tip area that has an additional two rigid layers laminated to it in the amplifier area
<azonenberg>
Also hmmm
<azonenberg>
Given that I have +20 dB of differential-to-single-ended gain with the LMH6401, I can afford to have some extra loss in the probe tip
<azonenberg>
I wonder if it might be worth trying to design a 20x tip which would give me a net 2x attenuation and 2K ohm differential impedance
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<azonenberg>
I've cracked the solder joints on two of these nice SMAs now
<azonenberg>
trivially reworkable but not visible as an obvious defect even under the microscope
<azonenberg>
I think this is caused by the loose fit of the edge launch
<azonenberg>
i will definitely want to glue them down in the future
<azonenberg>
in particular there's a gap between the bottom of the sma and the edge of the pcb
<azonenberg>
filling that will keep it from trying to peel off the board when torqued
<azonenberg>
So i wrote up a little tool that takes in two S2P's for inP-to-out and inN-to-out
<azonenberg>
And effectively averages the complex measurements to produce a differential-to-out S21
<azonenberg>
then outputs a touchstone file with differential S21 and S22 (for now I'm not calculating the reverse path or input return loss)
<azonenberg>
This is fine as I only need S21 for a de-embed of the forward path
<azonenberg>
This is what the forward path response of the current AKL-AD1 looks like
<azonenberg>
https://www.antikernel.net/temp/ad1-with-tip.png and this is the calculated response of the probe with an AKL-PT2 (again, assuming the PT2 and PD1 have near-identical response which may or may not be valid)
<azonenberg>
If we call -3 dB the average forward path gain, that would mean the overall system bandwidth is around 3.68 GHz
<azonenberg>
Slightly less than the 4 I was aiming for, but not awful either