<azonenberg>
ok WOW. Simulation isn't finished but i think i might have found my v1,4 AKL-PT1 design, no filtering needed
<azonenberg>
Switching from Vishay FC series 0402 resistors to CH series 0201 in an attempt to make the attenuator string electrically smaller (as an added bonus CH series has a 250 ohm so I can combine the 200 and 50 into a single 250 plus a 50)
<azonenberg>
and making a few other small tweaks to ground layout
<azonenberg>
So far the tip region is looking to be flat +/- 0.5 dB from DC out to past 16 GHz
<azonenberg>
now, this isn't game over - this is before adding loss from the probe body, and the actual tip pin is difficult to simulate (I have a rough approximation I'll bolt onto the end of this sim when it's done and see what the end result looks like)
<azonenberg>
But it's looking like a huge improvement
<monochroma>
:D
<azonenberg>
in particular, wrt getting rid of the 5+ GHz peaking Shahriar complained about in the review
<azonenberg>
So, it turns out there isn't exactly 5 GHz peaking
<azonenberg>
there's two separate effects going on
<azonenberg>
ok so, for starters, red is a simulation of the tip end of the v1.3 probe as-is. ENIG loss not modeled so the oshpark prototype will have increasing attenuation as frequency increases
<azonenberg>
(these sims all include FR408 dielectric loss, but for the tip region only - the whole probe is too big to fit even in sonnet gold at usefully small mesh sizes)
<azonenberg>
Note how it rises to 5 GHz, then dips
<azonenberg>
then plateaus around 9 GHz and rises again?
<azonenberg>
In the current view it's a little hard to see but once you know what to look for there's definitely more current there than there should be
<monochroma>
:O
<azonenberg>
suggesting that little bit of ground is resonating
<azonenberg>
Soo
<azonenberg>
you know what happens when you cut off the ground vertically and don't have that tail?
<azonenberg>
You get the blue trace
<azonenberg>
in the original graph i linked
<azonenberg>
it stays about the same until 4 GHz then keeps on climbing all the way up to -11 dB at 16 GHz
<azonenberg>
The difference between red and blue is entirely caused by resonances in that tiny stub
<azonenberg>
(I've been running sims almost nonstop for weeks making tiny tweaks to the layout to track down some of these things)
<azonenberg>
anyway so basically that little stub acted like a weak notch filter and concealed the true extent of the peaking
<azonenberg>
All of these sims also remove the deliberately added filter I had on v1.3 to weaken the peaking as well
<azonenberg>
Since i figured that was also concealing the real problem and i wanted to get to the bottom of it
<azonenberg>
Anyway, then blue to purple kept that stub gone and switched from 200-200-50 ohm FC series to 250-200 ohm CH series. The CH series resistors have looser tolerance so the DC gain accuracy drops from like 0.1% to 2%, but much flatter frequency response
<azonenberg>
And I can make the attenuator string smaller by going from 3 to 2 resistors
<azonenberg>
Then purple to cyan stays with CH series but shrinks from 0402 to 0201
<azonenberg>
I'm now running a sim of the cyan layout using RO4350B instead of FR408HR to see how much things change with less dielectric loss
<azonenberg>
then will concatenate this with a separate sim of the probe body to get end to end board level response. Then I can try and estimate contributions from the probe tip
<azonenberg>
this sim includes the PCB footprint for the tip sockets but not the needles themselves
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<_whitenotifier-5>
[starshipraider] azonenberg pushed 2 commits to master [+50/-1/±9] https://git.io/JtjUJ
<_whitenotifier-5>
[starshipraider] azonenberg 9a7d30f - Continued AKL-PT1 simulations
<_whitenotifier-5>
[starshipraider] azonenberg 4083bd6 - Lots more AKL-PT1 simulation work