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<azonenberg>
Sooo it looks like my probe issue was the resistor
<azonenberg>
I'm still getting some issues but it's way better than i had before
<azonenberg>
In the current configuration i'm going to say it's definitely suitable for recovery of gigabit serial data, you probably wont be doing in-system eye measurements with it but i'm not sure most probes can do that
<azonenberg>
i really do want to get a ZS1500 to compare against though
<azonenberg>
and one of those pico probes
<tnt>
I have a couple of 1152A, I'd be curious to compare against that. (I'd need to get glscopeclient running first though :p)
<azonenberg>
tnt: 1152a? are those active probes of some sort?
<azonenberg>
ah ok, so about the equivalent of the lecroy zs2500
<azonenberg>
i was going to get a zs1500 because my scope is only 1 GHz b/w
<tnt>
yeah, mine is only 500 MHz, I got that probe because I was able to get it cheap on ebay :p Technically my scope doesn't support it, but a bit of hacking and I was able to make it work just fine on there.
<azonenberg>
That might not be of much help then
<azonenberg>
because my probe seems pretty flat out to 500 MHz
<azonenberg>
it's ~500 - 1G i have problems with
<azonenberg>
then past 1G it actually seems to peak a bit then level off
<bvernoux>
just seen latest news about the probe
<bvernoux>
the signal is really better
<bvernoux>
I suspect it could be better with coplanar waveguide ...
<azonenberg>
Yes
<azonenberg>
I think i've gone as far as i can with the current PCB layout
<azonenberg>
For a coplanar waveguide is there any rule as to how wide the ground has to be?
<azonenberg>
the closed-form models all assume an infinite ground
<azonenberg>
but i'm not sure how it falls off as the ground gets smaller
<azonenberg_work>
Right now what i'm looking at doing is an 0.33mm wide pcb trace with 0.125mm horizontal spacing to the top layer ground (min oshpark clearance) and 0.18mm of FR408 prepreg to the layer 2 ground plane
<azonenberg_work>
saturn PCB gives 50.7 ohms for that
<azonenberg_work>
Then for an 0402 passive if i have ground on layer 3 (h=1.382 mm) but not 2, saturn gives me 62.9 ohm impedance
<azonenberg_work>
meanwhile, if i put my main reference plane on layer 3 the whole way along, a 1mm wide conductor is a near perfect match (51 ohms)
<azonenberg_work>
I think that's the way to go, because the 0402 is going to have an impedance step regardless (it's a resistor, duh)
<azonenberg_work>
and the 1.1mm MMCX center conductor will be nearly perfectly matched (49 ohms)
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<bvernoux>
for me just follow the PCb calculator like KiCad or other
<bvernoux>
or even better the one from PCB Toolkit v7.08
<bvernoux>
especially to match impedance
<azonenberg>
yeah i used saturn pcb
<bvernoux>
Conductor Impedance -> Coplanar Wave
<azonenberg>
yeah thats what i did, the problem is that it isnt a full field solver
<azonenberg>
and doesnt model non-infinite ground width
<azonenberg>
i just wasnt sure how big a deal that was
<bvernoux>
IIRC the computation is quite good and shall be checked with a VNA anyway at end
<bvernoux>
the main issue is accuracy of manufacturer to match the spec
<bvernoux>
which can be specified to respect 50 Ohms Impedance
<bvernoux>
but it cost more and cannot be done at OSHPark:(
<azonenberg>
Yeah thats why i wanted to go with a coplanar waveguide specifically
<azonenberg>
because with a nice wide pcb trace and ground plane, small etch changes will have much less of an effect on impedance vs microstrip
<azonenberg>
microstrip is fine if you're using a real controlled impedance fab (although i'd probably still go GCPW for this probe to reduce loss) but here, i need to design for loose tolerance
<bvernoux>
the best for impedance is RO4350 but I Imagine it is costly
<bvernoux>
IIRC Harmon Instrument has done some nice test with just OSHPark
<bvernoux>
even better with flexible PCB
<bvernoux>
as the permissivity is better than FR408 and also for coplanar it is better thinner ...
<bvernoux>
also a very nice tool => QucsStudio-2.5.7
<bvernoux>
it is even better than Qucs
<bvernoux>
ha ok so with FR408
<azonenberg>
Yes
<azonenberg>
I'm putting ground on layer 3 of 4
<azonenberg>
so one prepreg and one core between me and the solid plane, then ground on the top layer
<bvernoux>
ha with 4 layers PCB
<azonenberg>
it's a 2-layer board built on a 4-layer stackup
<azonenberg>
i'm only using layers 1 and 3
<bvernoux>
ok
<azonenberg>
so basically copper, prepreg, core, copper, prepreg
<azonenberg>
This gave 50 ohms for 1mm wide top-layer traces with 0.125mm clearance between top-layer ground and signal
<bvernoux>
I recommend checking with QucsStudio 2.5.7 => Line Calculator
<azonenberg>
The mmcx center ground is 1.1mm wide
<bvernoux>
it is more advanced
<azonenberg>
which saturn pcb calculates as about 49 ohms on the same config
<bvernoux>
it take into account also the length of conductor and lot of other things
<azonenberg>
I deliberately chose these design rules to be relatively tolerant to overetch at the fab because i know oshpark has poor line width control for this kind of stuff
<azonenberg>
So i targeted a config where a moderate overetch won't change impedance too badly
<bvernoux>
yes it is very interesting
<bvernoux>
especially to check on 3 boards to see differences
<bvernoux>
I will buy a batch of 3 also ;)
<bvernoux>
could you provide the different value you have used
<bvernoux>
G/W/H/L/T
<bvernoux>
T=Thickness of copper which should be 35um
<azonenberg>
My design rules are 1mm wide conductor, 0.125mm clearance to top layer ground, 1.382mm to inner layer ground, FR408
<azonenberg>
this is my current probe tip, the ground is in line with the signal
<bvernoux>
I can check what happen with it in Qucs
<azonenberg>
so its a little hard to se, but you can see the needle
<azonenberg>
The original resistor was a Yaego 953 ohm that had poor high frequency performance
<azonenberg>
the one I am using now is a Vishay FC series 1K
<azonenberg>
it's flip chip style mounting without wraparound leads
<azonenberg>
i really wanted 950 ohms but couldnt find anyone stocking it
<bvernoux>
you are using a 0603or less ?
<azonenberg>
0402
<bvernoux>
ok
<azonenberg>
Vishay models the 0402 flip chip package as 26.2 fF shunt capacitance pad to pad
<azonenberg>
then the nominal resistance value in series with 1.89 pH
<bvernoux>
yes very nice
<bvernoux>
as on previous resistor nothing was specified anyway ;)
<azonenberg>
The 1K is flat out to ~600 MHz then starts dropping off a bit, at 1K ohms it's about 900 ohms
<bvernoux>
as they are not for RF
<azonenberg>
So you will get a little bit of peaking around 1 GHz which is consistent with the data i am seeing
<azonenberg>
however i also am seeing loss from about 500 - 900 MHz which i suspect is PCB related
<azonenberg>
hence the respin
<bvernoux>
the 0402 Flip Chip version is even better in datasheet
<azonenberg>
That's what i am using
<bvernoux>
ha ok
<bvernoux>
and it is something like 10USD/resistor
<azonenberg>
not even closer
<azonenberg>
mouser has them for 2.05 USD @ qty 1, 1.70 @ qty 10
<bvernoux>
anyway at end it is still cheap with good performance to be confirmed with your latest design
<azonenberg>
(no stock at digikey)
<bvernoux>
ha good
<azonenberg>
Now i'm working on respinning the enclosure
<azonenberg>
the first round needed some machining in order for the MMCX mating half to fit (the female connector on the PCB fit flawlessly but the male connector on the cable collided with the enclosure)
<bvernoux>
the enclosure seems already very nice
<azonenberg>
Yeah i am overall pretty happy with it, it fits the Pico probe positioners beautifully
<azonenberg>
My plan is to keep the top half as-is but respin the bottom half with a bit more clearance
<azonenberg>
no other changes for the moment
<azonenberg>
it's starting to look like a proper piece of test equipment though, so i'm pretty happy
<bvernoux>
I need to buy some Pico prob positioners ;)
<azonenberg>
i just need it to *work* like one :)
<bvernoux>
I do not remember the ref
<azonenberg>
pico TA102
<azonenberg>
you can buy them from a bunch of different places for 25 USD each
<bvernoux>
I plan to buy things on Farnell so I will check
<azonenberg>
My enclosure is cylindrical and too big to fit into the round holes, but i have a rectangular cutout region exactly the thickness of the cutout on the hole at the tip of the positioner
<bvernoux>
I try to avoid at maximum mouser/digikey because it is awfull with tax/import in France
<azonenberg>
So it slides between the open jaws, then the cylindrical parts of the enclosure keep it from sliding front to back
<bvernoux>
ha nice
<azonenberg>
my first iteration was boxy, the rounding was just to make it more comfortable to hold freehand
<azonenberg>
it serves no other purpose
<azonenberg>
Also, i see them at Newark and Allied, not farnell
<azonenberg>
also tequipment.net
<bvernoux>
yes Farnell does not have it :(
<azonenberg>
sjelectronics.co.uk has them for 18 GBP though
<bvernoux>
like lot of other stuff digikey ... are too often mandatory
<azonenberg>
Since they're still EU, for now, can you avoid import tax for that?
<bvernoux>
yes until the brexit ;)
<azonenberg>
Hence "for now"
<bvernoux>
it is why UK is not good guys as they avoid tax ;)
<bvernoux>
they import from china and resell to EU without any tax ;)
<bvernoux>
they are the bad boys in EU but they are not the only one hehe
<azonenberg>
Ok so, respun pcb ordered from oshpark, i have enough resistors and mmcx's to make another two probes
<azonenberg>
now to fix the enclosure
<bvernoux>
do you plan to do some test with flexPCB ?
<azonenberg>
I have a separate flex pcb that i already did some time ago from a real fab with impedance control
<azonenberg>
that i want to try populating with the FC resistors instead of the cheap yaggos
<azonenberg>
yaegos*
<azonenberg>
have not done that test yet
<azonenberg>
they're a solder-in design with castellations at the tip then a tiny fr4 stiffener under the MMCX
<azonenberg>
gregdavill tweeted some nice pictures of them a few weeks back
<azonenberg>
i have 500 of them and they work great for lower speeds but with the cheap resistor performance in the GHz range was pretty bad