<azonenberg>
The order of v0.3 probe boards got here
<azonenberg>
The new MMCX footprint looks much nicer than the old one
<azonenberg>
I'm going to solder one up with the five resistors in a few, then try with no cap and with a discrete compensation cap
<azonenberg>
the distributed cap is on the v0.4 board which is still at fab
m4ssi has joined #scopehal
<azonenberg>
hmmm
<azonenberg>
There is still some other effect going on that i am not modeling
maartenBE has quit [Ping timeout: 245 seconds]
maartenBE has joined #scopehal
m4ssi has quit [Remote host closed the connection]
bvernoux has joined #scopehal
<azonenberg_work>
bvernoux: did you see my tweet thread from yesterday?
<bvernoux>
ha yes
<bvernoux>
very interesting
<azonenberg_work>
That weird step we've been tracing is an impedance mismatch for sure
<azonenberg_work>
but why the heck is the observed impedance not matching everything i calculate it should be??
<bvernoux>
I do not understand how do you have checked that you have Impedance of 30 Ohms in fact
<bvernoux>
instead of 50Ohms expected ...
<azonenberg_work>
I don't know for sure that I do
<bvernoux>
as it is very strange if it is true
<azonenberg_work>
What I can say is, the shape of the waveform is consistent with a length of 30-ohm trace equal to the length of my pcb from resistor to mmcx
<azonenberg_work>
followed by a 50 ohm line to the scope
<bvernoux>
do you have some capture with the probe and without on same signal with scope ?
<azonenberg_work>
additionally, when i machine away part of the trace to increase the impedance
<azonenberg_work>
the height of the step was reduced
<azonenberg_work>
Those are experimental results I can report as fact
<bvernoux>
maybe the most impact is on inductance/capacitance too ...
<azonenberg_work>
I am not confident enough to say that the actual impedance is indeed 30 ohms
<bvernoux>
as I doubt it do so much difference on impedance
<azonenberg_work>
the high frequency rolloff i saw on the 1-0-1-0 pattern was reduced
<azonenberg_work>
the hump height on the rising edge was reduced
<bvernoux>
I suspect capacitance and maybe other parameters are improved
<azonenberg_work>
This is my main evidence pointing to the impedance being wrong
<bvernoux>
inductance/impedance ...
<azonenberg_work>
My design parameters here were 125um gap to top-layer ground, 1.17mm gap to inner layer ground, and 1.1mm wide center conductor
<azonenberg_work>
Later simulations suggested 1.0mm would be closer to 50 ohms but it was too late and the pcb was at fab already
<azonenberg_work>
My latest version that i sent out last night reduces the center conductor to 0.75mm and increases the gap to 200um
<azonenberg_work>
plane spacing stays the same
<azonenberg_work>
Sonnet models that as having a 60 ohm impedance
<azonenberg_work>
vs the ~45 of what i built this time
<bvernoux>
maybe Sonnet is not very good to compute that strange
<azonenberg_work>
i deliberately went pretty aggressive with the impedance bump to make sure i see results
<azonenberg_work>
it might be an overshoot vs ideal match
<bvernoux>
or there is other parasitics things not taken into account
<azonenberg_work>
So, the two possibilities i'm considering is that there's some other parameter i didnt model
<azonenberg_work>
or that you need a thick-metal model to accurately model GCPWs
<bvernoux>
we will have the truth with S-parameters ;)
<azonenberg_work>
the free edition of sonnet does not support thick metal
<azonenberg_work>
when i buy the full version that won't be a problem
<bvernoux>
ha ok
<bvernoux>
you plan to buy the full version
<bvernoux>
?
<azonenberg_work>
I thought that a 30um conductor with 125um gap would not have major thickness variations
<bvernoux>
yes me too I was not thinking it will change so much things
<azonenberg_work>
Yes... most likely the "gold" version which has a 2GB ram cap, allows 3 conductor layers, unlimited ports, unlimited ideal components, thick metal, dxf import/export, etc
<azonenberg_work>
MSRP is 12.5 kUSD however the sales guy said that's negotiable for small businesses
<bvernoux>
especially here we have something like 500MHz Signal about 2ns ...
<azonenberg_work>
bvernoux: well the actual signal has much higher freq components
<azonenberg_work>
it's a SGMII idle pattern on a 7 series GTP with a ~40ps rise time
<azonenberg_work>
the 1 GHz scope front end is rounding that off
<bvernoux>
ha ok so it is more 1GHz in fact
<azonenberg_work>
there is definitely frequency content out well past 1 GHz
<azonenberg_work>
I almost certainly cannot afford the "pro" version of sonnet which is $CALL_FOR_PRICE
<azonenberg_work>
"level 3 gold" is in the same price range as my new scope
<bvernoux>
maybe Sonnet guys can make a huge effort to support open source things ;)
<azonenberg_work>
And if i can get them to take a bit off for me being a one-man shop it will be within the plausible range
<azonenberg_work>
Lol
<bvernoux>
with a special price
<azonenberg_work>
We'll see, i'm going to be getting a 30-day demo of pro in august-ish when i'm moved into the house
<azonenberg_work>
so that will give me a chance to run some much more detailed analysis
<azonenberg_work>
The other option i have available, should it be necessary, is to buy a 30-day timeboxed version of pro (the normal license is perpetual for that version)
<bvernoux>
yes maybe you could wait a bit I do some measurements
<bvernoux>
to start the 30days trial ;)
<bvernoux>
as we will have more details on what happen
<azonenberg_work>
Which is 1.5 kUSD
<azonenberg_work>
the way i see it, if i use it for more than a few months then i'm better off buying a perpetual license of gold unless i actually need the pro features
<bvernoux>
yes
<azonenberg_work>
Any edition below gold is pretty useless to me with the tiny ram caps
<azonenberg_work>
And nothing would stop me from buying a short term pro license on top of my copy of gold if i needed the extra features for a short time
<bvernoux>
I think you plan to simulate lot of other RF things too the hard things it to group all in the 30days window ...
<azonenberg_work>
Well yeah
<azonenberg_work>
as much as i would love to use OpenEMS for all this work
<azonenberg_work>
it's just not mature enoguh
<azonenberg_work>
the UI isnt there
<bvernoux>
yes OpenEMS is too much coding ...
<azonenberg_work>
(literally, the UI is nonexistent - you have to write python to do anything)
<azonenberg_work>
maybe in 5-10 years it will
<azonenberg_work>
but right now it simply isnt an option
<bvernoux>
yes and it is prone to lot of trial and errors ...
<azonenberg_work>
Anyway, so my current plan is to wait 2-3 weeks until the latest probe iteration comes back from fab
<azonenberg_work>
Test that
<bvernoux>
I started checking what we can do but like you when I have seen the python script ...
<azonenberg_work>
then probably send you one of those boards for VNA analysis
<azonenberg_work>
along with the other pile of stuff i have queued up for you
<bvernoux>
anyway congrats for the latest update and very good results
<azonenberg_work>
I keep thinking i'm ready for you to do the measurements
<azonenberg_work>
then the board comes back and i find problems :p
<bvernoux>
yes no problem I'm not limited by time to do measurements and even try things on the boards to see if that improve things
<azonenberg_work>
I can't believe i just sent out my fifth respin of a board that is literally a resistor and a coax connector
<azonenberg_work>
lol
<bvernoux>
yes but just res & coax change lot of things ;)
<bvernoux>
I was clearly not thinking it will have such big impact especially on freq < 1GHz ...
<bvernoux>
and it is very good to have an excellent probe for > 1GHz
<bvernoux>
do you have checked the probe tip sold by Pico ?
<bvernoux>
they are quite cheap in fact I will buy some to check if that change anything