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[starshipraider] azonenberg d03b1f2 - Added file missed in last commit
<azonenberg>
woop, just ordered the new AV1
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<darthrake>
irt AV1 on the old wireless router: did you do a comparsion to a near-field probe on a SA yet and see how much (if at all) the frequency moves?
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<nfbraun>
Hi!
<nfbraun>
I looked into 2X-Thru deembedding in the last days, but I don't really understand the assumptions it makes.
<nfbraun>
The IEEE 370 standard seems to actually state this assumption, but the implementation in scikit-rf gives unsymmetrical results, even with synthetic test data where the halves are symmetric.
<nfbraun>
I also don't understand why this assumption should be valid; it obviously is if the fixture consists merely of a transmission line, but if something happens at the launch from the connector into the TL, I would think it becomes unsymmetric.
<azonenberg>
nfbraun: So, the assumption is that you have three identical connector+TL networks
<azonenberg>
the first two are back to back, forming a 2x thru
<azonenberg>
the third goes to/from your DUT
<azonenberg>
assuming they are equal, given a measurement of the 2x thru you can calculate the response of just one half
<azonenberg>
then de-embed that from your DUT
<azonenberg>
So it's OK if there is mismatch at the launch from connector to TL as long as the mismatch is consistent and reproducible across all three copies of the circuit
<azonenberg>
Error will, of course, be introduced if the three copies are not identical
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<HexaCube>
ooooooh, well who knew this place existed!
<azonenberg>
o/ HexaCube
<azonenberg>
so anyway, re your question... there's a 100 ohm Vishay HML01 tip resistor which is on the order of $9
<azonenberg>
a bunch of jellybean passives like 1% 0402 resistors and 0.47 uF 0402 caps, let's call that no more than a dollar combined
<azonenberg>
the high freq trim cap is $9 @ qty 1 on digikey (assuming you're asking about low volume BOM not production qty)
<azonenberg>
output filter is $6.73
<HexaCube>
oh dang, $9 for a resistor alone, impressive :D
<azonenberg>
the SMPM connector is $9.38
<HexaCube>
what makes it so expensive if I may ask?
<azonenberg>
HexaCube: more annoying is the 100 unit MOQ
<azonenberg>
it's just an ultra obscure part thats made in low volume
<azonenberg>
you are basically paying them to tool up the line and put molds on machines etc just for you
<azonenberg>
so it's not actually $900ish per min order = $9 each, it's probably more like $2 each + $700 tooling
<azonenberg>
lol
<HexaCube>
hrmmm
<azonenberg>
(But i haven't inquired about higher qty pricing yet)
<azonenberg>
internally its an 0402 or MELF resistor with leads soldered/welded to it and then overmolded in plastic
<HexaCube>
what's the reason for picking that specific one then? low capacitance/inductance, or the fact it's a special size etc?
<azonenberg>
It's an axial lead resistor that's very tiny
<azonenberg>
not much competition in that space (0.9mm square or so iirc)
<HexaCube>
hmm, I see. Right I seem to recall you are using those to directly be soldered to what you're measuring? Or was that something else?
<azonenberg>
Yes
<azonenberg>
that is indeed the use case. this is how most big name solder-in diff probes work
<azonenberg>
my ground lead is extra lead wire from the resistors (they come with like 30mm of wire on them which is gross overkill, i trim to 7.5mm overall length and use a 7.5mm length of the extra wire as a ground)
<azonenberg>
anyway, moving down the bom, the 20k trimmer is $3.17
<azonenberg>
LM27761 is $1.68
<azonenberg>
the ADP7142 is $3.40
<azonenberg>
the OPA210 is $3.24
<azonenberg>
and the BUF802 is $5.06
<azonenberg>
so all told you're looking at on the order of about $52 in components per probe head in low volume
<HexaCube>
Reason I asked is because I keep having this idea of taking the basic idea of the PCBite probes and improving upon them heh
<azonenberg>
dropping to probably high 30s or low 40s in higher qty
<azonenberg>
Add to that the PCB (next to nothing, under a dollar each at oshpark) for probe head and mounting foot, a 55mm piece of 20 gauge copper wire
<azonenberg>
(to connect the head to the foot)
<azonenberg>
and the SMA-SMPM cable which is close to $100 in low volume but drops to $60ish in quantity
<HexaCube>
in which case your design is drastically exceeding anything I'd want, but I figure I could start off your design and cheapen down from there considering even 1/10th of the bandwidth is still plenty :D
<HexaCube>
interesting. Then again, at work I pay like 15 EUR for a plain M12 sensor cable hah
<azonenberg>
These are not cheap cables. at this price they come with serialized VNA measurements
<azonenberg>
(you can cut $5-10 off the price without that)
<azonenberg>
for a solder in probe, a highly flexible cable is crucial to avoid putting forces on the solder joints
<azonenberg>
but it also has to not be excessively lossy
<azonenberg>
micro coax tends to have high loss
<azonenberg>
KF047 is the best tradeoff i've found for this to date
<HexaCube>
Right, that's where I kinda wanna see if the PCBites can be improved
<azonenberg>
Anyway, so you could definitely cut a bit of cost by swapping the SMPM connector out for MMCX. MMCX is only good to 6 GHz which is fine for this probe
<HexaCube>
As they're generally just "dropped" onto pads, but I wonder if one could add a nice, compact way to 'preload' aswell
<azonenberg>
but i'm using SMPM on all of my designs for parts commonality
<azonenberg>
since the AKL-AD4 and AKL-PT5 are good to 7 and 8.5 GHz roughly
<azonenberg>
MMCX is iirc what the pcbite probes use
<nfbraun>
azonenberg: "assuming they are equal, given a measurement of the 2x thru you can calculate the response of just one half": But for that, you have to assume that each half of the 2x thru is symmetrical?
<azonenberg>
nfbraun: symmetrical meaning S21 == S12?
<HexaCube>
azonenberg: I suppose I wouldn't need the 1.8GHz lowpass either
<azonenberg>
The term I usually see used for that is reciprocal
<azonenberg>
HexaCube: sooo
<azonenberg>
that LPF is actually for EMC reasons
<nfbraun>
S21 == S12 and S11 == S22
<nfbraun>
The latter bothers me.
<azonenberg>
nfbraun: yeah i dont think that assumption is valid
<azonenberg>
most 2x thru algorithms i've seen use time gating on the total S11/S22 response to figure out the portion coming from one half
<azonenberg>
HexaCube: basically, the tip is pretty good at picking up 2.4 GHz and the amplifier is high impedance so it doesnt take much
<HexaCube>
azonenberg: ahhh, you're saying otherwise you're turning the probe/cable into a 2.4GHz radiator? :D
<azonenberg>
No, a receiver
<azonenberg>
the -3 dB BW of the probe is less than that anyway, but strong wifi can drown out or severely interfere with weak signals being measured
<azonenberg>
by explicitly filtering out the 2.4 GHz ISM band, you get much lower noise floor when near a wifi AP
<HexaCube>
right, but it shouldn't matter if the 'design goal BW' is way lower, no?
<HexaCube>
or am I missing something
<azonenberg>
well it depends on how high BW the scope you use it with is
<azonenberg>
if you have a 1 GHz probe with a 4 GHz scope that still passes 2.4 GHz you'll see it
<HexaCube>
right, that's what I meant
<azonenberg>
if you only ever use it with a cheap scope you can get away with more but the filter is cheap, i dont see it being worth cutting corners there
<azonenberg>
nfbraun: i think S11 / S22 *magnitude* have to match
<HexaCube>
I mean you said $6, that's 10% of the BOM cost (of the probe itself :P) hehe
<azonenberg>
but phase does not
<azonenberg>
HexaCube: yeah you could probably get away with a simple like first order r-c LPF or something
<azonenberg>
made out of a pair of 0402 passives
<HexaCube>
right that was what I was gonna ask next
<azonenberg>
i needed a sharp cutoff to get 1.8 GHz to pass and 2.4 to be notched out
<HexaCube>
I wonder if I could convince some prof at university to let me use whatever gear required to characterize a selfmade probe
* HexaCube
scratches head
<azonenberg>
if you're targeting like 300 MHz BW it would be a lot easier
<azonenberg>
anyway, so next up
<azonenberg>
my probe design as it stands uses the HML tip resistor to provide damping, there's quite a bit more L-C ringing without it
<azonenberg>
whether that is enough to matter at your freq of interest is hard to say
<azonenberg>
The biggest challenge though is grounding
<azonenberg>
you can put a pogo or whatever at the tip
<azonenberg>
but then you have to ground it somehow
<azonenberg>
that's where the pcbite stock probes fail
<azonenberg>
And it's a problem i have not yet solved
<HexaCube>
azonenberg: right, if I remember the slides of your probe-presentation correctly (and the theory behind it) - basically the closer that resistor is to the pad/point of measure, the better yeah?
<azonenberg>
Correct. that's why it's in the lead rather than on the PCB
<azonenberg>
But in the case of probes with capacitive inputs it also helps damp out ringing
<azonenberg>
anyway so one idea i had was to have some sort of compass-like structure like fancy diff probes have
<azonenberg>
where you have signal and ground points with adjustable spacing
<azonenberg>
i experimented a bit but never reached something i was happy with during testing
<azonenberg>
i eventually want to make an active voltage probe, a transmission line probe, and an active differential probe in pcbite form factor
<azonenberg>
i have an early prototype transmission line probe that was basically just a few resistors, a SMA, and a pogo
<azonenberg>
it fit a pcbite mounting arm, cost like $15 in parts to make, and was extremely flat to 3 GHz then had some peaking you could filter off
<azonenberg>
but the ergonomics were awful, it just had a copper wire dangling off the back that you had to bend to touch the pcb just right
<azonenberg>
if you didnt align it perfectly it wouldnt work
<nfbraun>
Ok. I saw the time gating thing as well, but that is at best an approximation, right?
<azonenberg>
nfbraun: I mean all de-embedding is approximations. the reference and actual trace will never be exactly equal in geometry etc
<azonenberg>
that said, the math behind it is where i get lost
<azonenberg>
it took me quite a bit of study of Dunsmore to even grok basic de-embedding given S-params of the other network
<nfbraun>
Of course. What I mean is that it will not give perfect results even with synthetic data.
<azonenberg>
HexaCube: anyway, so i am experimenting with a bunch of ways to get similar tip mounted damping without using the solder-in resistors
<azonenberg>
for a handheld or positioner based browser probe
<azonenberg>
i have three main tracks i'm pursuing at this time
<azonenberg>
rod-shaped RF resistors soldered into a cut probe needle
<azonenberg>
a flip chip 0402 soldered into a cut probe needle
<HexaCube>
azonenberg: what would one actually need in terms of gear in order to characterize a probe? A network analyzer and I guess a fast-edge generator?
<azonenberg>
and a full custom tip made of machined ceramic then metallized with nichrome and copper
<nfbraun>
Or at least that's what I think I found from my testing of the implemention in scikit-rf.
<azonenberg>
experimental ceramic tip, not metallized (yet)
<HexaCube>
wow, now THAT is fancy hah
<HexaCube>
did you machine that yourself?
<azonenberg>
A friend did, she had better luck than me. my first 3 attempts all snapped :p
<azonenberg>
you can see its not perfect, there's some roughness in the surface finish and then the point is not quite centered
<azonenberg>
because the dremel we used as a "lathe" has nontrivial runout in the spindle at these scales
<HexaCube>
so I seem to remember something about carbon tips
<azonenberg>
it's very difficult to machine a material that is harder than tungsten carbide and super brittle :p
<HexaCube>
was that also part of your slides or am I getting something mixed up
<azonenberg>
you basically have to use diamond tooling
<azonenberg>
There is. LeCroy has a patent on probe tips made of composite materials consisting of resistive fibers or particles in an insulating binder/matrix
<azonenberg>
the actual probe they ship uses pultruded carbon fiber impregnated with epoxy to form a 100 ohm resistor that is also a structural element of the tip
<electronic_eel>
is the ceramic tip sturdy enough not to snap easily when used for probing?
<HexaCube>
yeah I was wondering if you could sinter something like that
<azonenberg>
The patent expires in 2025 but one way i'm considering sidestepping it in the near term is this, using a fully insulating tip with thin film resistor deposited on the surface
<azonenberg>
at that point the probe is not made "substantially of" resistive material
<azonenberg>
nor is it fibers or particles in a matrix
<azonenberg>
so i'm clear on that patent
<azonenberg>
it would also eliminate solder joints and other potential weak points
<azonenberg>
as the tip is one monolithic piece
<azonenberg>
electronic_eel: Good question
<azonenberg>
i dont know yet
<azonenberg>
this is the only one that's survived machining and it's shaped wrong to fit in a socket
<azonenberg>
we're exploring better tooling to get more precise geometry and will then do some actual tests on un-plated ceramic blanks
<HexaCube>
I was also gonna ask, what's stopping you from in theory directly soldering a teeny tiny resistor to a pogo-pin
<azonenberg>
That is the other option i'm pursuing
<azonenberg>
i'm exploring two different routes here, one is a rectangular flip-chip RF resistor (Vishay CH0402)
<azonenberg>
the other is cylindrical rod resistors
<azonenberg>
the flip chip resistors do not have metallized end caps, only that top rectangle of metal is plated
<azonenberg>
so they're super weak and need glue or something to support them
<azonenberg>
the rod resistors in theory would be stronger
<azonenberg>
but i'm not a fan
<HexaCube>
(also followup question - couldn't you put the 100R directly on the board, at least with the boards you#re making yourself?
<azonenberg>
one vendor hasnt even got back to me yet
<azonenberg>
the other told me they could send me two 50 ohm samples
<azonenberg>
if i wanted higher values i'd have to order 250 of them at like $28 each
<azonenberg>
they do not have s-parameters, mechanical strength data, or any other design ifnormation
<HexaCube>
heh, fun
<azonenberg>
i would have to buy the whole production lot and hope it works
<azonenberg>
you see why i'm starting to explore a DIY thin film route :p
<azonenberg>
at least for R&D
<electronic_eel>
that they don't have any proper data on their resistors, but then want $28 each with such a MOQ is quite strange
<azonenberg>
electronic_eel: yeah
<HexaCube>
yeah, understsandable azonenberg
<azonenberg>
i asked specifcially for data on how strong the end caps were in shear
<azonenberg>
and the peel strength of the plating to the ceramic body
<azonenberg>
they just said "use data for alumina"
<azonenberg>
idgaf about compressive strength of the resistor body
<azonenberg>
thats not where it's going to fail
<azonenberg>
i want to know about the plating to ceramic bond :p
<azonenberg>
anyway this is active research with multiple paths being pursued in parallel
<azonenberg>
i dont yet know which way i'm gonna go
<azonenberg>
For lower BW you will likely be fine with a short pogo or rigid tip
<azonenberg>
this kind of stuff matters more as you get in the 5+ GHz range
<azonenberg>
Keep in mind i'm trying to push limits on probing tech here. my passive probes are already competitive with the best ones on the market in terms of bandwidth etc
<azonenberg>
and i'm aiming to go even higher
<azonenberg>
So you can get away with less crazy designs if you're not aiming to make the highest bandwidth passive probe in the world lol
<electronic_eel>
azonenberg: be careful, you might end up building your own probe tip manufacturing plant in your backyard ;)
<HexaCube>
thanks for going into so much detail azonenberg =) I appreciate it
<azonenberg>
but it only had collets, no chuck, and we didnt have one of the right size for our stock
<electronic_eel>
does the company that made your precision pcb rework mill also make lathes?
<HexaCube>
you can buy these shanks that come with collet holders, should just need some bearings and a way to attach a motor via some belt :D
<azonenberg>
electronic_eel: Yes. but i'd feel bad putting a sherline through this kind of abuse
<azonenberg>
water full of highly abrasive ceramic dust getting everywhere
<azonenberg>
i'd rather have a purpose built tool we dont mind abusing and can easily repair/replace parts on
<HexaCube>
and with the right bearings / motor you could also get to the really high RPMs needed for "proper" machining of that size
<azonenberg>
yes thats the other point against a sherline
<azonenberg>
they max out at like 3000 RPM
<azonenberg>
if we were gonna use a premade tool it would be like a watchmaker or jeweler's lathe
<HexaCube>
could probably even use a nice (oversized :P) BLDC, just gotta find a belt that can take the centripetal forces hah
<HexaCube>
for a janky-but-still-decent setup, I mean
<azonenberg>
Yeah. i mean the forces involved are extremely low
<azonenberg>
you'd be taking off microns per pass
<HexaCube>
exactly
<azonenberg>
(also ping monochroma in case you want to chime in)
<azonenberg>
not sure if she's awake but we'll see :p
<HexaCube>
or maybe you can find a used PCB-spindle on ebay/craigslist
<HexaCube>
wonder if there's actually a market for "pcbite, but more bang for the buck" geared towards... sophisticated hobbyists
<HexaCube>
via tindie or something :D
* HexaCube
still needs to find a way to earn money whilst studying
<electronic_eel>
i once played with the idea of buying a pcb drill/mill spindle. but they usually need specialized vfds and a ton of other custom stuff, like air pressure for the toolchanger, oil, cooling water and so on. so getting it up running would have been quite an involved setup. so i decided against it
<azonenberg>
electronic_eel: this was something along the lines of the circuitmedic microdrill
<azonenberg>
it looks like either the same thing or a clone of it
<azonenberg>
this is a brushed dc motor and an extremely good spindle
<HexaCube>
because it turns out designing PCBs for private individuals SUCKS
<azonenberg>
i ran mine up to like 45K RPM under a microscope with a steel blank in the chuck
<azonenberg>
it looked like it was standing still
<azonenberg>
i couldnt see any runout at all
<azonenberg>
no idea what kind of bearings they use but they're excellent
<azonenberg>
So a tool like that would do great for this
<azonenberg>
HexaCube: as far as a market, good question
<azonenberg>
sensepeek is very much aiming at the lower end according to my conversations with them
<HexaCube>
Because I generally think the PCBite idea is great
<azonenberg>
My personal plans for these probes is to keep the designs open source and sell assembled units at digikey once i'm satisfied with them
<azonenberg>
yes i love the positioner. i just think they need better probes in the same form factor
<HexaCube>
I just don't think the average joe wants to drop 200 bucks on just 4 100MHz probes
<azonenberg>
i actually had a nice conversation with iirc their CEO and one of their engineers
<azonenberg>
around christmas 2019 iirc
<electronic_eel>
what i was writing about was getting a used spindle off ebay. from some pcb fab that closed down here in germany. it was geared for industrial production, so in this environment all the custom stuff was ok
<azonenberg>
or was it 2018
<azonenberg>
They sent me some of the special solderable mounting nuts to play with
<azonenberg>
i came up with a prototype and emailed them asking to have a meeting to chat about my work to date and how we could improve it
<azonenberg>
then they ghosted me
<HexaCube>
Ouch :/
<azonenberg>
i keep meaning to reach out and see if maybe it just got caught in spam or something
<azonenberg>
but i've had my hands full with so much other stuff i didnt want to add to my workload
<HexaCube>
understandably so
<azonenberg>
HexaCube: anyway, so my target here would be more of very high end hobbyists / independent consultants / small businesses
<azonenberg>
people who might have scored a GHz scope on ebay
<azonenberg>
but can't afford a bunch of active probes to use with it
<HexaCube>
wait wait am I seeing it right that the only grounding option of those PCBites probes is that grabby-arm plus a dupont wire?
<azonenberg>
Yes :D
<azonenberg>
lol
<HexaCube>
O_o
<HexaCube>
that's a mighty weird decision
<azonenberg>
there is a bare copper pad on the back of the PCB where you can solder your own ground on
<azonenberg>
but they provide no stock accessories to go there
<azonenberg>
these are targeted at very low end hobbyists
<HexaCube>
azonenberg: at which point I find 40-50 EUR still kinda expensive - then again I keep underestimating how much it costs to make stuff like that
<azonenberg>
Yeah i think the price is very reasonable
<azonenberg>
anyway so for my own probes my very rough model for pricing is, look at a commercial product in the same performance class
<azonenberg>
chop a digit off the end
<HexaCube>
reasonable - sure; But 'low end hobbyists' don't wanna pay reasonable prices hah
<azonenberg>
and i want to come in at or below that
<HexaCube>
guess I should just see what I can come up with (at what cost) after I'm finally done with this damn LED-Matrix project <.<
<HexaCube>
after a freaking YEAR of having to redesign the damn board because chips go in- and out of stock
<azonenberg>
so for example, the ZS1500 from LeCroy is 1.5 GHz BW (i measure it as more like 1.2), 1M ohm || 900 fF, 10:1, only works with lecroy scopes, $2500ish MSRP
<azonenberg>
the AKL-AV1 is 1.8 GHz BW, 5M ohms || 400 fF, 10:1, works with any scope
<azonenberg>
and i figure $250 is a reasonable MSRP for it :p
<azonenberg>
given BOM of $50ish for the probe head, $60ish for the cable
<azonenberg>
then PCB costs, import tariffs on some of the components, shipping, labor for assembly, cost of packaging, fees charged by the retailer, etc
<azonenberg>
somewhere in the $250 range should be enough to make the project sustainable (i.e. not losing money) and bringing in just a little bit of profit per sale that i can sink into R&D for the next probe
<azonenberg>
Which is expensive for a hobbyist, sure
<azonenberg>
but dirt cheap compared to a new name-brand active probe
<HexaCube>
aye, fully agreed
<HexaCube>
and sorry for asking again I think the question kinda got swamped in chat - but if I were to make my own probe, how would I go to characterize it / gain any info about performance
<HexaCube>
I assume at the very least a Network Analyzer and a Fast Edge generator?
<HexaCube>
to measure flatness and er... what would you cause the other one, ringing?
<HexaCube>
(and well a scope to actually do measure the edge? :D)
<azonenberg>
So I generally do three different characterization procedures on my scopes
<azonenberg>
VNA of the probe across a 50 ohm line (port 1 = probe tip or as close as i can get on the fixture, 50 ohm termination from that line to ground, port 2 = probe output or as close as i can get to it with adapters)
<azonenberg>
fast edge step response
<azonenberg>
and eye pattern of serial data
<azonenberg>
on my probes*
<electronic_eel>
HexaCube: i think you'd need a probe fixture to go with your vna
<electronic_eel>
and the fast edge generator is only helpful if you also have a scope with enough bandwidth
<azonenberg>
Yeah. Fixturing is a challenge
<azonenberg>
I currently use a LeCroy PCF200 but there are losses in the fixture that i cannot de-embed
<azonenberg>
i want to get a fixture that i can more fully characterize with a 2x thru or similar
<HexaCube>
I'll just send them to you azonenberg harharhar
<HexaCube>
:P
<azonenberg>
lol i mean that is an option
<HexaCube>
not for a poor student like me :P But I bet I could convince an EE-Lab Prof or whatever to give me a few hours if I ask really nicely
<HexaCube>
they should have all the nice gear I'd hope
<electronic_eel>
the fast edge generator is quite cheap, you can get them from leo bodnar or even make them yourself
<azonenberg>
I generally offer steeply discounted or pro bono lab services to noncommercial/open hardware projects
<azonenberg>
if you're not making money on the project i'll do the lab work at no cost unless it's a major time commitment or needs expensive consumables
<azonenberg>
all i ask is you reimburse postage if you want the prototype back
<azonenberg>
otherwise i'll e-waste it here at no charge
<HexaCube>
azonenberg: right but shipping alone to the US is around 50 bucks or so I'd wager. Though might be way cheaper if fit into a small envelope
<electronic_eel>
the high bw scope to read the results with ... not so cheap or easily made
<HexaCube>
a very generous offer azonenberg, thanks and I might consider it if I actually do end up building the thing :D
<azonenberg>
HexaCube: you could also order parts to my place and ship me a bare board
<azonenberg>
or something likethat
<azonenberg>
if its not too complex i could assemble here
<azonenberg>
anyway, basically whenever the lab is otherwise idle i'll gladly make my resources available to the community
<HexaCube>
or even more interestingly, do both - measure it myself AND have you measure it, so I can see how close I can get to the proper measurement
<azonenberg>
such stuff is done on a best-effort basis and slotted in around billable stuff for work and my more important stuff
<HexaCube>
oh, I do have one last question, just out of curiosity - on your AKL-AV1, you're using a charge-pump for the negative voltage, and then two linear v-regs right?
<HexaCube>
actually nevermind I just answered my followup question myself I think :D
<HexaCube>
was gonna ask 'could one use a virtual ground too' but I suppose that won't work out due to the scope's GND reference eh
<HexaCube>
anways, thanks again for all the info and help =)