<azonenberg>
miek: ooh, looks like a good starting point
<azonenberg>
although if i were to build one, i'd be designing it for multi GHz bandwidth
<azonenberg>
i'd want something competitive with the lecroy/tek options
<miek>
yeah, i really like how it walks through all the design decisions and reasoning. i figure it shouldn't be too hard to adapt for higher BW and it might already be better (the author didn't have the kit to test too high)
<azonenberg>
Yeah
<azonenberg>
i would also redesign the power stage for our usb-pd interface
<azonenberg>
those AAs are massive
<azonenberg>
the board would probably be 1/4 the size
<azonenberg>
and have SMA input/output rather than BNC
<azonenberg>
Can you file a ticket on the starshipraider repo for a power rail probe design?
<azonenberg>
link to that, the TSP teardown of the tek one, the lecroy rp4030, and any other resources you can find
<azonenberg>
Hopefully i can find some time this weekend to look at the current status of the PD board and figure out what needs to be done
<azonenberg>
as of now i have one assembled board @david.lenfesty sent me and one bare board that needs to be populated
<azonenberg>
the parts are likely to be hard to find though
<azonenberg>
(yay semiconductor shortage)
<_whitenotifier-5>
[starshipraider] miek opened issue #6: Design a power rail probe - https://git.io/JOBsh
<azonenberg>
noopwafel: ping
<_whitenotifier-5>
[scopehal] azonenberg pushed 1 commit to master [+0/-0/±2] https://git.io/JOBrJ
<agg>
might try making a quick version of that too, though probably replace the AAs with a regular USB input or something off the scope front panel
<agg>
wonder if there's any mileage to be had in amplifying the AC path to overcome scope frontend noise?
<agg>
I guess just being 50r output would help a bunch.
<azonenberg>
Yeah
<agg>
would be nice to have my scope control the offset and so forth but seems like a lot of work to just mechanically interface, let alone the i2c or whatever protocol
<agg>
hurry up and release your entire ecosystem please azonenberg :P
<azonenberg>
lol
<azonenberg>
hurry up and get me some more minions then
<azonenberg>
and make this chip shortage go away
<miek>
i may be digging through the firmware on my scope to see if i could do offset control from it right now :)
<agg>
urgh this chip shortage is extremely boring
<agg>
really "enjoying" trying to predict what chips we might want for the next 12m, finding replacements/new suppliers for most of them already out of stock, then sitting like a dragon on a stupid hoard of some particular dcdc converter
<agg>
miek: oh, cool, which scope?
<agg>
from tsp's teardown of the keysight one it really doesn't look like it should be all that complex, but figuring it out without a probe to RE seems much trickier, I guess the scope fw is a good starting point
<miek>
agg: an agilent mso6034a
<agg>
and if I had an n7020a I wouldn't need to make one, heh
<azonenberg>
agg: yeah i just scored all of the HMCAD1520s I'd need to prototype a single ZENNECK channel
<azonenberg>
i have a couple of artix7s left over from another project
<azonenberg>
But the rest of the frontend is going to need more work still
<miek>
mine's a bit too old to support the n7020a though, so i don't know if i can persuade it to control an offset like that
<_whitenotifier-5>
[scopehal] azonenberg pushed 1 commit to master [+0/-0/±2] https://git.io/JOBDI
<_whitenotifier-5>
[scopehal] azonenberg 0a5b905 - eSPI: Added decode for channel 2 capabilities register. See #407.
<Degi>
Ah yes, though in that case the linked design probably doesnt work at all
<azonenberg>
and *especially* for anything within the loop bandwidth of the power supply, you want fairly high loading
<Degi>
I mean it has 100 nF and 10 ohm to 50 ohm scope...
<Degi>
Hm, for switching regulators you probably won't get problems with that unless you have like quite high / several nF capacitance... (though linear ones could be a problem)
<azonenberg>
Before we do anything else I want to do a VNA sweep of the RP4030 I ordered and see what shakes out
<Degi>
How come it has 1.2 x attenuation? I saw that mentioned on the other page too, is there a specific reason for that value?
<azonenberg>
That I'm not sure of
<Degi>
Hmm, I wonder how good the circuitcellar probe is
<azonenberg>
Good question. The guy who designed it couldn't test past 100 MHz
<azonenberg>
If somebody wants to build one off his design, i could run some VNA testing on it
<Degi>
I mean its similar to how I'd design it if I want a cheap probe which measures noise on a DC rail
<azonenberg>
although i dont have the time to pick out parts and order it etc
<Degi>
Hmm, maybe
<azonenberg>
if one shows up in my mailbox i'll measure and mail back
<Degi>
A bit of a problem seems to be compensation because there's going to be a dip or peak somewhere
<Degi>
(I once tried designing something similar in ltspice for an isolated probe)
<miek>
keysight's been a bit misleading, on their site they say 1:1 but it's actually 1.1:1 for the 2GHz probe, 1.3:1 for the 6GHz
<Degi>
(Where you split DC and AC paths)
<Degi>
huh
<azonenberg>
Yeah, i suspect we will need to do some de-embedding to get good results with it
<azonenberg>
this is what the real vendor probes do
<Degi>
Do they give you s2p data to deembed?
<azonenberg>
They apply unspecified correction filters to flatten response
<azonenberg>
it's done automagically
<Degi>
Ah, thats what the extra pins are for?
<azonenberg>
details of implementation are unknown AFAIK
<Degi>
Hm, how much dB of dip would be acceptable?
<azonenberg>
The probus interface provides +/- 12V power, a presence detect signal, ground, and I2C
<azonenberg>
in all active probes, the I2C gives access to a descriptor EEPROM that identifies the probe model, serial number, etc. Some of this eeprom format has been RE'd
<Degi>
I mean it could just read out a flash over I2C... (Well I don't have one to disassemble xD)
<Degi>
Neat
<azonenberg>
additionally, some probes also have other I2C functionality for things like gain or offset adjustment
<azonenberg>
e.g. the RP4030 has a... 30V, I think, offset range
<azonenberg>
i assume this is an i2c dac somewhere
<Degi>
Oh, that TSP video seems pretty near to the circuitcellar circuit
<miek>
the pricing on these is crazy to me. i get that it takes a bunch of design time, but for what's inside the n7020a $3.5k is absurd
<azonenberg>
Higher end WaveLink probes are individually calibrated and and some form of the frontend response is stored on the probe somehow
<Degi>
oof
<azonenberg>
Unsure if s-parameters or something simpler like FIR filter coefficients
<Degi>
Hmm, maybe s2p and scope converts to FIR coefficients depending on the frontend?
<azonenberg>
miek: and this is why i'm trying to design better probes lol
<miek>
:)
<Degi>
I could get like 3.5 rigols for that
<azonenberg>
degi: no, i think it's more likely they VNA the probe in the factory
<azonenberg>
then make a FIR filter with the inverse response
<Degi>
Hmm
<azonenberg>
and store the tap values on the probe eeprom
<Degi>
I mean that'd work too
<Degi>
Hmm
<azonenberg>
correction for scope frontend performance is done separately inside the scope
<Degi>
Maybe it can be reverse engineered by making a custom probe and then changing stuff in the EEPROM lol
<Degi>
Ah
<azonenberg>
There has been some RE of the probus eeproms already
<azonenberg>
I just havent had time to look at what's known
<Degi>
hm okay
<Degi>
How come they have 50 kOhm? To keep noise low?
<miek>
to avoid putting any significant load on the supply
<Degi>
I mean why not use like 50 GOhm?
<Degi>
(Or 1-10 MOhm like usual probes / multimeters)
<Degi>
(Also if you used higher resistances you could use higher voltages)
<Degi>
That's quite an FPGA board at 18:30
<Degi>
azonenberg, is 6 dB attenuation with 1 dB ripple good enough?
<Degi>
(from 5 to 7 dB)
<azonenberg>
Maybe? i mean i'd prefer flatter
<Degi>
Got to 5.7-6.8 dB
<Degi>
Kinda hard
<miek>
hm, i think the HRA marking on the opamps in the teardown points at the AD8065
<Degi>
5.94-6.21
<Degi>
But needs 470 nF cap which might be hard to get
<Degi>
(I guess you can combine film cap and SMD cap)
<miek>
i guess it's a good way to get a precision reference to the end of a probe while avoiding issues with noise or contact/lead resistance & keeps the expensive parts in the scope
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<azonenberg>
miek: well the RP4030 is a box that mates with a probus interface and has a SMA input
<azonenberg>
from what the drawings show
<azonenberg>
it looks like the N7020A is similar
<azonenberg>
now you know what's *really* interesting? the bandwidth of the various RP4030 accessories
<azonenberg>
So it looks like the probe itself has a SMA input but it comes with a SMA-MCX cable
<azonenberg>
They have short lengths of coax with MCX terminations on the end that are intended to be soldered directly to the DUT
<azonenberg>
that, and a MCX cable on the DUT, give full 4 GHz BW
<azonenberg>
the U.FL adapter only goes to 3
<azonenberg>
and - this is the interesting part
<azonenberg>
the handheld browser is only 350 MHz
<azonenberg>
what i find interesting is that the browser is actually a PP066 7.5 GHz transmission line probe
<azonenberg>
but with a straight-through wire instead of a resistor