Feasability DIY passive HiZ osci probe on PCB - Page 1
Forum section: electronics / projects

author: DerDaniel
author_rank: Contributor
timestamp: August 13, 2025, 09:24:43 am
content: Hello analogue/oscilloscope experts,The boring why:I have a circuit that I have to move quite a bit while measuring; hence, I can't use traditional osci probes. For low impedance sources I'm going to use transmission line probes. However I have a few HiZ sources that need measurement as well and the transmission line probes would overload/distort them. Luckily the signal timing is more important than the fidelity; hence, a little over/under compensation isn't too big of an issue.Thus my question for the analogue/oscilloscope experts:Is building a halfway decent HiZ passive probe on a PCB feasible?The circuit I'm thinking about is in the following. The connection between PCB and oscilloscope is a 30cm BNC to U.Fl cable.For compensating / figuring out the capacitor values for the cables, I would make a separate circuit on the PCB with a trimmer capacitor, connect it to the oscilloscope's compensation output, trim the cap and then use this value for the fixed caps (hence two caps, to have a little more flexibility)...Alternatively, I could use a trim cap at every test point and have a solder jumper, where I compensate the probe first with the jumper open and then close the connection to the DUT. But with the cost of one of these trim caps, I would like to avoid that.

author: tszaboo
author_rank: Super Contributor
timestamp: August 13, 2025, 09:33:21 am
content: You can place the parts, which are on the oscilloscope probe on the PCB. That's not the issue.The issue will likely to be the coax cable. They are not just a 50Ohm transmission lines, not in the traditional sense. They are a lossy transmission line, something like 300 Ohm in total on the center conductor.If someone knows which coax cable to purchase to achieve this, chime in I'm also interested.I would try to place BNC testpoints on the signals. And use one of these BNC to scope probe adapters to connect a regular probe.

author: Andree Henkel
author_rank: Regular Contributor
timestamp: August 13, 2025, 09:48:04 am
content: I wouldn´t bother.If you can work with the 1/20 division factor use a laxmux probe:https://www.lasmux.com/product/single-ended-active-probes/?attribute_coax-cable-connector=SMAhttps://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg4973839/#msg4973839cable to scope is 50Ohm impedance and you can choose if you want BNC or SMA at scope sideFor longer distance just add 50Ohm assembled coax cable in series to scope.for fixing the probe you need to use supplied acessories, best the solder pins. and some improvisation like cable ties, velcro tape, gaffa...Of course you get some additional damping and slightly lower bandwidth the longer the cable, but that probably doesn´t matter for you.As Low Z like 1k or 500R seems not want you want, but I improvise them: in these case i Solder 1k on board in front of the U.FL and then use the u.fl adapter cable works fine, but the signal Integrity with the LAXMUX is much better (I was measuring square waves up to 150MHz).

author: DerDaniel
author_rank: Contributor
timestamp: August 13, 2025, 10:04:13 am
content: Thanks for the ideas so far, but as I said, traditional probes are out of the scope (pun not intended) as they would fall off the PCB when it is moved (or bend / snap off in the worst case).

author: Andree Henkel
author_rank: Regular Contributor
timestamp: August 13, 2025, 10:15:57 am
content: is your pcb that small or what is the problem that you don´t find a way to fix a probe?

author: Andree Henkel
author_rank: Regular Contributor
timestamp: August 13, 2025, 10:42:01 am
content: Well, thinking about it: beeing able to buy u.fl to BNC cable with lossy transmision line and length like 30cm would inded be a nice feat...on the other hand your distance is just 30cm and not 1..1,5m like with passive 1:10 probe so maybe it works well enough for you, with just 50Ohm cablingwhat maximum frequencies / minimum rise fall times are you trying to measure?try it, report your results here pleasebut if it fails you need plan B:testing with probe.It´s called design for testability...

author: DerDaniel
author_rank: Contributor
timestamp: August 13, 2025, 11:12:37 am
content: It's a size issue on the one hand, on the other, it's a inertia problem. Four probes including leads are a substantial inertia on a moving bed or gimbal system. It will at least change the response of the system and might even overload the servos...I'll try it anyway, and I'll let you definitely know of the results. I just would have liked to know in advance if it's a lost cause or if it's a skill issue when I don't succeed.

author: tszaboo
author_rank: Super Contributor
timestamp: August 13, 2025, 11:28:14 am
content: Quote from: DerDaniel on August 13, 2025, 11:12:37 amFair enough, it's is a setup issue. And there is only so much you can do about it.PMK sells MMCX probes. I would talk to them if it's the right way of connecting your setup.

author: Andree Henkel
author_rank: Regular Contributor
timestamp: August 13, 2025, 12:52:31 pm
content: DerDaniel: slightly nitpicking on your second schematic: point to connect ground of "Probe Comp" is missingbut this opens next potential problem: proper connection / wiring of of probe comp output to your setup

author: DerDaniel
author_rank: Contributor
timestamp: August 13, 2025, 01:37:00 pm
content: The schematic is just exemplary. I'm not planning to go all the way in regards to RF design, because the "probes" will have 3 to 4 components (including the U.Fl connector) and won't see much more than a few MHz, hence impedance matching etc. won't really be necessary.Regarding the connection to the comp output of the osci, I'll screw the PCB to the two taps of the output with matching pads.But this is just for sanity checks. In the end, I'll probably calibrate with a square wave from the wave gen, that goes into a 50 Ohm load. I'll have that circuit anyway (which will probably see some RF designing), because I want to see a Bode Plot of the "probe".

author: Cpuch22
author_rank: Contributor
timestamp: August 13, 2025, 04:07:11 pm
content: Can you buy a cheap oscilloscope probe chop off then end and solder the cable to your board with whatever compensation components you need.What frequencies are you investigating, and what magnitude of timing is important to your measurement?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 13, 2025, 08:08:57 pm
content: https://electronics.stackexchange.com/questions/491355/oscilloscope-probe-points

author: DerDaniel
author_rank: Contributor
timestamp: August 14, 2025, 09:05:54 am
content: Quote from: Cpuch22 on August 13, 2025, 04:07:11 pmThis might actually be the solution, if the U.Fl to BNC cables do not deliver! I can get a pair of new 100 MHz probes of ebay for 20 bucks (doubt that they really do 100 MHz, but ¯\_ (ツ)_/¯), so not much loss if it fails.The only question is, how I can get a U.Fl connector onto the cable. The thickest cable supported by a crimp connector i could find was around 2 mm, which is a long way from the roughly 4 mm diameter of the usual scope probes.The signal with the highest frequency component is a voltage step with a fall time of around 1us. The timing of this signal is more important than the fidelity. Hence, I should be fine with 10 MHz.

author: tszaboo
author_rank: Super Contributor
timestamp: August 14, 2025, 10:14:50 am
content: Quote from: Cpuch22 on August 13, 2025, 04:07:11 pmThat's a good idea. Some of these 100MHz Chinese probes are like 20 EUR.I wonder if they use the right type of coax. If you measure it let us know.In fact, I can check one of these types for you this weekend.