Feasibility (or existence) of 10:1 scope probe PCBA for direct solder connect? - Page 1
Forum section: electronics / projects

author: globoy
author_rank: Frequent Contributor
timestamp: January 15, 2025, 04:10:35 pm
content: This is a how hard would it be or does such a thing already exist question.I envision a small PCB with two solder connections and a flexible, light-weight, coax cable back to the scope acting as a 10:1 probe that I can affix to the DUT using double-sided tape and very short soldered wire connections to the signal and a close-in ground to minimize the ground loop.It seems this should be as simple as a 9M resistor and trimmer. For frequencies up to 200 MHz or so how much care would need to be taken with PCB layout and/or PCB impedance control (would be nice to design and order some from one of the cheap PCB houses).Thinking about it just now as I write this, I guess I could solder wires to an existing scope probe but still that form-factor isn't always ideal. At least my Agilent probe coax cables are so stiff that they would make affixing the probe difficult.I know that small sockets exist for existing probes but as I mentioned often the probe sticking out of them is problematic. For some use cases I think it would be nicer to solder wires to the DUT.

author: David Hess
author_rank: Super Contributor
timestamp: January 15, 2025, 05:08:06 pm
content: I sometimes use a probe-to-BNC adapter to connect to a short BNC pigtail soldered into the circuit.

author: tggzzz
author_rank: Super Contributor
timestamp: January 15, 2025, 05:09:56 pm
content: Quote from: globoy on January 15, 2025, 04:10:35 pmGood luck trying to make a passive 10Mohm RF probe. If you succeed, patent the technique.Apart from that the answer is "no", for many reasons outlined in the references inhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/especially the "fundamental theory:" one. Yes, that's old, but it is complete and the physics hasn't changedThat page, and the references therein give alternatives. See especially the Z0/resistive divider probes - which arehigherimpedance than your 10Mohm probe

author: Martinn
author_rank: Frequent Contributor
timestamp: January 15, 2025, 05:19:10 pm
content: Passive probes use a lossy coax cable, otherwise you'd see ringing all over. You can't just use RG174.I remember having read that some Tektronix engineer pulled out the center conductor of a coax cable and replaced it with a resistive wire, inventing the passive probe in the process.- Martin

author: inse
author_rank: Super Contributor
timestamp: January 15, 2025, 05:50:16 pm
content: What I am occasionally use to move the probe away from the PCB, is a short (as short as possible) piece of twisted pair with a … just have a look at the photo.It is a compromise and surely not for 200Mhz but works rather well in the lab.

author: langwadt
author_rank: Super Contributor
timestamp: January 15, 2025, 05:54:59 pm
content: Quote from: inse on January 15, 2025, 05:50:16 pmif you circuit can handle the load this can work to very high frequencyhttp://jahonen.kapsi.fi/Electronics/DIY%201k%20probe/

author: Martinn
author_rank: Frequent Contributor
timestamp: January 15, 2025, 06:58:45 pm
content: Quote from: langwadt on January 15, 2025, 05:54:59 pmZ0 probe, I love those. Buy a RG174 BNC cable (rare), cut it in half and have two high performance probes. Particularly useful for measuring power supply ripple (if you can live with the 21:1 attenuation).You can build many of those and solder them right into your circuit.

author: tggzzz
author_rank: Super Contributor
timestamp: January 15, 2025, 07:30:01 pm
content: Quote from: langwadt on January 15, 2025, 05:54:59 pmThe load is 10*betterwith a Z0 probe than a 10Mohm at VHF and above. You need a 50ohm input, not 50ohm//15pF, or use an inline 3dB or 6dB pad.FFI, see the references in the link I gave.