Building Solderable In-Circuit Oscilloscope Probes

RE: Building Solderable In-Circuit Oscilloscope Probes

jc2048 — Wed, 10 Feb 2021 22:16:11 GMT

This is interesting. I've seen the idea mentioned, but never tried it myself, so it's very useful to see your results.

Resistive probes are described in online and offline literature.

Howard Johnson and Martin Graham, in 'High-Speed Digital Design', simply go for a 1k resistor (ie 21:1) and use the oscilloscope to turn that back into sensible values [presumably they had a fairly good oscilloscope to play with].

Henry Ott has an interesting variation on the theme in 'Electromagnetic Compatibility Engineering'. He suggests two of them, side-by-side, with 450R resistors (so 10:1), to make a differential probe for looking at differences between ground signals in separate parts of a plane (two 50R input channels, with subtraction for the differencing). He tacks the outer of the shields together every few inches [there's no connection for the screen at the probe end - the return currents just run around the end to form their own loop, if you see what I mean - if this isn't very clear, say, and I'll draw it - I don't want to reproduce his picture because it's still in print]. He also suggests using it with a 'high-frequency 180 degree combiner' to feed it to a single-ended spectrum analyser [that part looks very radio-engineerish to me, because he starts going on about insertion losses, and that kind of thing] which is probably more up your street than mine [as I don't even have a spectrum analyser].

One reason 20:1 might work well is to do with the kind of capacitance you get at the input of the oscilloscope. For my scope, it's 11.5pF. If the end-to-end capacitance of the probe resistor is of the order of 0.5pF, then that's in the right kind of area for a 20:1 capacitive divider to match the resistive one. If you wanted to experiment, simply trying 1206, 0805, and 0604, and seeing which worked best might be one approach, though the difference might be too slight to see. Just a thought.

RE: Building Solderable In-Circuit Oscilloscope Probes

hlipka — Wed, 10 Feb 2021 21:35:58 GMT

Cool idea, I need to try that. I would try to use some heat shrink around the SMD resistor instead of epoxy, this might give better stability with the wires (and can also double as color-coding).

RE: Building Solderable In-Circuit Oscilloscope Probes

DAB — Wed, 10 Feb 2021 20:07:00 GMT

Very good idea.

Well done.

DAB

RE: Building Solderable In-Circuit Oscilloscope Probes

14rhb — Wed, 10 Feb 2021 13:57:41 GMT

Hi Shabaz,

This was a great read and some nice clear photos, thank you. Oscilloscope probes are often overlooked IMO they are a very important aspect of getting a credible reading but we often just use what was provided or hanging up on the workshop wall. What I like about your approach is that a low-cost test jig/harness could be made for working on fine pitch ICs or they could be tacked onto the IC legs. That said my home 'scope only has two inputs and I've got three proper probe sets [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b30dc0bf-9058-4a1a-a8ec-56f5a29b89cf/contentimage_5F00_938.png:16:16]

I wasn't aware of the plastic material - Playdoh to me was a soft non-setting material that children used to create things from, a bit like bread mix after it was kneaded properly - it had a nice unique smell as well ! I looked online and found the following called Aquabeads. Is that the same product that you used?

RE: Building Solderable In-Circuit Oscilloscope Probes

navadeepganeshu — Wed, 10 Feb 2021 04:36:22 GMT

Cool idea! Very nice surgery [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b30dc0bf-9058-4a1a-a8ec-56f5a29b89cf/contentimage_5F00_28294.png:16:16]

RE: Building Solderable In-Circuit Oscilloscope Probes

jw0752 — Wed, 10 Feb 2021 03:53:06 GMT

Thanks for the great ideas and explanations. Shabaz strikes again!

John

RE: Building Solderable In-Circuit Oscilloscope Probes

genebren — Wed, 10 Feb 2021 02:11:01 GMT

Great work! The traces look great, especially the resistive and active probes. This is something that I will need to remember.

RE: Building Solderable In-Circuit Oscilloscope Probes

baldengineer — Wed, 10 Feb 2021 01:39:08 GMT

Excellent work! I've been thinking about how to solder passive probes into a circuit for while. (PMK makes an accessory for it, but almost no scope manufacturer that uses their probes stocks or sells it.)

So, for circuits where 1 kOhm loading is acceptable, this looks like an excellent DIY option! Two questions.

1. On the construction, why the 'polydoh' blobs and not just larger hot glue blobs?

2. When doing the comparison, why did you use 1 Mohm with the coax and not 50 ohm?

3. Okay, I lied, one more question. If someone wanted to make these probes but didn't have your fancy ferrules tool, could you offer another suggestion to try?

I really like this idea, I'm very likely to try building someone in the near future.

(Okay, final question. If a certain someone made a video about building a DIY scope probe that looked almost exactly like this and gave you credit, would you be mad?)

RE: Building Solderable In-Circuit Oscilloscope Probes

kmikemoo — Wed, 10 Feb 2021 01:35:59 GMT

Brilliant and very cool! Thanks for sharing. [View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b30dc0bf-9058-4a1a-a8ec-56f5a29b89cf/contentimage_5F00_11380.png:16:16][View:/resized-image/__size/16x16/__key/commentfiles/f7d226abd59f475c9d224a79e3f0ec07-b30dc0bf-9058-4a1a-a8ec-56f5a29b89cf/contentimage_5F00_11380.png:16:16]