Show me your custom scope probes for connecting to prototype circuits.  - Page 1
Forum section: electronics / testgear

author: LinuxGuy123
author_rank: Regular Contributor
timestamp: January 25, 2025, 07:34:34 pm
content: I prototype a lot.My desk and bench (where I prototype) are a rat's nest with coiled scope probes. The probes on my Rigol are 4 feet long even though the scope is a foot away from the circuit. I am always trying to connect a 4 foot long scope lead with a big probe to a tiny pin on a small board 6 inches in front of my scope.I'm thinking of building a 4 channel pod that sits 12 inches in front of my scope. It will break out the 4 BNC coax channels to pin headers and then I can connect to my circuit via jumper wires rather than scope probes. I'd like it to be switchable from 1:1 to 10:1, but I could live without that. Thoughts ?If my oscilloscope has an input impedence of 1M, does a 10:1 probe just insert a 9M resistor into the circuit ?Another thing I would like to do with the pod is put a fuse in the probe ground circuit, so I never fry my scope or accidentially kill a power supply. A 300ma fuse would probably work. It would mess up the probe circuit characteristics so I could have a jumper to short it for use at higher frequencies.A great thing about a scope pod is that I could have multiple of them and have one mounted and attached to the circuit I am working on, so if I leave it all I have to do is detach the BNCs and all the probe connections stay attached. To resume using the scope with it I would just have to reattach the BNC leads.What thin coax is used for scope probes ?Thoughts ?What do you use for connecting your oscilloscope to small circuit boards ?Thanks

author: joeqsmith
author_rank: Super Contributor
timestamp: January 25, 2025, 08:07:42 pm
content: For home, I basically do what you describe 99% of the time. Just OEM probes. I have a fair selection of high voltage; active single ended and differential; passive 10X, resistive. Just depends what I am trying to measure. List and show a few of them in Lasmux's thread about his active probe.https://www.eevblog.com/forum/testgear/12-ghz-active-probe-project/msg4988716/#msg4988716I do have a home made holder to attach a single probe to the board. I used a Harbor Freight dial indicator arm for it. I attempted to look at the radiation pattern from a BLE antenna and soldered the coax directly to the board.https://www.eevblog.com/forum/testgear/sub-$1000-vna-for-antenna-matching/msg4550168/#msg4550168Many times I will solder test wires to the board and probe them. All depends...

author: bdunham7
author_rank: Super Contributor
timestamp: January 25, 2025, 08:24:37 pm
content: The answer to your questions complex and "it depends..."What voltages and bandwith are you anticipating? If under 10V and 1MHz, then direct connections are fine, perhaps even optimal. IF over ~30V or 10MHz then 10x probes are necessary. The 10x probe is more than a series resistor, the probe also forms a capacitive divider with the input capacitance of the scope. This is how it maintains a good frequency response into the 100MHz+ region. Note that the circuit loading at higher frequencies is dominated by the capacitance and the actual impedance will be far less than 10M. The actual probe cable is a special resistive variety that minimizes dispersion at the cost of some amplitude loss. This can be accounted for in the design of a 10x probe, but not 1x. Note that a direct connection is not the same as using a 1X probe, direct is much better. You should not use actual scope probe cable for anything you are likely to build and if you did, the very fine center wire is difficult to deal with.A breadboard jumper pack and a basket of IC grabbers like these are all you need for most prototyping work unless you are really working with small stuff.https://www.ebay.com/itm/10Pcs-Tektronix-Tek-30V-IC-Grabber-Clip-Probe-Accessory-the-black-one-/401862203176?_ul=INAs for what I do, I often solder in test points to grab onto or probe. For higher frequencies and small circuits, Tektronix makes some nice miniature probes with sharp tips that can be used manually or even in a holder if you get extra fancy. It's very difficult to do this with sensitive or HF circuits because any probe connections you add will change the circuit. ProbeMaster has a 10M 100X 3.5pF probe that is excellent for low loading of mid-to-high frequency circuits, say 10 to 100MHz. Higher than that and things get difficult.

author: tautech
author_rank: Super Contributor
timestamp: January 25, 2025, 09:10:55 pm
content: From an old post when just getting into more complex and miniaturized PCB's:Quote from: tautech on June 30, 2014, 08:28:43 pm

author: tggzzz
author_rank: Super Contributor
timestamp: January 25, 2025, 09:15:50 pm
content: Quote from: LinuxGuy123 on January 25, 2025, 07:34:34 pmUsually I use either Z0 or *10 "high" impedance probes, with the lead snaking over the bench and/or over the edge towards the floor.If you want to roll your own, you will have to specify your circuits risetime (not frequency/period), voltage, and tolerable load. The basic physics of those will dictate the possibilities.Little has changed in half a century, so this is an excellent tutorialhttps://w140.com/tekwiki/images/6/62/062-1146-00.pdfyou will set probes aren't bits of wire:)FFIhttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

author: LinuxGuy123
author_rank: Regular Contributor
timestamp: January 25, 2025, 09:44:28 pm
content: This forum never disappoints. So much knowledge and experience here. I'd be using my probe pod for embedded work. Most signals are <12V, < 1MHz. If I need to look at something fast, I'll use a conventional probe.Digging through my electronics supplies I found a bunch of female SMA bulkhead connectors, male SMA cable connector and some LMR 100 coax. I ordered BNC fittings. They should be here tomorrow. My plan is:scope <- BNC to male SMA cable > <female SMA bulkhead connector> <jumper to PCB pin> PCB pin or socketThoughts ?

author: tggzzz
author_rank: Super Contributor
timestamp: January 25, 2025, 09:53:39 pm
content: Quote from: LinuxGuy123 on January 25, 2025, 09:44:28 pm100MHz is simply an invalid characterisation of digital signals. Risetime is the important parameter. Hint: the physics neither knows nor cares when thenexttransition might occur.For a little theory and some measurements, seehttps://entertaininghacks.wordpress.com/2018/05/08/digital-signal-integrity-and-bandwidth-signals-risetime-is-important-period-is-irrelevant/

author: bdunham7
author_rank: Super Contributor
timestamp: January 25, 2025, 09:59:09 pm
content: Quote from: LinuxGuy123 on January 25, 2025, 09:44:28 pmKeep your BNC-to-SMA cables short, 12 inches or less if you can. Where possible, using breadboard jumpers or my favorite, solid copper wire salvaged from POTS cable, twist the two wires together and only separate them far enough to connect them to the necessary parts on the circuit board.Does your scope have a 50R input option?

author: shabaz
author_rank: Frequent Contributor
timestamp: January 25, 2025, 10:21:20 pm
content: The photo shows what I sometimes use; I don't know if it's useful for your needs or not. But maybe it is worth examining for some general construction ideas, even if you implement something different.I used RG-178 cable, it's a bit thinner than the LMR-100 you mention, but not so thin as to be difficult to assemble.The plastic blob contains a 953 ohm 0805 resistor, and there's a short (maybe 10-15 mm) length of wire on the end.The braid was bent back, and then the metal ferrule crimps onto the braid, and then the flared end was filled with a bit of solder.In use, the ferrule gets soldered onto a nearby ground, and the 'scope is set to 50 ohm termination (I sometimes use a BNC T-adapter and a decent external 50 ohm termination, if the 'scope doesn't support it built-in. Possibly debatable if that's good enough; I'm not suggesting you do as I do; I'm just indicating what I do if I just want a quick connection and a semi-plausible result for some signals). And the 'scope is set to 20X.EDIT: Regarding the ferrule, a 2.5 mm^2 one fitted RG-178 coax. For the plastic blob, I used Polydoh (it comes in small round pellets, which can be heated with a hot air tool, and then just mash them with fingers (it's hot but doesn't burn when there's just a few pellets), and press around the resistor into a ball. If it doesn't look right before it solidifies, it can be re-heated to soften it again to tidy it up.

author: LinuxGuy123
author_rank: Regular Contributor
timestamp: January 25, 2025, 10:44:13 pm
content: Quote from: tggzzz on January 25, 2025, 09:53:39 pmMy bad. I meant 1 MHz. Some SPI bus blocks are faster than this but I can hook up directly to those. Most stuff that I am looking at is interfacing slow peripherals.I edited my post above to read 1 MHz.

author: LinuxGuy123
author_rank: Regular Contributor
timestamp: January 25, 2025, 10:48:21 pm
content: My scope is an Rigol MSO5000. It does not have 50 ohm input but Rigol sells the ADP0150BNC 50 ohm adapter. Not sure that helps if the scope internal are not 50 ohm.

author: tggzzz
author_rank: Super Contributor
timestamp: January 25, 2025, 10:50:21 pm
content: Quote from: LinuxGuy123 on January 25, 2025, 10:44:13 pmMy point is unchanged if you have a 1Hz signal. Signal integrity and latch/register hold times are completely independent of repetition interval.What is the risetime and falltime on your signals? That will indicate the frequencies in your system.What capacitive and resistive load are they specified to drive? That will indicate how much different types of probe will change the UUT.N.B. a 50ohm inline termination will turn a scope's 1Mohm//15pF BNC input into a 50ohm//15pF input. How much that affects your measurements will depend on your signal's risetime and the cable length.

author: LinuxGuy123
author_rank: Regular Contributor
timestamp: January 25, 2025, 10:57:40 pm
content: What a great setup !How would you set this up if you had a scope with a high impedence ? I just ordered some RG178 because of your post.

author: bdunham7
author_rank: Super Contributor
timestamp: January 25, 2025, 11:19:24 pm
content: Quote from: LinuxGuy123 on January 25, 2025, 10:57:40 pmIt really is, I'm impressed and I'm probably going to duplicate it. The small and solderable (if needed) design is exactly what you need for faster signals although the overall design could be adapted, sans resistor, for a regular 1M input. I doubt you'll need this level of performance though if you just have SPI and similar signals in the <1MHz range.A Z0probe like this is the reason I asked if you had 50R inputs. A 50R adapter (you don't need the Rigol one) is an OK substitute to a point, but on an input that is not back terminated it might not look so good at higher frequencies >100MHz.

author: shabaz
author_rank: Frequent Contributor
timestamp: January 26, 2025, 12:23:29 am
content: Thanks! I've edited in some constructional detail regarding ferrule size and plastic for the blob to the comment, in case it helps.