Probe bulkiness... - Page 1
Forum section: electronics / testgear

author: alank2
author_rank: Super Contributor
timestamp: October 25, 2014, 04:05:28 am
content: Hi Everyone,So, one thing that irritates me is when I end up having scope probes piling up on top of each other trying to connect them to a project. I recently found the thread here about making your own probe which intrigues me because I'd like to make a probe that is more compact.My goal is to have it simply end with a 2" lead for the signal and a 2" lead for the ground, each with a male pin that fits into a breadboard or at an angle into a test point, or uses with a small female to female adapter and connected to a header post, etc. I get that this might not be a very high bandwidth thing to do, but that is a trade off I think is worthwhile for how compact and easy to use it would be.Now, with that said, I've seen a few types of probes one can make like this.Low capacitance probe - A 50ohm piece of coax with a 950ohm (20X) or 2450ohm (50X) resistor inline for use with the scopes 50ohm termination mode.Upsides - easy to make, and high bandwidth.Downsides - must be mindful of how its low impedance (1K or 2.5K) will load the circuit, and you must enable 50ohm mode.1X probeUpsides - easy to make, high input impedance (1M)Downsides - bandwidth is limited and will get attenuated above 30MHz or so. I was viewing a 80Mhz square wave with it that didn't look too bad though.10X probeUpsides - medium bandwidth, high input impedance (10M)Downsides - will require cutting a real probe and trying to duplicate the components with wire ends.So, I am thinking about getting some of the cheap probes from eBay with the capacitance adjustment in the BNC connector and cutting them off at the probe end and putting a 9M resistor in them with my 2" leads. Do you guys think this will work reasonably well? Will I need some capacitance across the 9M as well? Other thoughts?

author: tautech
author_rank: Super Contributor
timestamp: October 25, 2014, 07:51:20 am
content: You stated "project".That would imply a prototype for proof of concept and testing.There are a couple of ways to satisfy your requirements.Test Points, that is physical points for test connections. Google "SMD test points". Or TH types of several designs.Test Pads. Normally incorporated in the final design for post production checks/tests using a "bed of nails" test jig. These can be soldered to for a short lead connection to std probes.Grabbers, in many shapes and sizes, the small Tek ones I find the most handy and quite capable of connecting to SOIC or SOT packages. ~ US$5 ea and 5 is all you might need.Agilent did some grabbers that clipped on their probes instead of the hook end that would easily do DIP and the likes of TO-92

author: Bored@Work
author_rank: Super Contributor
timestamp: October 25, 2014, 08:47:29 am
content: Just as an example, ProbeMaster has several attachments for their probes to add 0.025" receptableshttp://www.probemaster.com/product_info.php?products_id=250http://www.probemaster.com/product_info.php?products_id=251Note the "cost".Other manufacturers offer similar attachments for their probes. So invest a bit more in a reasonable probe and get the corresponding attachments fitting them. Signal integrity will of course go down, but should be good enough for simple breadboard work.The simplest DIY probe would probably be a Z0, Lo-Z, etc. probe (google those terms).

author: Aleksolder
author_rank: Contributor
timestamp: November 24, 2014, 01:54:30 pm
content: I'm in the same boat, understand your description perfectly, and can't make heads nor tails of all the theory and religion on making probes. What did you end up doing?

author: tggzzz
author_rank: Super Contributor
timestamp: November 24, 2014, 03:04:28 pm
content: Quote from: alank2 on October 25, 2014, 04:05:28 am... for frequencies below a few kHz.QuoteI suggest you google for HP and Tektronix literature about what's in a scope probe. Even documents produced in the 1960s are highly relevant to your question.There are alsomanythreads about such things on this forum. (No, I'm not going to poorly repeat things that are better explaned in other threads!)

author: alank2
author_rank: Super Contributor
timestamp: November 24, 2014, 08:55:43 pm
content: Quote from: Aleksolder on November 24, 2014, 01:54:30 pmStill working on it - but so far the best I have is having cut off an older probe master 150 MHz probe and putting a 7.5pF cap in parallel with a 9.1M resistor and then going through a 100R resistor to the tip. This turned out remarkably well and works pretty nicely. The quality of the coax and its lossy-ness is very important. The probe master one I cut measures about 300 ohms through it vs. the junk P6100's off of eBay that measured 100 ohms through them and were useless. At this point I'm thinking about a VERY tiny 1/8 or 3/16" wide super tiny pcb that allows for a strong attachment to the coax (soldering the braid to the pcb) and has room for the two in parallel and one in series components. Still trying stuff..

author: tautech
author_rank: Super Contributor
timestamp: November 24, 2014, 09:19:11 pm
content: Would the grabbers etc concept in this post be of interest/use?https://www.eevblog.com/forum/repair/suggested-for-a-sticky-part-one-comments-or-additions-please/msg471163/#msg471163

author: Aleksolder
author_rank: Contributor
timestamp: November 25, 2014, 12:58:29 am
content: Thanks for the suggestion tautech, but this is what we (at least I) are trying to achieve (see attached photo). Plug directly into breadboard. The photo shows my test leads for my DMM, and they're great, use them plenty. I want this for my function generator and my 100MHz scope.

author: Aleksolder
author_rank: Contributor
timestamp: November 25, 2014, 02:13:26 am
content: Here's what I've come up with, but I'm not an EE. I believe this is in line with the OP's objective, and so is not a hijack.Function gen output leads, end to end makeup:a. Standard BNC connection, crimp mount, no fancy bits inside, it's just a connector, though a good one, eg Amphenol's 50ohm 31-315-RFX www2.mouser.com/Search/ProductDetail.aspx?R=31-315-RFXvirtualkey52310000virtualkey523-31-315-RFXb. 50 ohm coax any any length (possibly even the resonant length) from 20cm to 100cm (8" to 39"), core is 7x0.02mmsq (7x34AWG). Microphone cable, in fact; beautifully flexible, small diameter, locally available.c. At the test tip, soldered to the braid and to the core each a one 1cm (1/2") clip of stiff wire such that used on leaded resistors and shown in the photo I posted above, just big enough to fit snugly into the breadboard holes, and bent 90deg, so the cables will lay flat.Scope 10x test leads, end to end makeup the same as the func gen cable (including the fact that it is not the fancy resistive cable, just 50ohm) except:a. At the tip of the core, just before the stiff wire tip, a pair of standard SMD metal film resistors summing in series to 9M.b. Parallel with the 9M resistor pair, a trimmer capacitor of around 20pF. Though it would be better to move this to the BNC if possible, as the OP suggests.I plan to pack the tip network as a squashed bug into heatshrink, maybe cut a hole for the trimmer socket, try to match the photo I posted above.The 50ohm cable will probably be a showstopper for the scope probes (not really sure), but perhaps can be overcome by a matching network, or by using some fancy resistive cable cut out of off-the-shelf cheapie 10X probes.I have researched this, thinking it would be a breeze, and found endless horrors of highly arcane theory and, I suspect, and rather heavy dose of religion.My func gen is 30MHz with 50ohm output and the scope is 100MHz with 1M 16pF input and no 50ohm setting.

author: alank2
author_rank: Super Contributor
timestamp: November 25, 2014, 02:49:24 am
content: Here is the test one I have right now:I ordered some small pcbs from osh park so I can try to make one of these that has some physical strength as well. (0.8" x 0.160")Thanks,Alan

author: Aleksolder
author_rank: Contributor
timestamp: November 25, 2014, 03:29:51 am
content: That's excellent, Alan, time to shop. Can you say whether my function gen leads are correct?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 25, 2014, 04:05:50 am
content: Depends on the project I guess. When I do my stuff on large PCBs with lots of room, typically I plop down a PCB mount SMB connector with a 475R resistor tapping the signal of interest. Then I just bring a SMB-SMA coax to the scope. I can snap in the cables, they stay connected and they can rotate to accommodate turning of the PCB.That's great for 50 ohm input scopes, and I have a bunch of pass-through 50 ohm terminators if I don't have a 50 ohm scope.On smaller boards I put Hirose H.FL cables. That's more expensive though.

author: tggzzz
author_rank: Super Contributor
timestamp: November 25, 2014, 10:00:08 am
content: Don't forget the rule of thumb that 1mm of wire has an inductance of 1nH. Couple (ho ho) that with the input capacitance of the wire and you have a tuned circuit that will ring/resonate. For a standard 10*/15pF probe with a ground lead that will bea round the 80-120MHz region.Provided your signals' rise/fall times are greater than, say, 10ns you probably don't have to worry.There are many threads on this forum documenting these effects.

author: artag
author_rank: Super Contributor
timestamp: November 25, 2014, 11:48:50 pm
content: I suppose this set is out of the question ?http://www.ebay.co.uk/itm/HP-Agilent-1155A-Low-Mass-Active-Probe-2-Channel-750-MHz-/371186208071?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item566c6c0d47