Sensitive LCR Meter Measurements - Page 1
Forum section: electronics / testgear

author: mawyatt
author_rank: Super Contributor
timestamp: July 23, 2024, 07:19:35 pm
content: Decided to post this after doing some evaluation measurements over here:https://www.eevblog.com/forum/testgear/lcr-cal-test-pcb/When doing sensitive High Z LCR measurements (for example, small C at low F) one must be aware of the impinging E fields.Kelvin Clips like the TH26011CS and others generally do a good job but can fail at higher impedances often encountered during small capacitor measurements. If one studies the TH Kelvin Clip construction carefully, the entire length of the handle half is unshielded right to the clip tips. This allows these conductors to radiate and be susceptible to localized disturbances such as your hands, affecting any attempted quality measurements. Since most don't have "Calibrated Hands" this effect can't be totally eliminate by careful calibration unless one wants to glue their hands to the Kelvin Clip HandlesTonghui specs the TH260011CS to 100KHz which seems reasonable, they have a version that specs higher but we don't have such so can't say what is different, altho likely the shields from the cable extends closer to the Kelvin Clip tips. There may be others from quality sources that also a specified to higher frequency use.This is one reason we prefer the use of "dedicated" fixtures like the TH26048A and SMD types (TH26008) which attach directly to the LCR meter BNCs, since these have a shielded enclosure for the conductors with keeps most of the fields contained and repeatable.The SMD Fixture types do have "exposed conductors" without shields in the SMD Plungers, altho the wiring to BNCs is all within the case shielding and the plungers are somewhat fixed just above the case ground shield so the E fields are sorta "bent" down towards the case cover shield. One can see the effects by moving your hand close to the plunger to observe the effects, however not nearly as sensitive as the TH Kelvin Clips mentioned.There are also some nice SMD Tweezers cables like the TH26009B for use with LCR meters, these are rated to 15MHz and carry the shields right to the tweezer tips.There's also the issues at the opposite end of impedances like the low Z stuff, but we'll pause on that for the moment.Anyway, we've known this for some time (shame on us for not posting earlier) but never considered posting until making the measurements mentioned, hopefully saving others from additional LCR measurement uncertainty.Edit: When posting below we recalled an illustration that Hioki had which showed a conductive work base connected to the "Guard Terminal" on the LCR meter for sensitive measurements (it's in the IM3536 users manual). We found our PCBite plate is conductive and connected it to the LCR Meter "Guard Terminal" as shown below. Results improved considerably wrt to TH260011CS Kelvin Clip hand location sensitivity with the Guarded Conductive Plate, altho close by the handle still showed high sensitivity as would be expected since the plate can't have much effect for the near field of the handle conductor withinBest,

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 23, 2024, 07:30:14 pm
content: So are we gonna add some shielding to our kelvin clips now? 😉

author: mawyatt
author_rank: Super Contributor
timestamp: July 23, 2024, 07:46:42 pm
content: We're not!! Just going to be careful of use and understand the sensitivity of the measurements at handLater we might investigate Kelvin Clips with higher rated frequency use, but even then one is still stuck with the cable length effects, so best to just stay at low frequencies and be aware of the actual Kelvin configuration limitations.Honestly was surprised to find the Tonghui Clip handles totally unshielded, that was something we didn't know until evaluating your PCB with the 1pF test cap, and trying to determine the source on measurement uncertainty and noting how sensitive hand location was. I mean within ~100mm of the clips, and began to show effects as we would move our hand to write the measurement value only to see a different value on the IM3536!!Operating on a conductive workbench plate would likely help, but we don't have such. Hioki has an illustration of using a conductive plate attached to the "Guard" terminal which we should probably do. Need to find such tho!! Edit: Found one, the PCbite base is conductive, added above!!Best,

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 23, 2024, 08:30:39 pm
content: Did the plate help with the 1pF test?

author: mawyatt
author_rank: Super Contributor
timestamp: July 23, 2024, 08:35:18 pm
content: Yep, sure didSee edit in OP.Best,

author: Sensorcat
author_rank: Regular Contributor
timestamp: July 23, 2024, 08:50:46 pm
content: Quote from: mawyatt on July 23, 2024, 07:19:35 pmI'm not sure if I understand correctly: Does that mean the full path from the little box, where the big and the four small cable meet, to the tips, unshielded? If yes, that's quite disappointing! Raises the question if other manufacturers do better...

author: mawyatt
author_rank: Super Contributor
timestamp: July 23, 2024, 11:15:49 pm
content: Quote from: Sensorcat on July 23, 2024, 08:50:46 pmNo, the 4 wires are shielded but at the Kelvin Clip handles where the wire enters the handle, the wires are not attached to any shield there, just at the box. The center conductor goes to the long metal "blade" that becomes the Kelvin Clip tip at the handle. So the entire length of there handle is not shielded and why your hand has such an influence on the measurement.See image below.The popular cheap Kelvin Clips you often see, are no better. Same unshielded conductor thru the handle to the Clip Tip.Also, see OP for information on using Guard Terminal to help suppress external measurement influences for high Z measurements.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: July 25, 2024, 03:58:11 pm
content: Using LCR Meter Guard Function for SMD Fixture, see:https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/Best,

author: nctnico
author_rank: Super Contributor
timestamp: July 25, 2024, 09:43:39 pm
content: Did you see this thread:https://www.eevblog.com/forum/testgear/zurich-instruments-mfia-impedance-analyzer-(z-1m-1t)-review-teardown/? There could be some interesting information to be obtained from the review and teardown.

author: Martin72
author_rank: Super Contributor
timestamp: July 25, 2024, 10:03:13 pm
content: Quote from: Sensorcat on July 23, 2024, 08:50:46 pmThat's why I'm a friend of measuring components with as few cable lengths as possible, the higher the measurement frequency.So it's no surprise, for example, that the ET4410/4510 LCRs perform rather poorly at 100kHz if they are supplied with cables that TongHui/Sourcetronic specify as max. 10kHz.

author: jonpaul
author_rank: Super Contributor
timestamp: July 25, 2024, 10:11:11 pm
content: See HP Impedance Measurement Handbook of 1980s.https://www.keysight.com/us/en/assets/7018-06840/application-notes/5950-3000.pdfDepeneong on the Z range either 4 term (kelvin) or guarded measurement is used, but not both at one time.J

author: mawyatt
author_rank: Super Contributor
timestamp: July 26, 2024, 01:24:40 am
content: Quote from: nctnico on July 25, 2024, 09:43:39 pmDid not see this until now, that's a really nice instrument, wish we could afford such!!Thanks for the noteBest,

author: mawyatt
author_rank: Super Contributor
timestamp: July 26, 2024, 01:47:01 am
content: Quote from: jonpaul on July 25, 2024, 10:11:11 pmActually the technique shown in OP by Hioki works quite well indeed with Kelvin Clips and using the Guard Plate. We did so with Hioki IM3536, Tonghui TH26011CS Kelvin Clips and PCBrite Conductive plate as mentioned.We really should look into getting some Kelvin Clips rated higher than the TH26011CS tho, they are rated only to 100KHz.Best,

author: Martin72
author_rank: Super Contributor
timestamp: July 26, 2024, 10:06:40 pm
content: I don't know how well Tonghui accessories are available in the states, but Sourcetronic ships worldwide:https://www.sourcetronic.com/shop/en/kelvin-clip-terminal-st26011bs.htmlRated up to 5Mhz(pic shows the 300kHz version).

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 26, 2024, 10:28:06 pm
content: Quote from: Martin72 on July 26, 2024, 10:06:40 pmI would ask them to confirm that. Every TH version that looks like that, including the same model number as the URL maxes out at 100kHz.https://www.tonghui.com.cn/en/product_list_192.html

author: Martin72
author_rank: Super Contributor
timestamp: July 26, 2024, 10:44:27 pm
content: Jeez, this is a lot they offer...And you're right, not a single cable goes over 100kHz(except the ones with a smd tweezer) - that needs to be clarified, I'll write to sourcetronic.

author: mawyatt
author_rank: Super Contributor
timestamp: July 26, 2024, 10:52:03 pm
content: Yeah, those are ones we've been looking at, rated to 5MHz. They look exactly like the CS versions we have (rated to 100KHz).Even the Clips look identical and from the images looks as if the Kelvin Handles are the same with the "exposed" unshielded conductor blades.Begs the question, what's the difference, maybe the cables....or maybe they are just tested at 5MHz??Best,

author: Martin72
author_rank: Super Contributor
timestamp: July 26, 2024, 11:01:22 pm
content: I'll find out, the guys from Sourcetronic are even in the same city where I live...Maybe it's actually the cables.10kHz/100kHz is still more “LF”, where the properties of coax cables still play a subordinate role.But what do you do in the Mhz range, where cables with their pFs already play a parasitic role....

author: Martin72
author_rank: Super Contributor
timestamp: July 27, 2024, 10:31:16 pm
content: I have now written to them and asked for a data sheet, together with an explanation as to why.The things look so similar, but it may well be that the cables from Sourcetronic do not come from TongHui or are “different” and are manufactured by TongHui for Sourcetronic.We will know soon.

author: mawyatt
author_rank: Super Contributor
timestamp: July 27, 2024, 11:24:33 pm
content: Quote from: Martin72 on July 26, 2024, 11:01:22 pmIt's not so much the cables capacitance as theexposed cable signal fieldsthat are an issue. If the cable shields were carried up to the Kelvin Clip tips like the Tonghui TH26009B Kelvin Tweezers, this wouldincreasethe over cable capacitance but produce better results because thecable signal fieldsare no longer exposed along the handle. So the Kelvin Clip Handles wouldn't have exposed signal fields which your hand changes and thus changes the measurement underway, only the Kelvin tips would have exposed fields but these are near the DUT, and far from your hands and not influenced by such.Honestly quite surprised they are made this way with the long exposed conductive blade to tip instead of a short conductive tip connected to a shielded wire going thru the handle, as it wouldn't be difficult to carry the cable shields right to the short conductive Kelvin Clip tips.All the Kelvin Clips we have, and we have quite a few expensive and cheap, are not shielded up to the Clip tips with exception of the Tonghui Tweezers, probably reason they are expensive and rated to 15MHzBest,

author: mawyatt
author_rank: Super Contributor
timestamp: August 02, 2024, 04:44:31 pm
content: Following up on the issue with the Kelvin Type clips we decided to do a quick experiment to see how well the quality Tonghui TH26009B Shielded Tweezers would compare to a direct connected LCR Meter SMD fixture shown here:For some reason link doesn't work, so search for:"cheap-smd-lcr-fixture-the-good-bad-and-ugly!"Honestly, we don't have a quality SMD fixture (know we should have, but don't), however we do have a pair of cheap clone types that have been modified (see above) that produce highly repeatable results. Like < 0.01pF with small capacitance SMD chips, so no fear in utilizing these SMD fixtures as a reference to compare the TH26009B Tweezers with.First off we setup the SMD fixture and did the usual Short and Open Cal with the IM3536. For the Open we pried the plunger Opening ~ same as 0603 device, to Cal out the parasitic capacitances at that spacing (important if one is doing small capacitance measurements like 1pF!!). Precision C0G type 0603 values of 1, 10, 100 and 1000pF were selected for comparisons.After the SMD fixture measurements we setup the TH Tweezers on top of a conductive plate (PC Bite) connected to the IM3536 Guard Terminal. Operating with the IM3536 Guard Enabled has shown better repeatability with all cable type clips with high Z measurements, so we use this when necessary. We also spaced the Tweezers ~ same as 0603 spacing for Open Cal for same reasons.Here's the IM3536 results at 1MHz:Capacitance SMD Fixture TH26009B1pF 1.0004pF 0.9937pF10pF 9.9692pF 9.9303pF100pF 99.9551pF 99.7126pF1nF 992.236pF 992.838pFThe results produced were stable and little variations.This shows that quality Tweezer Kelvin Types like TH26009B which employs shielded conductors right to the tweezer tips can produce good results with small DUT capacitances with carefully attention to setup and measurement details.However, because the Kelvin connection is not made at the very tip end, one would expect results with very small impedances to suffer. Here even the SMD fixtures mentioned above aren't going to be optimum as the Kelvin connection on these types is at the plunger body and not the plunger tip. This was one reason we developed the experimental fixture type shown here and it's various refinements:https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/msg5061028/#msg5061028Anyway, seems the Quality Tweezers like the TH26009B which carry the signal shields to the tips are capable of quality, repeatable measurements with small capacitance DUTs (high Z), even at higher frequencies.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: August 02, 2024, 11:49:18 pm
content: This effort created another where we wanted to see how well things worked with low Z SMDs, so selected some precision low ohms 2512 SMDs.After a few futile efforts we gave up on the TH26009B Tweezers, too difficult to create a good repeatable Zero Ohms Cal, and the readings were unstable jumping around too much for our liking. We needed calibrated handsMaybe if we had more patience, but were somewhat in a hurry so proceeded to the SMD Fixture mentioned above in "Good, Bad and Ugly" which for some reason link didn't work, so you'll need to do a search for the thread. This fixture directly mounts to the IM3536 and should produce better results, which it did but far from what we would have liked!!Some difficultly with good Zero Ohms Cal, remember we're talking about sub-milliohms now!! Readings took awhile to stabilize also, so room for improvement!!Then we engaged the Special SMD Fixture approach indicated here:https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/msg5061028/#msg5061028This technique works with the Split Kelvin Connections Force-Sense right up to the DUT SMD and allowing the DUT SMD end caps to make the Force-Sense Kelvin connection. Getting a good Zero Ohm Cal is still tricky, but nowhere as difficult as with the Standard SMD Fixture, or the Tweezers, and getting good stable readings is a breeze comparativelyWe first did the Special SMD Fixture, then the Standard SMD Fixture.Afterwards we decided to reinstall the Special Fixture, do a quick Zero Cal, and rerun the test DUT Resistors, note the repeatability in the Special SMD Fixture results.Resistor Valuein Milliohms Special SMD Fixture (Repeat see text above) Standard SMD Fixture1 0.80 (0.80){0.82}<0.81>~1.235 4.84 (4.82){4.82}<4.86>~5.4510 9.90 (9.87){9.86}<9.92>~11.9815 14.70 (14.71){14.72}<14.73>~13.2920 19.83 (19.84){19.85}<19.85>~17.8440 39.87 (39.88){39.88}<39.88>44.8750 49.87 (49.91){49.91}<49.93>48.29100 100.21 (100.23){100.21}<100.27>99.28"~" indicates some variability in readingsAnyway, the Special SMD fixture seems to be a good candidate fixture for low Z SMD LCR Meter measurements and curious to see if the Guard Plate under the SMD measurement provides additional benefit (see link above).Edit: 4~5 hours later redid the measurements shown as{#}for additional results.Edit: From<#>post 24 below.Best

author: exmadscientist
author_rank: Frequent Contributor
timestamp: August 03, 2024, 12:01:41 am
content: This seems like a good thread for dumping one of my old observations:When using an Agilent E4980A LCR meter or its predecessor the 4284A, with leads longer than I'd like but difficult to make better, doing the open and short correctionstwice(so open-short-open-short or vice versa) gave significantly better results, especially in terms of repeatability. I can only guess that it's easier to dial in a small correction than a large one, and the first round correction makes the second round smaller?Either way -- I've done that ever since when using an LCR meter and trying to get any level of precision.

author: mawyatt
author_rank: Super Contributor
timestamp: August 03, 2024, 12:24:15 am
content: Good point, will give that a try next time and see if it improves the Calibrations. Not sure how these Calibration Measurements are utilized internally within the LCR Meter, but doing it twice certainly can't hurt!!Best,

author: mawyatt
author_rank: Super Contributor
timestamp: August 03, 2024, 04:43:13 pm
content: Just turned power on to the IM3536, no Calibration (using previous Calibration from yesterday), and started measuring the pervious 2512 Precision Resistors.This is with no warm up, no Calibration, and just quickly making measurements.Note results added in<#>in above post number 21.Here's what we use for Zero Ohm Calibrations, short lengths of #12 Cu wire with ends filed smooth for Standard SMD Fixture. For Special SMD fixture is a short flat rectangular conductor slightly bent and ends rounded, also shown is an "L" #12 Cu wire we use for Kelvin Clips.2512 test resistors are 1,5,10,15,20,40,50 & 100 milli-ohms.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: August 04, 2024, 04:38:29 pm
content: After using the Special Fixture with the IM3536 this morning we decided to try the Tonghui TH2830 LCR meter while riding out Tropical Storm Debbie. Here we'vecopied from post 21 aboveand added theTH2830 Resultswith the Special SMD Fixture.We first did the Special SMD Fixture, then the Standard SMD Fixture. Afterwards we decided to reinstall the Special Fixture, do a quick Zero Cal, and rerun the test DUT Resistors, note the repeatability in the Special SMD Fixture results.Resistor Valuein Milliohms Special SMD Fixture (Repeat see text above) With TH2830Standard SMD Fixture1 0.80 (0.80) {0.82} <0.81>[0.79]~1.235 4.84 (4.82) {4.82} <4.86>[4.81]~5.4510 9.90 (9.87) {9.86} <9.92>[9.82]~11.9815 14.70 (14.71) {14.72} <14.73>[14.69]~13.2920 19.83 (19.84) {19.85} <19.85>[19.83]~17.8440 39.87 (39.88) {39.88} <39.88>[39.88]44.8750 49.87 (49.91) {49.91} <49.93>[49.96]48.29100 100.21 (100.23) {100.21} <100.27>[100.25]99.28"~" indicates some variability in readingsEdit: 4~5 hours later redid the measurements shown as {#} for additional results.Edit: From <#> post 24 below.These results hint at the good repeatability with this Special SMD Fixture when using the same LCR Meter for low Z DUTs, but also show good agreement between the different brand quality LCR Meters we have.Hopefully others can benefit from this interesting concept of keeping the Kelvin Force and Sense connections separate and allowing the SMD DUT conductive (solder) end caps make the Kelvin Connections, seems to work a treat in our case!!BTW we've been "pinged" about making this Special SMD Fixture available as a kit. A couple possibilities are worth considering, one would use the same repurposed LCR Kelvin Clip Box with Lever Arm BNCs like shown, the other a custom enclosure design which would be an aluminum "U" design like the repurposed LCR Kevin Clip Box and Lever Arm BNCs (important for a quality design). 3D Printed components would be created as needed.Anyway, let us know if interested.ArghCounsel has advised against this, due to liabilityBest,

author: Martin72
author_rank: Super Contributor
timestamp: August 07, 2024, 10:25:07 pm
content: Quote from: Martin72 on July 27, 2024, 10:31:16 pmAs I didn't get an answer from them either privately or from work, I called them "today".They are in the process of getting this information, because they don't have it themselves....

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 07, 2024, 10:34:58 pm
content: Translation: TongHui definitely made it, and didn't supply good information. 😉

author: Martin72
author_rank: Super Contributor
timestamp: August 07, 2024, 10:38:36 pm
content: That's how I see it too, because when I brought the name into play, they reacted rather narrow-mindedly.

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 07, 2024, 10:43:39 pm
content: That's because they have a lot of experience dealing with them. 😉

author: EC8010
author_rank: Frequent Contributor
timestamp: August 08, 2024, 03:35:53 pm
content: I stopped using leads like those shown when I discovered that the capacitance of their coaxial cables gets converted into 100nH of series inductance. Until I discovered that, I thought all electrolytic capacitors had an ESL of about 100nH.I've found the best way to measure components is to hang a die-cast aluminium box directly off the analyser's four BNCs and have the Kelvin clips on unscreened leads. I also got fancy and bolted a vertical steel bar down the middle of the back of the box. Why? If you cut two pieces of tin plate and fold a right angle on their ends, then a few neodymium magnets to hold the tin plate in place allows you to have a variable length screen between the two Kelvin clips. Provided the screen is adjusted to the right length, I find my stray capacitance is only about 5fF - allowing me to measure little capacitors accurately. My large box is 8" x 5" x 3" so I bolted it to the ends of a pair of 1/2" steel rods that go under the analyser to take the weight off the BNCs.

author: mawyatt
author_rank: Super Contributor
timestamp: August 08, 2024, 05:43:21 pm
content: Not sure "what leads" you are referring to?If it's those in post #21 & #25 that come out the side of the small fixture box, they are not coaxial and in fixed position within a sturdy small shielded enclosure that directly attaches to the LCR meter with the 4 BNCs. The H and L sides are shielded within the box as shown. Since these leads don't move when installing the fixture, nor when making measurements, their effects are somewhat removed by the proper Open/Short Calibration.The "trick" with this fixture is allowing the SMD DUT end terminals to make the actual Force and Sense connections for Hcur, Hpot and Lcur, Lpot respectively. This effectively removes the contact issues with SMD devices and has shown excellent repeatability with very small Z DUTs.This fixture works well up to 8MHz limit of our IM3536 we've found, even with low Z (or higher Z components). We developed this specialized fixture for low Z SMD components, and it easily outperforms our best Tonghui Kelvin Clips for SMD devices, especially with very low Z components.What will be interesting (if the custom PCBs ever arrive) is will driving the Guard section underneath the active PCB contact area (the IM3536 has Active Guard capability, which significantly improves Kelvin Clip use as previously mentioned) improves results, this area is vacant of any direct PCB conductor with the present custom PCB.Anyway, in our case this unique SMD fixture has produced excellent results and displaced the normal SMD LCR Fixture and Kelvin Clips for low Z SMD component measurements. Still use the regular SMD fixtures for higher Z DUTs and rarely employ Kelvin Clips for anything other than leaded components (which is what they are intended for).BTW curious as to your LCR Meter setups, please post some imagesBest,

author: EC8010
author_rank: Frequent Contributor
timestamp: August 08, 2024, 06:16:52 pm
content: Quote from: mawyatt on August 08, 2024, 05:43:21 pmI was thinking of the leads shown in the photograph in the first post.This is my large box. The Kelvin clips are Mueller. The Hameg measures R + jX at 68 frequencies from 20Hz to 200kHz (excluding mains frequency) and reports directly to a spreadsheet. Spreadsheet subtracts a S/C calibration run, or on high impedance stuff an O/C calibration run of 5fF in parallel with the instrument's noise floor. Spreadsheet then least squares fits a model to the curve, calculates manufacturer's uncertainties and adds error bars. So far, I've about six different models for capacitors and inductors. It's noticeable that tightening the two screws at the bottom to tightly close the lid reduces measured uncertainties due to hum. The two tin plate screens slide against one another to give variable length.

author: mawyatt
author_rank: Super Contributor
timestamp: August 08, 2024, 06:36:07 pm
content: That's a nice setup!!You've effectively eliminated most of the disturbances by placing the DUT inside the shielded enclosure. Only issue might be the variation in movement of the DUT and Kelvin Clips & Leads between the Open Cal and Changing DUTs since within the box you have parasitic capacitance in all directions to the shield. The movable shield is clever, can you maintain the DUT and Kelvin Clips leads positions well enough for pF measurements with good repeatability?Also understand your statement about a bar for support, that aluminum box looks pretty heavy hanging on those 4 BNCs!!Nice workBest,

author: EC8010
author_rank: Frequent Contributor
timestamp: August 08, 2024, 07:03:11 pm
content: Thank you. Yes, there's stray capacitance to the shield. Unavoidable. For the smaller box, I lined it with 10mm thick hard foam. No idea what the plastic is, but (being foam) it will have a nice low permittivity and prevent the DUT getting too close to the shield. I didn't have any more hard foam to use for the large box.Surprisingly, repeatability isn't a problem provided I'm careful not to let the movable shield get too close (<2mm) to the DUT and provided I arrange the DUT to hang (if its smallish) such that a little movement isn't a problem. The biggest problem is that the Mueller Kelvin clips don't always make a good contact - they have flat faces rather than corrugations to make their contact and their springs aren't all that powerful. The limiting factor for 1pF capacitances is actually the instrument itself. The impedance of 1pF at any frequency below 200kHz is quite high and that dominates the uncertainty at 1% (if I remember correctly). In general, the box allows the instrument to perform to its specifications. Which is nice.The box itself isn't heavy, but I often test big heavy lumps like transformers in there, perhaps weighing a kilo or two, which would definitely be unkind to the BNCs. Those M5 screws with penny washers go into the rods.

author: mawyatt
author_rank: Super Contributor
timestamp: August 08, 2024, 07:43:30 pm
content: We created some crude LCR Meter plotting software in Python, here:https://www.eevblog.com/forum/testgear/lcr-meter-plot-software/Obviously we're not that proficient in coding, and need to spend some time wrt the UI, adding GUI would be nice, but GUI is something we've never done before. Was "hoping" some clever software types would pickup on this but never happened, so it's in our "to do list", but way down the list!!Best,

author: EC8010
author_rank: Frequent Contributor
timestamp: August 08, 2024, 08:40:57 pm
content: You've got the tricky bit done; getting the data out of the instrument. That was done for me. But I wrote the spreadsheets that calculate impedance from the model and then allow the Solver to do the least squares fit to find model values.I looked at your impedance plots - they're all familiar, although they go to a higher frequency than mine. ESR is pretty much constant with frequency. I think what you are seeing when it apparently increases at low frequencies is imaginary capacitance, which, being imaginary, is at right angles to real capacitance, putting it on the real plane and looking like resistance, but inversely proportional to frequency. It's in series with capacitors and quite a good indicator of the dielectric. The ideal capacitor has imaginary capacitance that is much larger than the real capacitance, thereby making its series contribution small.

author: mawyatt
author_rank: Super Contributor
timestamp: August 10, 2024, 10:18:52 pm
content: V5 of the Special SMD fixture PCB arrive and assembled, see preliminary results here starting at #22:https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/msg5600601/#msg5600601Best

author: Martin72
author_rank: Super Contributor
timestamp: August 20, 2024, 06:49:15 pm
content: Quote from: Martin72 on July 27, 2024, 10:31:16 pmST26011A 300kHzST26011B5Mhz300kHzST26011C 300kHzThe website will be changed accordingly.But there will still be no data sheets.

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 20, 2024, 07:32:48 pm
content: Quote from: Martin72 on August 20, 2024, 06:49:15 pmDid they note any differences between them?

author: Martin72
author_rank: Super Contributor
timestamp: August 20, 2024, 07:54:34 pm
content: After they hadn't contacted me for 3 weeks, they called me today.It was an interesting conversation...They all had the same bandwidth of 300kHz.I was like ah OK, and what about data sheets?They didn't really go into that, instead they told me that they had looked at Keysight, among others, where it was also only 300kHz....I think they didn't get any information from the actual manufacturer.It will be some OEM stuff where they order their name on it.Since the R&S adapters/cables look exactly the same, I would be inclined to ask them for data sheets.

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 20, 2024, 08:37:30 pm
content: Quote from: Martin72 on August 20, 2024, 07:54:34 pmThey do look identical.https://www.rohde-schwarz.com/us/knowledge-center/videos/r-s-lcx-test-fixtures-video-detailpage_251220-1386305.htmlHowever, I found nothing else on their website except for that video.ETA: Here's the only "datasheet" I could find. More like sales brochure though:https://assets.testequity.com/te1/Documents/pdf/rs/lcx_datasheet_en_3609-8309-32_v0100.pdf

author: mawyatt
author_rank: Super Contributor
timestamp: August 20, 2024, 08:48:47 pm
content: All those R&S fixtures look like Tonghui. Wouldn't be surprised if R&S LCR Meters are from Tonghui, probably similar to some TH types but with different front panels and maybe different firmware with same PCBs??Best,

author: mawyatt
author_rank: Super Contributor
timestamp: August 20, 2024, 08:51:38 pm
content: Quote from: Martin72 on August 20, 2024, 06:49:15 pmNot surprisedPersonally don't think these types of Kelvin Clips are good enough for repeatable results at 300KHz, certainly not the cheap clips you often see with lesser instruments!!Best,

author: Martin72
author_rank: Super Contributor
timestamp: August 20, 2024, 09:03:54 pm
content: I think so too.The 5Mhz was simply utopian for this type of line.With fixtures you can easily outperform this bandwidth.Oh yes, the fixture from Sourcetronic is on its way.

author: mawyatt
author_rank: Super Contributor
timestamp: August 20, 2024, 09:04:32 pm
content: BTW we did some measurements the other day with the cheap common 4 Wire Kelvin Clips and our DMM3500 for some low valued precision 2512 SMD chip resistors. Was a real PITA, even had a couple chips "flip out" with these clipsDid find them after crawling around on the floor the next morning thoThen we used the Special Custom Split-Kelvin SMD Fixture with the DMM6500 and this was much easier, and we appreciated why we developed this Specialized SMD FixtureAlso tried the Standard type SMD Fixture and couldn't get anything reasonable as the Zero Cal is too much dependent on the Zero DUT. Just not repeatable down to the levels necessary for good low milli-ohm measurements IMHO. This isn't surprising as these fixtures are more for higher Z components and the termination of the Force and Sense at the Plunger Base doesn't compensate for the variations in contact resistance at the plunger tip to DUT interface.Best,

author: Martin72
author_rank: Super Contributor
timestamp: August 20, 2024, 09:07:46 pm
content: Quote from: mawyatt on August 20, 2024, 08:48:47 pmI don't think so.The LCX meters are manufactured in Malaysia, just like the inexpensive spectrum analyzers from R&S.The test leads/fixtures will certainly be from Tonghui.I wanted to buy an LCX100.But only the LCX200 can be upgraded to 10Mhz.However, all LCXs can be upgraded with a graphics plot.

author: hpw
author_rank: Frequent Contributor
timestamp: August 21, 2024, 07:00:51 am
content: Quote from: mawyatt on August 20, 2024, 09:04:32 pmA week ago, I liked to measure a larger Tantalum Cap (as 2512 SMD sized) using my SMD fixture.Conclusion:- 2512 sized: impossible to have any contact- 402 sized: impossible to have not slipped them under the contact needle- also missing a nice leakage measurements of 30 old Tantalum (100u / 10V) ...Did then with an Siglent 4000 Power supply on 5V & 1K resistor & SDM3065 current graphs almost down to ground noise.

author: mawyatt
author_rank: Super Contributor
timestamp: August 21, 2024, 01:13:12 pm
content: Quote from: hpw on August 21, 2024, 07:00:51 amIf you were using the typical SMD LCR Fixture, ones we have can be "opened up" to accommodate the 2512 by sliding the stationary plunger back giving more space. You can use a Zero Ohm 2512 for the Short Cal, or better yet a small section of #12 Cu solid wire, or small rectangular Cu plate.Even with this effort it's still difficult to get a good Zero Cal reference for reasons previously mentioned, the Custom Split Kelvin SMD fixture is a better choice IMO.0402 SMDs are indeed difficult. We found that some of the SMD fixtures don't have the plunger thin shaft to tip seated directly on the Insulated "V" shaped support. Placing "shims" under the support can help, we even 3D printed a custom "V" support for one of our cheap SMD fixtures.If one is constantly working will small 0603, 0402 SMDs, might be worthwhile to create a custom 3D printed "V" support for this, even honing down the plunger tips to a smaller diameter (please don't do this to a quality fixture, use one of the cheap types!).BTW how do you like the 4000?Best,

author: hpw
author_rank: Frequent Contributor
timestamp: August 21, 2024, 02:54:16 pm
content: Quote from: mawyatt on August 21, 2024, 01:13:12 pmYes, as at best to have various "V" shaped plug's for that matter.>> SPD4121XWell, still missing the service manual so how to custom calibrate as an opened task even no copy calibrated to user settings copy menuThe GUI somewhat different as RIGOL, so to think different. In addition, RIGOL one channel with low voltage.I needed for oven 4 individual channels, as to go to four times 12V/1.5A.IMHO also the connections and cables should have 4 individual color pairs, otherwisered/black to label.

author: Martin72
author_rank: Super Contributor
timestamp: August 22, 2024, 06:09:43 pm
content: The ST28006A test fixture arrived today and I'll be taking it home with me at the weekend.Here are 2 pictures, from the outside the almost 270€ expensive ST version and mine from Aliexpress hardly differ.But after opening it....

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 22, 2024, 06:20:15 pm
content: How do the plastic holders compare in shape?

author: Martin72
author_rank: Super Contributor
timestamp: August 22, 2024, 06:32:18 pm
content: I'll take direct detail/comparison pictures at the weekend, but what I noticed was that the spring is smoother on the ST fixture.

author: mawyatt
author_rank: Super Contributor
timestamp: August 22, 2024, 07:28:50 pm
content: You can adjust the spring tension by removing the plunger case and/or the end cap. We prefer more pressure on the DUT for better DUT terminal contact.Make a shield like the ST version, you can use a Cu plate/foil or just Aluminum L Bracket. Also make sure the 4 BNCs are electrically well connected to the case, and the cover has good electrical connection to the base case thru the 2 screws holding it on.A little rework and the cheap SMD fixture will be almost as good as the expensive oneBest,

author: Martin72
author_rank: Super Contributor
timestamp: August 22, 2024, 07:42:40 pm
content: I think I will install some kind of second housing made of sheet brass.But first I will repeat the measurements on the ST2830/ET3503, i.e. 10p, 100p and 1n, once with my fixture, then with the ST fixture.Let's see if we can see anything.

author: mawyatt
author_rank: Super Contributor
timestamp: August 22, 2024, 08:03:02 pm
content: A little EM analysis will show that all the case/ground/shield currents return thru the 2 end screws, make sure this connection is really good, one reason we prefer the 4 screw version!!Best,

author: Martin72
author_rank: Super Contributor
timestamp: August 22, 2024, 08:08:54 pm
content: That's a pretty exciting thing, I like that.My 2nd edition of the power inductor checker has been set up and will soon be put into operation.If it works as intended this time, I will publish it in the forum.

author: mawyatt
author_rank: Super Contributor
timestamp: August 23, 2024, 12:26:50 am
content: Quote from: Martin72 on August 22, 2024, 06:09:43 pmMartin,Something doesn't look right here!!The ST28006A image shows two end brackets to hold the cover. Each support bracket is bolted to the cover with 2 bolts, yet the image of the ST28006A only shows 1 set of dual bolts at one end of the cover, where's the other bracket attachment bolts at the other end???Best,

author: mawyatt
author_rank: Super Contributor
timestamp: August 23, 2024, 05:09:52 pm
content: Here's some plots made with the IM3536 and the Custom Split-Kelvin SMD Fixture. These are 1% 2512 0.001 ohm and 0.01 ohm Resistors.Edit: Added 50 and 100 Milliohms, later the High "Q" 100nF C0G Capacitor. Note the artifacts in the upper right of the -2 plot around 7.4MHz, likely due to a fixture/setup resonance.Best,

author: Martin72
author_rank: Super Contributor
timestamp: August 23, 2024, 07:53:21 pm
content: I have now also taken the ST fixture home with me, before that I measured 3 capacitors at 100khz at work, once with the ST, then with my nameless one.The results are congruent, which suggests that the “denser” structure of the ST does not yet play a role in the frequency.Tomorrow I will do the same with the ET3503, but then of course also at 300kHz.When I got home, I photographed both fixtures again, the pictures will be split accordingly due to the maximum number.

author: Martin72
author_rank: Super Contributor
timestamp: August 23, 2024, 07:56:30 pm
content: The two fixtures photographed from the outside...

author: Martin72
author_rank: Super Contributor
timestamp: August 23, 2024, 08:00:41 pm
content: Finally, the clearly different inner life of the two.

author: mawyatt
author_rank: Super Contributor
timestamp: August 23, 2024, 08:39:33 pm
content: How is the bracket attached to the cover in the Left side of image 4788 and Right side of image 4789?There's no thru bolts or screws shown from the top side of the cover image for this bracket attachment!!Are these something like a welded stud to the cover bottom, then smashed/expanded to hold the bracket??Best,

author: Martin72
author_rank: Super Contributor
timestamp: August 23, 2024, 09:17:46 pm
content: QuoteNormally with screws through the housing, as these also fix the spring mechanism on top of the cover.This is not necessary on the other side, where threaded bolts are welded on and the bracket is fixed with nuts.Mechanically completely plausible.

author: mawyatt
author_rank: Super Contributor
timestamp: August 24, 2024, 01:05:24 am
content: Quote from: Martin72 on August 23, 2024, 07:53:21 pmWith the 10pF Caps this is showing ~20ff difference, or 0.2%.Since you used the same 10pF Cap (C0G), same quality LCR Meter, same basic setup, this leaves this Fixture as the main source of the difference. The difference effects can now be broken down into variations in Cal and main differences in physical fixture (lack of shield).A simple experiment can verify IF the shield is worthwhile.Setup the measurement after lengthly warmup with a very large average (~100), do a spot Open Cal at highest frequency the LCR supports (300KHz) with a small piece of plastic about same size as 10pF Cap DUT for proper plunger spacing with the ST Fixture. Now record the result, then replace the Fixture with the No Name and repeat, record the result. Note the small plastic spacer will allow the 2 fixtures to have ~ the same spacing and remove this uncertainty, however they will increase the fringe capacitive effect, so don't rely on this for accurate measurements, only relative differences. The ~100 averages should give good mean values for comparisons.Now carefully remove the No Name cover and install a temporary shield. Use some art & crafts thin Cu plate and cut a shield and tack solder in place (BNC lug) at the ST Fixture shield location, careful not to disturb the wires. Make sure it's mechanically stable, you can use some hot glue if needed.Put the cover back on and reinstall the fixture, then repeat the measurement procedure. Suspect you'll see a value closer to the ST fixture measurement, and this should verify the benefit of the shield.After all, don't think Tonghui (and others) would use a shield if it provided no measurable benefitBest,

author: Martin72
author_rank: Super Contributor
timestamp: August 24, 2024, 07:58:49 am
content: QuoteI didn't say that either, see the corresponding post.Nor did I think so.

author: mawyatt
author_rank: Super Contributor
timestamp: August 24, 2024, 12:51:58 pm
content: Quote from: Martin72 on August 24, 2024, 07:58:49 amOf course you didn't!!Just say'n it's there for a reason for others that might not have followed the entire SMD fixture threads.BTW is the shield and case aluminum or steel?The Tonghui Kelvin Clip base fixture that connects to the LCR Meter has a Steel case, the cover is Aluminum. Suspect this base is for rigidity holding the 4 BNC connectors, rather than any H field shielding. This doesn't have a shield partition, but the 4 wires are coaxial shielded up to the BNC connection with shields soldered directly to BNC lugs.Best,

author: Martin72
author_rank: Super Contributor
timestamp: August 24, 2024, 01:50:24 pm
content: I can test this later with a magnet.The separation/shielding in the ST fixture should be made of tinplate, aluminum is not so easy to solder.It “feels” like the NoName is even heavier, although there is less “inside”.Whether the relatively low frequency of 100kHz already makes a difference would have to be compared with the nomogram, where the accuracy of the ST2830 actually lies when 10p is measured at 100kHz.Or you can approach it in comparisons, from say 1khz upwards - people with an LCR sweep would be fine there....Martin

author: Martin72
author_rank: Super Contributor
timestamp: August 24, 2024, 06:38:03 pm
content: QuoteIt is heavier..And so are the contact resistances, here as an example from ground to ground.

author: mawyatt
author_rank: Super Contributor
timestamp: August 24, 2024, 06:49:46 pm
content: 3 ohms is considerable, try measuring between the static plunger and the BNC shell for each. The lever arm doesn't really matter as it's not conducting nor shielding any signal. The BNCs provide the connection back to the LCR Meter are important wrt any shield currents getting back to the instrument.BTW our "Good" Standard SMD fixture weights ~291g and is all Aluminum, our "Other" Standard SMD fixture has been repurposed to include external DC Bias Capability within, it's also all Aluminum and weighs ~318g but has a lot of extra "stuff" inside for the added DC Bias Capability!Best,

author: Martin72
author_rank: Super Contributor
timestamp: August 24, 2024, 07:42:48 pm
content: QuoteI had measured everything, that was just the most “drastic” difference, which I then photographed.I am no stranger to good shielding in principle.

author: mawyatt
author_rank: Super Contributor
timestamp: September 02, 2024, 04:00:14 pm
content: Along the lines of sensitive measurements with typical Kelvin Clips utilized often with LCR Meters and DMMs, here's some information that users might find interesting wrt to very low Z measurements.A note clipped from a Hioki data sheet illustrates the importance of "Proper" Zero/Short Meter/Accessory Compensation. We've added some notes of our own to help illustrate the effects of reversing the Kelvin Clips and why the proper orientation for a quality Zero compensation is important for quality low Z measurements.With a proper Clip orientation with the Kelvin tips Hcur and Lcur directly on top each other as shown, the LCR Meter Forcing Current flows thru from the High Force side (Hcur) directly to the Low side (Lcur) and little current flows towards the Sense terminals (Hpot & Lpot) because of the high impedance these terminals present. In our notes because of this Ix ~0 since Id and Ic ~0. Under these conditions then Lpot and Hpot sense the same voltage Vx which is the mid terminal connection point between Hcur and Lcur, and the meter computes the effective Z as ~0 since Z = Vdut/Idut and Vdut is VHpot-VLpot or 0 since they each read as Vx as shown in the schematic.When the Kelvin tips are reversed with Hcur now directly connected on top to Lpot, then Hpot, and Lcur, the full DUT Force current flows thru all 4 Kelvin Tip ends and introduces a voltage drop between Hpot and Lpot which creates a DUT effective impedance as shown as Rx in our notes.Of course things get a little more involved and as frequency increases things can get way more involved, but this serves to illustrate the concept.To show these effects we've employed our Tonghui TH2830 LCR Meter with TH26011CS Kelvin Clips. These images show the Kelvin Clip connections on a Guard Plate, proper and reversed orientation and the subsequence meter readings.First a Zero/Short Compensation was preformed with the Proper Clip orientation, then the Clips disconnected and reconnected to show the Zero Reading, then Clips were reversed (L Kelvin Clip flipped over) and measurements repeated. 16 averages were used at results shown at 100Hz, repeated measurements with connect and disconnect showed reversed connection variation from 2 to ~4.5 milli-ohms, whereas with proper orientation these variations were under 20 micro- ohms (Note the Th2830 resolution is 10 micro-ohms).Would be interesting to see what others find with their LCR Meters and Kelvin Clips.Anyway this is a subtle point wrt to Kelvin Clip usage for low Z measurements, and hope some find this useful.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: September 02, 2024, 04:33:33 pm
content: We've added results for the TH2830 Rdc Measurements and the Keithley DMM6500. The TH2830 larger Rdc 46 micro-ohms with Proper Orientation is due to the Thermal EMF in the Kelvin Clips and Instrument, this effect is negated with AC measurements.Note negative readings with reversed Kelvin Clip Zero measurement, reason is left as reader exerciseBest,

author: EC8010
author_rank: Frequent Contributor
timestamp: September 03, 2024, 05:14:11 pm
content: I had to stop and think a bit about those Kelvin clip strictures.Seeing Martin72's internal photographs, I had to open up my SMD fixture. It's identical to his expensive fixture (although the screen is steel) but has Hameg HZ188 written on the outside because it came with a Hameg 8118. I seem to remember seeing a Digimess LCR tester from before 2007 that had a similar jig.

author: mawyatt
author_rank: Super Contributor
timestamp: September 03, 2024, 06:01:34 pm
content: Thanks to Martin for showing the ST versionThink the ST and many others from Tier 1 sources (R&S) are directly rebrands from OEM Tonghui, the lower Tier sources are various clones of the Tonghui with varying degrees of quality and various rebrands.We have a couple clone fixtures, one is very well made, the other not so!! At lease on the inferior SMD fixture, the plungers seem the same and good quality, can't say so for the case thoWe decided to "repurpose" the inferior SMD fixture into a self contained SMD fixture which will allow external DC Bias applied to the DUT. This works really well and is compact and more convenient for SMD devices than using the DC Bias Adapter we developed for LCR Fixtures, which works well with other LCR Meter fixtures such as, Leaded, Kelvin Clip & Tweezers.Also check out the Split-Kelvin SMD Fixture here:https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/So we've pretty much got all the bases covered for SMD devicesBTW if you use Kelvin Clips please note posts 71 and 72, the Zero Ohm Compensation does make a difference with Kelvin Clip Tip Orientation as shownBest,

author: Martin72
author_rank: Super Contributor
timestamp: September 03, 2024, 07:52:10 pm
content: The people from ST called me again and told me the same thing, somehow they don't seem to talk to each other.However, the second employee had confirmed that TongHui manufactures everything - for almost all other manufacturers.But they still haven't corrected the 5Mhz...

author: EC8010
author_rank: Frequent Contributor
timestamp: June 20, 2025, 11:54:19 am
content: Having invested in a Hioki IM3536, I needed a new test box, so I took the opportunity to improve it from the one shown earlier in this thread.Box rear 2: Four BNCs, each secured by button head M3 screws. They have smaller sockets than cap head screws so the hex driver doesn't foul the BNC and the heads don't get in the way of you securing the BNC. Because you can't get at them, the inner two BNCs have had their knurled portion cut off by a cutting disc in a multitool. Cut deep slots in the ring and the knurled section, rotate the BNC outer by 180 degrees and repeat, insert gash screwdriver into slot and rotate it to crack the remaining material and release the knurled ring. Underneath is a circlip holding a spring and rubber washer. Robust side cutters can cut the circlip to release the unneeded parts. The central vertical shiny strip is a piece of (magnetic) stainless steel to allow the movable screen to be secured inside. The brass hinges on the left are mounted on spacers to move their pivot points outside the edge of the box and allow the lid to close without fouling. Don't assemble with sweaty hands as brass in contact with aluminium easily starts electrolytic corrosion - especially if acid is present (sweat). The support rods have penny washers to spread their load on the die-cast box.Box rear 1: The support rods extend to the back of the Hioki. The (16mm diameter) Perspex ferrules are epoxied to the rods and fit the vertical space between the bottom of the Hioki and the surface it sits on, preventing vertical movement of the rods, and therefore strain on the front panel BNCs. In my situation, I didn't need ferrules at the front because the front panel of the Hioki and the Tek curve tracer clamped the support rod at the front, but a different situation might need extra ferrules. The rods take the weight of the box and DUT off the BNCs. The rods are set in their slightly odd horizontal positions to avoid fouling the Hioki feet.Box inside: A four-wire connection is made using Mueller Kelvin clips with minimum length extra flexible hook-up wire. The solder joints to the BNCs are sleeved with glue-lined heat shrink sleeving to take the stress off the wire entering the soldered joint. There are two folded tin plate screens. The fixed screen is bolted to the top of the box with two M3 screws and to the back of the box by a nut on one of the BNC securing screws. The fold to the top of the box needs to align with the draw of the die-cast box. Care was taken to eliminate gaps as far as possible. Both screens overhang the front of the box by 1.6mm to allow for the recess within the lid. The movable screen has a 10mm flange folded onto it to allow two 8mm cylindrical neodymium magnets to clamp it firmly to the box's rear steel strip. The flange is on the opposite side to the fixed screen's flange in order that it always sits flush to the box. A third magnet clamps the movable screen to the fixed screen. Thus, the movable screen can move up and down so as to minimise stray capacitance between the two Kelvin clips yet forms a leak-free variable length screen. Stray capacitance of <2fF is achievable, which can be zeroed out by the Hioki's open circuit calibration.The purpose of the box is to enable measurements primarily limited by the test equipment. The earthed conductive box electrostatically screens the DUT and also shields it from draughts. Spacing between the BNC centres is 22mm and seems to be an informal standard used by Hameg, Hioki, and HPAK. Lead length to the DUT is essentially zero, and residuals (lead inductance and skin effect) can be zeroed by the Hioki short circuit calibration. Fitting and tightening the three screws on the left hand side of the lid improves measurement quality at high impedances. The movable screen should be adjusted to minimise stray capacitance with the DUT connected. Heavy components should not be hung by the leads but sat on a spacer to support them from the bottom of the box. A scissor stand would be ideal if it would fit in the box, otherwise, blocks of wood work. Transformers usually need their casing to be earthed - an earthing lead has not yet been fitted to this box.

author: mawyatt
author_rank: Super Contributor
timestamp: June 20, 2025, 12:31:23 pm
content: That's a really nice fixture!! Well doneWith the internal magnets watch out for the effects on transformers or large inductors where the core might be some ferrous metal and the fixed magnetic field could alter the core characteristics.Anyway, this fixture does a really good job of controlling the fields around the leaded DUT and should produce excellent repeatability.Best

author: EC8010
author_rank: Frequent Contributor
timestamp: June 20, 2025, 03:11:04 pm
content: Thank you.Good point on the magnets and nearby transformers. Up until now, the DUTs have been big enough that they'vehadto sit some distance away. Perhaps a V4 of the jig would mount the steel strip internally, allowing weaker magnets to be used. Another possibility might be to machine a housing that was a pole piece for the rear of the magnet to bring it round to the front, with a nonmagnetic annulus between the two. That way, the external field would be restricted and a smaller weaker magnet could be used. Hmmm. Perhaps a 4mm diameter neodymium, sleeved in 6mm brass, then an 8mm mild steel cup to bring the rear pole to the front. That's worth a try.