EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: mawyatt on November 23, 2022, 08:10:55 pm
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Thought this might be a useful recollection after reading:
https://www.eevblog.com/forum/testgear/best-cheap-lcr-meter-for-small-capacitors-and-inductors/25/ (https://www.eevblog.com/forum/testgear/best-cheap-lcr-meter-for-small-capacitors-and-inductors/25/)
We have a couple cheap non-OEM SMD Fixtures for use with lab grade LCR meters, OK know it's not wise to strap on a cheap fixture to a quality lab type instrument, here's the story. We did have a quality OEM set of Tweezers (Tonghui TH26009B) that helped with some SMD component selection and verification for a major project. When the precision SMD components we required became unavailable we were forced (actually major redesigns 4 times to accommodate available components!!) to use what was available and do selection. These were thousands of components and after using the OEM Tweezers for awhile (tedious as heck!!) decided to get a proper SMD Fixture, however the OEM SMD fixtures were unavailable with deliveries "expected" in many months, so we took a change on a couple available cheap SMD Fixtures that "looked" OK.
The build quality isn't great in both fixtures, but sufficient. After some time playing around before committing to actual use (precision component selection) they seemed OK, but a little concerned about the case lid grounding. One lid was completely painted, the other partially painted but both relied on mounting screws for grounding, fixed this with a knife & drill bit scraping some paint off. Also noticed the internal BNC grounds were not connected on one that had the star ground washers, so we corrected this by soldering in grounding wires. Then repositioned the internal wires towards the middle of the enclosure to minimize parasitic shunt capacitance, this is between the internal shield between the L and H sides.
After we suffered thru the measurements long ago we began to notice more uncertainty in the readings, especially in one fixture, then it began to fail DC Short calibration. We found the threaded insert that is orthogonal to the plunger contact, where the Force and Sense wires come together between the BNCs and plunger was slightly loose. So was the other plunger insert. The plungers were removed and these inserts well tightened and everything reassembled with good results again!! Then we began to experience some increased uncertainty with the other SMD fixture and found the same result, loose threaded insert. Both Fixtures perform well now and serve our needs.
A couple tidbits on getting repeatable results with these SMD Fixtures (probably holds true for the OEM fixtures as well). When you do a Short and Open cal the fixture moveable plunger moves, this also moves the internal wires slightly. For best results we often use a similar size SMD Zero Ohm for the Short calibration, then use a spacer on the plunger arm to open the contacts to about the same space as the expected DUT. Because the wires move, the internal fields in the fixture change slightly influencing the results. Using a similar size Zero Ohm SMD for Short Cal and same contact spacing for Open Cal you are helping keep internals similar for the DUT measurements.
Of the two SMD Fixtures shown the one in front with the 4 BNC lever arms is the better build and mechanically more stable, however both achieve very repeatable and comparable results on either of our TH2830 or IM3536 Lab LCR meters. Recall both were under $100, not a OEM replacement, but if one doesn't mind tinkering around then not too bad a alternative.
BTW the Ugly one has a grey 3D printed SMD support, we needed to replace the original because it was useless for small devices which is what we normally use.
Anyway, hope this helps some folks considering a cheap SMD Fixture for LCR meters.
Best,
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Hers's what the insides look like. First is the better of the two cheap SMD Fixtures IMO. Note the baffle shield between L and H sides and how the Force and Sense wires go from the BNCs to the plunger insert, they are soldered here and this becomes the effective end of the Force/Sense capability. What this means is that the rest of the plunger from this point on becomes part of the DUT, and can only be compensated for by means of Open & Short calibration.
BTW it's a warm fuzzy when you can get a 0603 4.7pF COG to agree within 10fF between instruments & SMD fixtures (4.6912pF vs 4.6927pF), even when swapped (4.6905pf vs 4.6898pF). Repeatability between same instrument and fixture is even better ;D
Best,
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Upon viewing the second image above it became obvious that the internal wires could use a little trimming to shorten up and reduce the uncertainty regarding the internal E fields and reduce the series Z of the fixture. Honestly, don't know why we didn't do this long ago, but we didn't ???
Here's the internal view with shortened leads.
While playing round with this fixture decided to see how well it would work with a KS34465A DMM reading small capacitance, so gathered up some BNC adapters and cable and kludged (hooked) things up as shown in second image. Used the Math MX+B function and Long Averaging Function, and after allowing things to settle did a careful Null (the setup is floating so hand position can influence Null) to remove the residual capacitance (~68pF) with the fixture Open with a spacer in the Lever arm to position the plunger contact tips about the same spacing as the intended DUT, which in this case was the 0603 SMD 4.7pF reference cap mentioned earlier above. Believe it or not here's the result, 4.804pF with last digit bouncing round some but without any trickery :-+
Then we decided to kludge up the adapters for the DE-5000 as shown in the 3rd image, note we used the Guard to the Fixture case ground. With the KS34465A the fixture "floats" and is more susceptible to surrounding effects, with the DE-5000 Guard utilized this effect is reduced considerably. Also note the DE-5000 is using direct Banana to BNC, this defeats the 4 wire Internal Capability but we don't have the unique Split-Banana plugs, so did the best with what was on hand. This resulted in a stable reading of 4.69pF!!
Edit: Added 1, 10, 100pF COG SMD Caps measured with TH2830 & DE-5000 with SMD fixture as shown. 1.01927/1.00pF, 9.95836/9.95pF and 101.080/101.17pF. This was done without an additional Cal on the DE-5000 after a couple hours off after initial Open/Short Cal ~3 hrs earlier, the TH2830 was left On the entire time of ~3 hrs with only initial Open/Short Cal.
Anyway, just some fun stuff with these fixtures and other instruments. As always, YMMV!!
Best, and Happy Thanksgiving
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The kludge setup with the DE5000 above got us thinking. We acquired the DE-5000 long ago on the reliable recommendations provided (ones that had them and made comparative measurements), and planned on using this for some component selections (mostly small Cs), also got the TL-22 tweezers and then modified a TL-21 for proper Kelvin Clips. We found the Kelvin clips were not much better than the TL-21, also have a bunch of 6 1/2 digit DMMs with 4-Wire Kelvin capable and cables so no need for precise resistive measurements from the DE-5000 which isn't anywhere near the DMMs anyway. So the Modded TL-21 with attached Kelvin clips just never got used as intended.
After realizing we could use the SMD Fixture with the DE-5000 as shown above, and get good results (mainly for SMD capacitors), the thought of making an adapter with the TL-21 to 4 BNCs for use with the SMD Fixtures. Also, the core DE-5000 is a true 4 wire system, so we didn't want to defeat this as we did above with the BNC kludge adapters. So we cut the Kelvin clips off the modded TL-21 and reconfigured it with 4 BNC female connectors (could have used male but then would require the BNC barrels) we had on hand. The cables were kept short, maybe a little too short as you can see, and the workmanship isn't good (need to redo this sometime), but the resul is very good!!
We did a cal and pulled a Venkel 0805 COG 0.3pF reference cap we measured with the TH2830 at 0.31914pF, the DE-5000 shows 0.31pF!!
Best
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I like the idea of a spring loaded fixture like these for rapid change out. I built something similar but for characterizing parts in several MHz. They work alright. You can see the one in the first post.
https://www.eevblog.com/forum/rf-microwave/shunt-capacitance-of-1206-smd-resistors-jeroen-belleman-december-2010/ (https://www.eevblog.com/forum/rf-microwave/shunt-capacitance-of-1206-smd-resistors-jeroen-belleman-december-2010/)
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Nice DIY fixture!!!
Agree, these work very well and much preferred than having to solder the DUT into a test fixture, then insert fixture into measurement setup, then remove the test fixture, then unsolder the DUT to utilize.
The OEM types of these SMD Fixtures, like the Tonghui TH26008A, claim a frequency range of DC to 120MHz, although we only have a LCR meter (IM3536) capable of 8MHz.
We've utilized these SDM Fixtures to cycle thru many devices (more than I care to remember!!) and they made quick work of a boring procedure and produced good repeatable results!!
For under a $100 and a little rework not a bad addition to one's fixture tool kit, apparently even if they have a handheld LCR meter :-+
BTW the design of these SMD fixtures is not obvious from the images. They use a nicely machined (brass?) and plated "plunger" that slides inside a hollowed tube insulated sleeve that extends longer than the plunger. The tube slides inside the larger shinny (silver color) plated metal support (brass?) which is secured to the Lid. The plunger insert has a deep threaded hole orthogonal to travel, where a long brass thin bolt or rod is attached thru the insulator tube which has a hole. This rod fits within a ~ 13mm slot cut in the Al top cover, on the underside two wires, Force and Sense are soldered to the rod end and go to the BNC connectors, and allows the plunger to slide along the slots.
One plunger side is held with a thumb screw pressing into the insulating sleeve, the other is arranged with a spring loaded arrangement and a lever arm and the insulated sleeve extends well past the end to hold the Spring and Lever Arm. So the only active metal is the two plungers and they are arranged so the spring loaded lever plunger is on the Low or L side, and the other the H side of the LCR Meter. Very clever arrangement that is mechanically & electrically stable by maintain good control of the fields inside and outside the fixture. Likely this may have been copied from an old HP LCR fixture.
Here's what the inside of the modified TL-21 looks like.
Best,
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Here's my shunt fixture. The Teflon looks hacked up because it was cut with a knife.
https://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/msg2974530/#msg2974530 (https://www.eevblog.com/forum/projects/20db-rf-attenuator-seeking-feedback-to-improve/msg2974530/#msg2974530)
A member here was trying to make a fixture with pogo pins. I'm not sure how that worked out for them.
I have used some test PCBs to characterize parts by soldering them. Agree, it's a pain and having to insert the test board after changing components is hard on connectors. My balls are not big enough to solder with the test board still attached to the test equipment.
Istvan Novak published three articles in EDN on low cost fixtures. The very first was using solder wick and coax to make shunt thru measurements.
https://www.edn.com/solder-wick-trick-characterizes-bypass-caps/ (https://www.edn.com/solder-wick-trick-characterizes-bypass-caps/)
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I have used some test PCBs to characterize parts by soldering them. Agree, it's a pain and having to insert the test board after changing components is hard on connectors. My balls are big enough to solder with the test board still attached to the test equipment.
I have created some boards with SMB connectors for such purposes. Quicker to connect / disconnect. But there are also plug-in SMA adapters. Recently I received a few of those.
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One of the nice things about these fixtures is they are 4 wire Kelvin type fixtures, so the cabling, connectors and such are effectively removed. The Force and Sense wires converge at the threaded rod tip just before it's screwed into the tapered plunger, so this leaves just the tapered end of the plunger from the thread rod not sensed on each plunger. When doing the LCR Open and Short Calibration this finishes up the fixture normalization task fairly well. Some of the lab grade LCR meters utilized SD ADCs (up to 24 bits I believe) and can resolve into the Giga ohm impedance magnitudes, so can accurately resolve small capacitances at low frequencies, this is what we needed for work as everything was high impedance.
Anyway, this little DE-5000 LCR meter has surely earned it's keep along side the two lab grade meters, one of the better instrument values around IMO.
Best,
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I have created some boards with SMB connectors for such purposes. Quicker to connect / disconnect. But there are also plug-in SMA adapters. Recently I received a few of those.
Have those as well, think just about everyone does, but way too much of pain to use, especially considering how repeatable these SDM fixtures behave when properly setup and utilized.
Best,
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Using the VNA to make shunt thru measurements is similar to your low frequency Kelvin type connections. The problem is the two ports of the VNA share a common reference which causes a loop and leads to errors at lower frequencies. At higher frequencies, the cables are lossy which takes care of the problem. One way to solve the ground loop problem is by placing a common mode transformer in-line with the measurement port. The problem is designing such a transformer.
You mentioned starting out using the tweezers. I have a set of Kelvin tweezers for my RLC meters. They are fine for larger surface mount parts. Not sure using them would be faster than the fixture you're using. Sounds like in your case, they were not. In one case, I guess you have to place the parts on a non-conductive work surface and clip on. The other, you have to insert them into the fixture. Certainly both options are faster than soldering.
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Many of the SMD were 0603 even 0402, had difficulty handing these small SDM with the OEM Tonghui Kelvin LCR tweezers, very tedious and error prone, and the cheap no-name types are really difficult. With the SMD fixtures you just pull the lever, remove the previous device (use small SS quality tweezer like Techni-Tool), grab and drop in the next SMD and get the measurement. The fixture Insulated "V" groove places the SMD in the correct position for the plunger to engage. On one of the SMD Fixtures we 3D printed a custom "V" groove insulator because the one provided wasn't working well with the smaller SMD components, this is the Grey V Insulator in the images above.
For low Z components like very small inductors (nh), the VNA is a better instrument, but our precision needs were Cs and Rs, not Ls. We even had to study the voltage effects & thermal (self heating) on such, may recall we developed a fixture to allow supplying an external bias voltage with the LCR fixtures, but that's another story!!
Regarding the transformer, yep that's a difficult problem, ran into a similar issue with the Open Loop Bode Plots using a transformer made from a CM Core.
Best,
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Also noticed the internal BNC grounds were not connected on one that had the star ground washers, so we corrected this by soldering in grounding wires.
Note that some LCR meters do not want all of the BNC barrels to be grounded and interconnected at the fixture, so check before you do this. HP's do, but Wayne Kerr comes to mind as one that does not.
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Also noticed the internal BNC grounds were not connected on one that had the star ground washers, so we corrected this by soldering in grounding wires.
Note that some LCR meters do not want all of the BNC barrels to be grounded and interconnected at the fixture, so check before you do this. HP's do, but Wayne Kerr comes to mind as one that does not.
Thanks, we did!!
Both of the Lab Grade LCR meters we have (Tonghui TH2830 & Hioki IM3536) all input BNC grounds are connected, and on the IM3536 they are also connected to the "Guard" Input, the TH2830 has no "Guard" Input.
Interestingly, the TH2830 has the BNC grounds also connected to the Mains Ground return, while the IM3536 does not, it's galvanically isolated.
Best,
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I received my test fixture a few days ago. It appears to be exactly the same product presented by mawyatt in previous posts (to be exact, the one described as the better of the two...)
As can be seen from the image, the right support is isolated from the case by means of two insulating washers.
This solution, in my opinion, has no reason to exist, given that it exposes the measurement system to inevitable interference from the surrounding environment. I will remove the two washers making sure the related support is electrically connected to the case.
What do you think?
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Unless the plunger assembly is different than what we have those bolt insulators should not be used. The plunger assembly has the center "shaft" insulated from the plunger casing with the white plastic coaxial pieces in each plunger assembly. The conductive plunger shaft sits inside the coaxial plastic surroundings, so the plungers are isolated from the plunger casing and aluminum box.
You want everything at ground potential inside and outside the box as best you can, this also means the plunger case and box "lid" where the attachment screws create a ground connection to the box. The box is at ground due to the 4 BNC connectors. Make sure the 4 leads (2 pairs) that go thru the box to the plunger are tight, these were loose on one fixture version we had and caused measurement repeatability issues. The leads are soldered to a single long threaded brass insert that screws into each plunger shaft, this was loose and needed tighting.
Also make sure everything is mechanically stable including BNC connectors. Having a stable electrical and mechanical environment as "seen" by the DUT is important for accurate and repeatable measurements.
Once you have reworked these inexpensive fixtures, they preform well we've found :-+
Best,
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Thanks a lot for your prompt reply.
As you suggest I'll remove the two insulating washers and I will check that everything is tightened properly...
I'll use this tool with my Fluke PM6304 (and may be with the ET432 too). To do this, I'm going to make a pair of interface adapters.
I measured the distance between the BNC connectors of this tool and it seems to be exactly 22.0mm (66.0mm exactly between the central pins of the two outermost connectors).
The doubt I have is the following: is this the standard in use (i.e. a pitch of 22.0mm) or rather the most used standard is the 7/8" (i.e. 22.225mm)?
When making interface adapters, to increase their compatibility with possible further tools, I would like to be sure to use the most commonly used pitch!
On your branded LCR analyzer could you please measure with a caliper the distance between the two outermost connectors and tell me if it is closer to 66.0mm or 66.7mm? I don't have analyzers that use this type of BNC interface (My LCR meter PM6304 uses a proprietary interface based on a Lemo connector...).
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I finished reworking the test fixture and the result is very satisfying.
Here are some of the upgrades made (some of them absolutely necessary!!):
1) The right nylon insulating cylinder was reworked (it had an anomalous hole in correspondence with the tightening knob which caused the short circuit between the measurement contact and the case...)
2) To avoid excessive mechanical play, I replaced the M3 screw of the lever with a more suitable M4 one
3) Replaced the rigid PVC internal cables with extra-flexible silicone sheathed cables
4) Replaced the four support rubbers with more suitable and professional products
5) Repainted the entire unit with two-component polyacrylic varnish (RAL 7035). I did this last operation at the same time as painting the containers used for the two interface cables.
For the pair of interface cables I used Hammond die-cast aluminum enclosures (1590G2) and Amphenol BNC connectors.
Now I have really stable and repeatable measurements with both LCR analyzers at my disposal.
Thanks to those who presented this low cost test fixture (with many defects but... all fixable).
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Very nice work!! Like the cables and the box approach with the handheld.
Must apologize for not responded to your question, completely missed it :-[
We also measure 66mm, and 22mm between BNC centers.
An area we've had some difficulty in making quick but repeatable measurements is with some SMD devices where the connection pads are on the bottom, common to some surface mount Polymer Caps, Tantalum and Inductors we use. These required some fiddling to get just right in the SMD fixture "V" groove, the "V" we 3D printed works better for these devices but still isn't as good as we would like.
Been thinking about this for awhile now, and liked the way the plungers Lever Arm worked with our custom Tek577 fixture shown below.
https://www.eevblog.com/forum/projects/smd-test-fixture-for-the-tektronix-576-curve-tracer/msg4553639/#msg4553639 (https://www.eevblog.com/forum/projects/smd-test-fixture-for-the-tektronix-576-curve-tracer/msg4553639/#msg4553639)
So we came up with a similar type fixture to use with these SMD devices and bench type LCR meters. PCB should be heading to fab house this week along with some other projects. This allows the plunger Lever Arm to be on either side and the PCB can be flipped over for another set of surface contacts for larger devices. For first try we'll just go with standard plating, and if it works well may consider gold plating for the contact area.
Best,
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Cables inside the sheath and box approach are absolutely necessary for repeatability of ac measurements (especially when measuring low impedances where the "weight" of the cables inductance is more evident...).
Thanks for your kind reply. For the two boxes I used the pitch measured on the test tool at my disposal, i.e. 22.0 mm. (I've been lucky!!)
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For the best results the cables should be low capacitance shielded types, where each signal (L-Cur, L-Pot, H-Pot, H-Cur) are within individual coaxial shield cables. Placing these 4 cables inside a sheath is a good idea and reduces measurement uncertainty due to cable movements. Using a box as the breakout for the BNC to fixture connectors provides better mechanical stability and thus better measurement repeatability.
As a side note, in the SMD test fixtures, when one does the "Open" calibration the calibration should be done with the fixture plunger spacing the same as the expected DUT. The reasoning is that the plunger fringe capacitance should be the same as when the DUT is evaluated, and the internal two wires on the plunger movement side (L-Cur and L-Pot) slightly move and the fringe capacitance to the fixture interior case (ground) slightly changes. Same goes for the calibration "Short", use a Zero Ohm Short of the same spacing as the expected DUT, here we usually use Zero Ohm SMD devices.
All good concepts for better repeatable measurements :-+
Best,
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The use of a box for bnc connectors has a big advantage: it prevents movements between the "Force" and the related "Sense" cables, thus avoiding altering the short circuit trim parameters.
However, I do not agree with the procedure you use for the "Short Circuit Compensation".
For this step you must not use any thickness between the measurement points, otherwise you lose an important parameter of the DUT which substantially coincides with the equivalent series inductance associated with its physical length. (as example, think to the series resonance of a capacitor... ).
More, during SC Trimming the effects of the fringe capacitance to the fixture interior case is absolutely negligible (due to the voltages near zero...).
Even leading companies like R&S and Keysight require short circuit trimming with the tips touching each other.
Personally, in some specific cases, in order to avoid including in the measurement the contact resistance between the tips and the DUT,
I carry out the short circuit trimming by interposing between the tips a thin layer of tin (of negligible thickness compared to the length of the DUT).
Different ideas that I hope will help both...
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As a side note, in the SMD test fixtures, when one does the "Open" calibration the calibration should be done with the fixture plunger spacing the same as the expected DUT. The reasoning is that the plunger fringe capacitance should be the same as when the DUT is evaluated, and the internal two wires on the plunger movement side (L-Cur and L-Pot) slightly move and the fringe capacitance to the fixture interior case (ground) slightly changes. Same goes for the calibration "Short", use a Zero Ohm Short of the same spacing as the expected DUT, here we usually use Zero Ohm SMD devices.
I was recently measuring some single digit pF 0402 caps on my CKT5000 LCR meter with Tonghui SMD fixture, and I also realised I needed to space the fixture tips in a consistent manner for the open calibration.
I used a small offcut of 0.030" (0.75mm) thick styrene sheet and got the high precision measurements that I expected for those tiny caps that I subsequently lost... :-DD
I didn't use a 0R SMD part for the short calibration as I thought the imperfections in that part and the extra contact resistances would be more significant than getting the spacing right - but I didn't waste time on testing that theory. :-DMM
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I have one of these test fixtures too, the cheapest I could find on Aliexpress, and with some tender love, it works quite well on my HP 4276A LCZ meter.
I improved the grounding inside, cleaned out the overspray from inside with acetone to allow proper contact to the case for the BNC's and the divider panel, tightened everything up, and stuck it all back together with stainless steel screws just for a bit more pretty.
I am wondering though, is it worth using coax (shield connected at the BNC end) for the internal wiring?
Maybe using some RG316 or similar. Would this help improve things, or not worth the effort?
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However, I do not agree with the procedure you use for the "Short Circuit Compensation".
For this step you must not use any thickness between the measurement points, otherwise you lose an important parameter of the DUT which substantially coincides with the equivalent series inductance associated with its physical length. (as example, think to the series resonance of a capacitor... ).
More, during SC Trimming the effects of the fringe capacitance to the fixture interior case is absolutely negligible (due to the voltages near zero...).
Even leading companies like R&S and Keysight require short circuit trimming with the tips touching each other.
Personally, in some specific cases, in order to avoid including in the measurement the contact resistance between the tips and the DUT,
I carry out the short circuit trimming by interposing between the tips a thin layer of tin (of negligible thickness compared to the length of the DUT).
Different ideas that I hope will help both...
Depends on what you are measuring, if tiny inductance then yes don't use a Zero Ohm Shunt SMD as the length invokes inductance, however if measuring small SMD resistors of small values then using the Zero Ohm Shunt SMD allows one to partially emulate the plunger contact pressure and resistance better.
Edit: Using Terman Formula as an estimate of the self inductance of a rectangular conductor of width and length of a SMD (est conductor thickness of 0.1mm) we get as an approximation:
0603 40pH
0805 54pH
1206 94pH
(Results similar with Grover formula for circular wire of dia as width and same length)
Contact pressure and resistance is a built-in issue with these plunger type SMD fixtures with the ones we have and varies quite a bit, especially with SMD length. When the SMD device is soldered into a PCB the entire end surface is electrically engaged, however when measuring with the fixture only the plunger tiny contact area is measured, so our thought was mimicking this with a same size Zero Ohm SMD would better compensate for such during the Short Calibration.
Of course YMMV.
Best,
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I have one of these test fixtures too, the cheapest I could find on Aliexpress, and with some tender love, it works quite well on my HP 4276A LCZ meter.
I improved the grounding inside, cleaned out the overspray from inside with acetone to allow proper contact to the case for the BNC's and the divider panel, tightened everything up, and stuck it all back together with stainless steel screws just for a bit more pretty.
I am wondering though, is it worth using coax (shield connected at the BNC end) for the internal wiring?
Maybe using some RG316 or similar. Would this help improve things, or not worth the effort?
Interesting thought, not sure if there would be much benefit for using coaxial cables, since the entire interior space is surrounded by a ground shielded case, thus any slight wire movement moves within a grounded enclosure and invokes a tiny shunt parasitic capacitance change. However, the coaxial cable would also move but the internal capacitance would likely change less than the open wire, so might be worth a try.
Best,
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As a side note, in the SMD test fixtures, when one does the "Open" calibration the calibration should be done with the fixture plunger spacing the same as the expected DUT. The reasoning is that the plunger fringe capacitance should be the same as when the DUT is evaluated, and the internal two wires on the plunger movement side (L-Cur and L-Pot) slightly move and the fringe capacitance to the fixture interior case (ground) slightly changes. Same goes for the calibration "Short", use a Zero Ohm Short of the same spacing as the expected DUT, here we usually use Zero Ohm SMD devices.
I was recently measuring some single digit pF 0402 caps on my CKT5000 LCR meter with Tonghui SMD fixture, and I also realised I needed to space the fixture tips in a consistent manner for the open calibration.
I used a small offcut of 0.030" (0.75mm) thick styrene sheet and got the high precision measurements that I expected for those tiny caps that I subsequently lost... :-DD
I didn't use a 0R SMD part for the short calibration as I thought the imperfections in that part and the extra contact resistances would be more significant than getting the spacing right - but I didn't waste time on testing that theory. :-DMM
What we use for the Open Calibration to get the spacing right is just an insert in the lever arm behind the plunger where the spring is between the plunger case and the lever arm. Just a Bourns plastic blue trimmer tool works well, slide back and forth to get the approximate desired DUT spacing. This keeps everything away from the plungers and they then produce the parasitic capacitance that gets normalized during the Open Cal. We also have achieved some good results with tiny SMD small valued capacitors, and get consistent repeatable results with quality C0G/NP0 capacitors using this technique.
Regarding the Short, Cal. Those Zero Ohm DUT imperfections, and the plunger contact variations is what we wanted to normalize during the Short Cal. Our feeling was (and certainly questionable) that these variations and SMD Zero Ohm imperfections were likely to mimic the actual small resistance DUT parasitic resistance under measurement than just doing a Short Cal with the plunger tips touching, of course as always YMMV.
Best,
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Just did a quick test using our IM3536 LCR meter, this meter can deliver higher DUT test currents and why we prefer it for low impedance measurements.
Using the cheap SMD Fixture measuring a 1206 1 ohm 1% resistor:
DMM6500 using Kelvin Clip leads 990.7738 mohms with 32 uohm SD
IM3536 with fixture direct plunger Short Cal 971.50 mohms
IM3536 with 1206 Zero Ohm Short Cal 981.85 mohms
Also a 1206 2.49 ohm 1% resistor:
DMM6500 2.489757 ohms
IM3536 direct Short Cal 2.46736 ohms
Im3536 using Zero Ohm 1206 Short Cal 2.48366 ohms
Think this shows that we shouldn't rely on the cheap SMD fixtures for low impedance measurements regardless of how the Short Cal is performed, altho in our case the use of a Zero Ohm Shunt seems to help a little.
If you study the fixture design and construction this should be no surprise as the "Kelvin" type connections don't actually make it to the plunger tips where ideally they should be, but are terminated at a long brass headed thin bolt that is screwed into the bottom of the plunger. So from the threaded bolt end to the plunger tip where the DUT makes contact is a single connection and has considerable impedance.
This is one reason we developed the Lever Arm based LCR meter fixture shown above which allows the "Kelvin" connections closer physically and electrically to the DUT, and the PCB test pad has a much larger surface area for the SMD DUT to get pressed against. Also you can create a higher contact pressure with the lever arm adjustment. We anticipate this will provide a better "fixture" for low impedance DUTs and physically larger SMD DUTs with improved contact impedance and area with better repeatability for low Z components. The PCBs went off for fabrication today, so we should have some answers soon.
Edit: As a note, we haven't been able to get good repeatable low impedance measurements with these cheap SMD fixtures and would not recommend considering them for low Z measurements. We've experienced considerable uncertainty with low Z measurements due to the above mentioned shortcomings, however they have provided excellent performance and repeatability with higher Z measurements.
Best,
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These test fixtures are not created with the aim of measuring very low impedance smd components.
In the Keysight one for example (whose cost exceeds a thousand dollars), the connection for the "sense" return path is made directly on the BNC connectors of the case, thus leaving uncompensated the part of the cable between the BNC and the mobile measuring cylinder. Evidently, their intention is to favor other aspects of the measurement tool...
If you wish to cancel the contact resistance, i.e. make it almost irrelevant by the pressure applied on the component, the only solution is to avoid the direct connection between the Force and Sense signals. If you haven't already done so, I would try to divide the PCB contact area with the component into two isolated sections, one dedicated to the force and the second to the sense. During measurement, the metallization of the DUT will connect them together...
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Have seen the mentioned Keysight fixture with the plate across the Force and Sense BNCs, never understood the thinking behind this.
Understand the issue of separating the Force and Sense contact points and trying to figure out if splitting the DUT contacts in half would help solve this and having the DUT make the Force and Sense connections, but decided against that for the PCB because of the issue how to do the Open Cal. Even considered using resistors between the Force and Sense that would allow an Open Cal and the DUT would short them out.
For now the cheap Fixtures have served well, but not applicable for low Z measurements, while hopefully the Lever Arm type fixture will perform better for low Z components.
Might be worthwhile to rethink this Force-Sense splitting for the next version, if we ever do another version.
Best,
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If your main goal is to measure low impedance components, the "open" calibration assumes a marginal role and in some cases can be avoided.
Anyway, it is always possible to use a high value resistor to perform the open calibration.
As example, if you intend to characterize resistors with a value lower than 10 ohms and you carry out the open cal using a 10 Mohm resistor, the induced error on the measurement should be less than 1 ppm...
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That's a good idea just using a much larger Z device that "bridges" the gap.
Curiosity got to me and I got a couple 100 ohm 1206 resistors and placed them in the "teeth" of some Kelvin Clips, this separates the Force and Sense functions by 100 ohms. Did a quick Open Cal on our TH2830 (can't use the IM3536 at this movement, fan makes too much noise and bothers family!!), then normal Short Cal and measured 0.1 and 1 ohm Dale 1% resistors (large aluminum cased type), and a Caddock 10K 0.01%. Then repeated this using a normal Open Cal.
This wasn't anything other than to "see" how the Open Cal responded to the Force and Sense being separated by 100 ohms, and suspected little variation, which is what we got!!
So now we got the "bug" to roll a version with separate Force and Sense lines to the DUT and shunted by a couple resistors. Since our PCBs are in the production "Que" at the fab over the weekend, we might sneak a quickly prepared version of the LCR Fixture PCB with separate Force and Sense DUT PADs. Maybe a version without the on PCB BNCs, where these would be in a box rather than on the PCB (thinking here is better mechanical stability when pushing the Lever Arm). Might even split the DUT PADs uneven to give emphasis to the Force side which handles the current.
Anyway, thanks for the dialog :-+
Best,
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This wasn't anything other than to "see" how the Open Cal responded to the Force and Sense being separated by 100 ohms, and suspected little variation, which is what we got!!
I have no idea what the input impedance of the sense line might be. If this were ideally infinite, the error that emerges by carrying out the open cal using two 100 ohm bridge resistors would essentially be linked to the capacitance of the measurement cable and should decrease reducing the test frequency.
If the discrepancy you observe is also present in the DC measurements, this means that the instrument's equivalent input resistive component has its weight!
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This wasn't anything other than to "see" how the Open Cal responded to the Force and Sense being separated by 100 ohms, and suspected little variation, which is what we got!!
I have no idea what the input impedance of the sense line might be. If this were ideally infinite, the error that emerges by carrying out the open cal using two 100 ohm bridge resistors would essentially be linked to the capacitance of the measurement cable and should decrease reducing the test frequency.
If the discrepancy you observe is also present in the DC measurements, this means that the instrument's equivalent input resistive component has its weight!
Exactly, and we got what was expected. Little variation in DC and a small variation at 10 and 100KHz due to capacitive (Kelvin cable) effects. Since the intent of these custom LCR fixture(s) (working on PCB layout of the split Force/Sense DUT Pad version now) is for low impedance devices and the fixture doesn't have the long cable and capacitance of the Kelvin Leads we expect the overall results to be better than our cheap LCR SMD fixture for low impedance SMD devices.
If this behaves as expected, then "tempted" to cannibalize one of the cheap LCR SMD fixtures to get those nice male BNCs with the lever arms (can't find these anywhere) and maybe use the fixture base with the plunger mechanisms removed as the base for the low impedance SMD fixture, or just mount the fixture PCB on top of a metal case with female BNCs and use BNC male to male adapters to interface with the LCR Meter female BNCs.
Anyway, we'll see how this progresses as time permits, as we have a "paying" project with PCBs in fab now which must take precedence.
Best,
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Update.
We got a change to try the split contact area concept for the Lever Arm type SMD fixture. Still needs some work, as this was just a kludge setup using a split BNC cables for the Force and Sense leads to the LCR meter. Later when time permits we may roll a direct BNC to LCR meter fixture with the split contact area.
Here's some results with low valued 2512 resistors using a zero ohm 2512 for short cal. on the TH2830, and for comparison using a SMD fixture on the IM3536. Number is () is repeated measurement by removing DUT and later reinserting such.
Type TH2830 IM3536
2512 1% 10mohm 9.949mohm (9.951) -9mohm (6.68) unstable readings
40mohm 39.840mohm (39.848) 21.38mohm (31.54) somewhat erratic
50mohm 49.993mohm (49.994) 42.09mohm (45.39)
100mohm 100.317mohm (100.332) 81.52mohm (93.57)
Getting good repeatable results is much easier, altho still a task, with the Lever Arm Fixture for these low valued resistors.
Here's what things look like and we'll be working with this later when time permits, and likely rolling a direct BNC connecting PCB.
Anyway, just an update, hope some folks find this interesting as we've found getting good repeatable low Z measurements with SMD is not trivial.
Edit: These are DCR measurements. Also no disrespect for the standard SMD fixture, these aren't intended for low Z measurements. If one studies the design the Kelvin type connections are terminated on a long thin brass bolt and screws into the bottom of the brass plated plungers, so everything after the thin brass bolt head is not part of the Kelvin measurement. This includes the plungers, and the plunger to DUT contact surfaces, which can only be compensated for by the Short Cal. Whereas the Lever Arm structure with the split contact area, the DUT contact "ends" make the Kelvin termination, and thus should improve results for low Z components, which apparently works is some cases as we've demonstrated.
BTW, if some folks want to participate we might roll the next PCB with gold contacts, which should improve contact area performance.
Best,
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You fixtures look absolutely identical to the Rohde and Schwarz LCX-Z3 SMD fixture I have... which is US$1700ish. Do you have a link to where to get yours? Would be very interesting to put both on the R&S ZNB VNA and see if there is any difference....
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The R&S is very likely a rebranded of either a Tonghui (TH26007 or 8 ) and other source SMD fixture. There are many sources available from AliExpress and range in $ from $70~200 with slightly different characteristics. Study the images as they have slightly different configurations and BNC interfaces, but any like below should work. We have two types, but this is the only one we can find we ordered, the other is so old we have no record.
https://www.aliexpress.us/item/3256801507452996.html?spm=a2g0o.order_list.order_list_main.426.758318022GzISR&gatewayAdapt=glo2usa4itemAdapt (https://www.aliexpress.us/item/3256801507452996.html?spm=a2g0o.order_list.order_list_main.426.758318022GzISR&gatewayAdapt=glo2usa4itemAdapt)
You may need to check inside the case on the AliExpress types, one of ours the thin brass bolt that attaches to the plungers was loose and giving erratic readings.
Best,
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Ok, this is getting really interesting indeed!!
While waiting for Amazon to deliver we decided to use the Hioki IM3536 with the kludge fixture above, 1m cables included. These cables are just 2 wire with shield cable we had, and have the wires to 2 BNCs and shield to BNC shell. The wires are not heavy just small #22~24 or so.
After some thinking about the Short calibration and attempting to remove the effects of DUT contact area, the method we were using was to use a Zero ohm 2512 SMD as the Zero Ohm DUT, however we have no means to "know" how close this SMD is to 0, and thus just "assumed" it to be 0.
Also remember we are using a split contact concept with the fixture to help deal with the SMD end terminations, and the Force and Sense lines are not brought together by the fixture, the idea is the DUT end terminations make the Force Sense connections on each end of the DUT. This helps reduce the effects of contact to the SMD device by making the DUT perform the Kelvin connection.
Back the unknown Zero ohm SMD, we decided to use a quality 1 milliohm 1% 2512 as the Zero Ohm DUT and do a short Cal with this "knowing" it's 0.001 ohms. Then we just add 0.001 ohms to the readings, or just RTFM and use the reference and scale functions, no time for the manual now tho.
So does this work? You bet, stunningly good we might add, absolutely superb repeatability in the milliohm regions!! Here's some additional 2512 1% low valued resistors measured with the Hioki IM3536, set to Rdc, 16 average, 0.1 ohm range. Repeatability is excellent too!!
Value (milliohms) Reading (milliohms) Added for DMM6500 (milliohms)
10 10.04 10.026
15 14.97 14.868
20 20.06 20.016
40 39.90 39.916
50 50.03 50.065
100 100.25 100.242
Later we'll shorten the leads, as they a much too long at over a meter, and maybe spend so time RTFM ;)
BTW this doesn't require a heavy scaled ultra low impedance fixture with massive terminals and wires, just some simple thin cable, few BNCs and a kludge PCB with a $1 Lever Arm used for holding flat items, a 2512 resistor is a flat item...so why not?? Well, we must thank Sir Lord Kelvin again :clap:
Anyway, if you have a quality lab grade LCR meter, might want to give this Lever Arm with Split Contact SMD Fixture a try, doesn't cost much and it just works :-+
We tried to measure these low ohm resistors with the nice Tonghui $200 tweezers, repeatability is hopeless below ~0.1 ohm.
Best,
Edit: added DMM6500 readings above, had to go from center point BNC to single banana on all 4 BNC centers and no ground connections. Another kludge setup but giving good repeatable results!!
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We have two types, but this is the only one we can find we ordered, the other is so old we have no record.
https://www.aliexpress.us/item/3256801507452996.html?spm=a2g0o.order_list.order_list_main.426.758318022GzISR&gatewayAdapt=glo2usa4itemAdapt (https://www.aliexpress.us/item/3256801507452996.html?spm=a2g0o.order_list.order_list_main.426.758318022GzISR&gatewayAdapt=glo2usa4itemAdapt)
Well, what model do you really use : Quad BNC to lock or Quad with only 2 to lock.
See pictures... as Ali comes up with two different models.
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We use both types with 2 and 4 locking BNCs. The one shown below with the 2 lock BNCs we replaced the plastic DUT positioner with a 3D printed version as the supplied plastic positioner didn't do an acceptable job with smaller SMD devices.
There are various types with 2 and 4 locking BNCs with different plastic positioners available and different cases, note the case ends where the tap holding the spring is part of the top lid or a seperate "L" shaped bolted to top lid.
Anyway, these are the two we have, and both types work fine with our TH2830 and IM3536 lab LCR meters, acceptable for our needs with the previous mentioned interior details, modifications, and such.
Best,
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In general with those fixtures, 4 tabs usually denotes a higher quality fixture. 2 tabs is usually generic / lower budget option.
However, some sellers on Ali show the 4 tab version in their ad, then send you the 2 tab version. Somebody tried that with me, and I filed a complaint with Ali and got a partial refund. My SMD fixture cost me less than $20. 😉
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Here's the final rendition of the Lever Arm SMD fixture. This is mounted on a cheap AliExpress LCR Kelvin Fixture with the Kelvin cables removed, and a small PCB mounted to the top lid as shown below. At first we were going to use coax from the BNCs to the PCB, but decided to just use unshielded wire since this is intended for larger SMD of the low impedance type and not high Z components, nor high frequencies.
The cheap fixture makes a good host for this, and the Lever Type BNCs are nice and why we wanted to mount the PCB on a fixture base with these type BNCs.
Here's some images of this version.
Best,
-
Couple more.
Best,
-
The R&S is very likely a rebranded of either a Tonghui (TH26007 or 8 ) and other source SMD fixture. There are many sources available from AliExpress and range in $ from $70~200 with slightly different characteristics. Study the images as they have slightly different configurations and BNC interfaces, but any like below should work. We have two types, but this is the only one we can find we ordered, the other is so old we have no record.
https://www.aliexpress.us/item/3256801507452996.html?spm=a2g0o.order_list.order_list_main.426.758318022GzISR&gatewayAdapt=glo2usa4itemAdapt (https://www.aliexpress.us/item/3256801507452996.html?spm=a2g0o.order_list.order_list_main.426.758318022GzISR&gatewayAdapt=glo2usa4itemAdapt)
You may need to check inside the case on the AliExpress types, one of ours the thin brass bolt that attaches to the plungers was loose and giving erratic readings.
So have this one now |O
1) The new series have thick painting as it gets
2) one BNC connector do not have any ground
3) the isolation / separation wall even too no grounding
4) only the connection shifting parts 4 screws have ground
Good:
- SMD parts remain now on the plastic parts as before, any artillery books for small parts to read :-+
- 4 BNC to nonnect
The measurements as using coax cable as from a different fixture/tweezers. The a single cable about 80pF (as equal to RG178, RG316).
A lower cable cap. and may shorter to use.
Measurements (currently without the fixing the grounding issue) using new calibrated my HANTEK 1833C:
1) 10K SMD 805 0.05% is almost at 100Hz/100kHz equal ... OK
2) 1pF +/- 0.25pF SMD 603 as 8..9pf as TH26009C tweezers ... NOK :palm:
So the question rises, what lowest cap value as in xx pF is possible accurate to read.
Hp
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That depends on how well you've configured your SMD fixture & setup. You need to make all those ground connections and make sure you have a stable mechanical as well as electrical environment. The case cover, all BNCs, the internal shield all need to be well grounded. Sounds as if you have some prep work ahead on the SMD fixture. We used a large drill bit to "clean" the cover holes for bare metal contact, as well as shield mounts.
Best repeatable results are with direct LCR connection, external coaxial cables add uncertainty, so keep them short, with quality stable cables, and when comparing measurements don't move the cables and/or fixture (as coaxial cables flex the internal capacitance can change). Also the E Fields around the open section of the fixture can affect readings on low cap values, just measure a 1pF 0805 C0G/NP0 and wave your hands around near the fixture top. This will convey how far the fields reach outside the fixture.
To direct answer your question, we have a 0.3pF & 4.7pF 0603 C0G/NP0 that we use with other caps as in-house references. For example, we measured with TH2830 0.3159pF, 0.3174pF (with DC Bias adapter), then another time 0.31614pF and DE-5000 (using BNC adapter) 0.32pF, then 4.6938pF for TH2830 and 4.6942pF with Hioki and on and on scattered throughout the years. All within a few FF over the years, so very good repeatability and instrument agreements IMO.
With all this in mind we have no problem achieving repeatability well below 1pF (actually into FF ranges for small values), as confirmed by our Tonghui TH2830 and Hioki IM3536, even achievable with short cables :-+
Anyway, the key message if one wishes repeatable measurements, then a stable mechanical and electrical environment is paramount ;)
Best,
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Also should mention, when doing the "Open Cal", use something to space the fixture plungers the same distant apart as they will experience when engaged with the DUT. We just use something that keeps the Lever Arm at the proper spacing, a pencil works!!
Best,
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Also should mention, when doing the "Open Cal", use something to space the fixture plungers the same distant apart as they will experience when engaged with the DUT. We just use something that keeps the Lever Arm at the proper spacing, a pencil works!!
I was just about to write this myself. But a pencil??? :-//
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So have this one now |O
Can you post a photo? How many retainer arms does it have? 2 or 4?
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That depends on how well you've configured your SMD fixture & setup. You need to make all those ground connections and make sure you have a stable mechanical as well as electrical environment. ....
Best repeatable results are with direct LCR connection, external coaxial cables add uncertainty, so keep them short, ....
Anyway, the key message if one wishes repeatable measurements, then a stable mechanical and electrical environment is paramount ;)
Yes, did some mechanical work, not at the best and now after open & close calibration things looks any better. Bingo!
Now the height of the two mechanical connections seams to be a bit off, as I had to open those parts.
Attached my copper work, so all grounding should now covered in once :D
Yes, the cables could be shorten, as not RG178 = silver conductor, current multi strand copper.
Thank you for your help
hp
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Interesting! You don't usually see the 4 retainer clip version for that price.
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That look good, just make sure the cover makes good contact with the main case. Also make sure the main case is making good contact with the copper surfaces.
Actually you don't need to go to this extreme, just making sure everything has a good electrical ground connection is sufficient, this effectively creates a somewhat like Faraday Shield around everything inside the case. You want all the parasitic capacitances to remain fixed, and a good mechanical structure keeps things from moving around.
You folks with these SMD fixtures should consider the DC Bias Adapter, kinda interesting to "see" how the DC Bias capacitance changes with ceramic caps, diodes and such!!
Good work and keep us up to date on your progress :-+
Best,
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Also should mention, when doing the "Open Cal", use something to space the fixture plungers the same distant apart as they will experience when engaged with the DUT. We just use something that keeps the Lever Arm at the proper spacing, a pencil works!!
I was just about to write this myself. But a pencil??? :-//
Yeah, works well. It just sits behind the Lever arm between the arm and the low side brass plated (silver) plunger housing. Anything will work as a spacer, since it's not electrically involved.
Best,
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Yeah, works well. It just sits behind the Lever arm between the arm and the low side brass plated (silver) plunger housing. Anything will work as a spacer, since it's not electrically involved.
OK, I was imagining the graphite being placed between the contacts... :o
I don't know if your original description made it clear otherwise.
On the topic of your DC bias adapter, I've got some of your PCBs on the bench (5pcs added onto a JLC order) but I've not yet reviewed the BOM or ordered the parts to make one up.
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Yeah, works well. It just sits behind the Lever arm between the arm and the low side brass plated (silver) plunger housing. Anything will work as a spacer, since it's not electrically involved.
OK, I was imagining the graphite being placed between the contacts... :o
I don't know if your original description made it clear otherwise.
On the topic of your DC bias adapter, I've got some of your PCBs on the bench (5pcs added onto a JLC order) but I've not yet reviewed the BOM or ordered the parts to make one up.
The DC Bias PCBs were designed to fit inside this type 65x110x50 enclosure (sawed in half). Use the latest files for the PCB, they have the BNC terminals correctly spaced and added the fast charge capability (and a few other minor changes) mentioned earlier. We haven't used the latest PCB yet, waiting on a PCB order to piggyback on, which should be soon!!
Anyway, hopefully this DC Bias Adapter works as well for you as it has for us!!
Best,
https://www.aliexpress.us/item/3256801465819165.html?spm=a2g0o.order_list.order_list_main.638.27e31802vJOM1n&gatewayAdapt=glo2usa4itemAdapt (https://www.aliexpress.us/item/3256801465819165.html?spm=a2g0o.order_list.order_list_main.638.27e31802vJOM1n&gatewayAdapt=glo2usa4itemAdapt)
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Good work and keep us up to date on your progress :-+
So on second tweak as |O while the connections wires are too thick, also shows the common 2mm diameter connection connector as one was loosely too. :palm:
Finally, last picture, RG178 inner silver wire as more flexible and in addition with torx screws for better alignment. Enlarged also inner holes to 4mm diameter as better to adjust.
May at last, shorten or use different connection cables :phew:
BTW: The Hantek uses for cap measurements only 2 wires and each time switched on, the open calibration is a requirement.
Hp
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It seems like those screws are always loose. I tightened them by unscrewing the parts so they could be turned without damaging the wires.
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Yes, one of ours was also loose and causing erratic readings. These look different than what we have, ours are just long thin brass bolts without the insulator. These look better with the insulator!!
BTW this image illustrates why getting repeatable low Z measurements is difficult with these type fixtures. The effective Force and Sense point is the top of the brass screw insert where the two wires are soldered, this leaves the entire electrical path from the solder connections to the plunger tip not sensed.
We developed another inexpensive DIY type SMD LCR fixture for this reason where the Force and Sense electrical connection is made by the DUT SMD terminal, this has shown promise with repeatable low milli-ohm SMD DUTs readings.
Best,
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It seems like those screws are always loose. I tightened them by unscrewing the parts so they could be turned without damaging the wires.
The loose part(s) are the cause of the not flexible and more harder cable (even after longer time as SMD size movements). In addition caused by the "Heat shrink tubing" as cable would not brake at the solder points.
The other question about the used Chinese Gold as may thin as it gets??
That's why I used the more flexible inner RG178 coax cable.
Hp
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@mawyatt
I would be interested in purchasing a PCB from you, especially with the gold contacts, if you move forward with the design. I'm in the US. Please let me know if you make them available.
Thanks,
gitm
Anyway, just an update, hope some folks find this interesting as we've found getting good repeatable low Z measurements with SMD is not trivial.
Edit: These are DCR measurements. Also no disrespect for the standard SMD fixture, these aren't intended for low Z measurements. If one studies the design the Kelvin type connections are terminated on a long thin brass bolt and screws into the bottom of the brass plated plungers, so everything after the thin brass bolt head is not part of the Kelvin measurement. This includes the plungers, and the plunger to DUT contact surfaces, which can only be compensated for by the Short Cal. Whereas the Lever Arm structure with the split contact area, the DUT contact "ends" make the Kelvin termination, and thus should improve results for low Z components, which apparently works is some cases as we've demonstrated.
BTW, if some folks want to participate we might roll the next PCB with gold contacts, which should improve contact area performance.
Best,
-
@mawyatt
I would be interested in purchasing a PCB from you, especially with the gold contacts, if you move forward with the design. I'm in the US. Please let me know if you make them available.
Thanks,
gitm
Anyway, just an update, hope some folks find this interesting as we've found getting good repeatable low Z measurements with SMD is not trivial.
Edit: These are DCR measurements. Also no disrespect for the standard SMD fixture, these aren't intended for low Z measurements. If one studies the design the Kelvin type connections are terminated on a long thin brass bolt and screws into the bottom of the brass plated plungers, so everything after the thin brass bolt head is not part of the Kelvin measurement. This includes the plungers, and the plunger to DUT contact surfaces, which can only be compensated for by the Short Cal. Whereas the Lever Arm structure with the split contact area, the DUT contact "ends" make the Kelvin termination, and thus should improve results for low Z components, which apparently works is some cases as we've demonstrated.
BTW, if some folks want to participate we might roll the next PCB with gold contacts, which should improve contact area performance.
Best,
More details over here for those interested.
https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/msg5061028/#msg5061028 (https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/msg5061028/#msg5061028)
If we can get a few folks in US interested (shipping overseas is too expensive and better for those folks to just direct order PCBs), we'll consider ordering a small batch of the PCBs.
Best,
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I wouldn't mind a PCB or two but I'm in Japan. Have you released design files or Gerbers etc?
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Here's some additional work we've done with these cheap SMD LCR Fixtures.
The DC Bias Adapter is a good addition and works well with all our Bench Type Lab Grade LCR Meter fixtures, however it is somewhat bulky, especially for use with SMD Fixtures.
Since we often work with SMD components, we decided to modify one of our 2 SMD Fixtures to allow direct External DC DUT Bias, similar to what the DC Bias Adapter does. To make this more useful want all the components inside the Fixture case.
Since this Bias function doesn't have to work with all the various LCR Attachments like Kelvin Clips, Tweezers, Leaded Component Fixture, and so on, and we aren't interested in working with very large value capacitors for example (this is a SMD fixture after all!!), keeping things simple and small was a priority.
The SMD fixtures are structured so the Force and Sense signals are terminated at the plunger midpoint, thus there is always a Force and Sense connection, whether a DUT is installed or not. This simplifies the DC bias setup and LCR Meter protection scheme.
We used a 10uF Mylar Film (250V) for the series Hcur decoupling and a 1uF Mylar Film (400V) for the Hpot decoupling. Meter protection is utilized on the Hcur and Hpot with back-to-back Zeners (24V) for Hcur and back-to-back diode isolated Zeners for Hpot. The diode isolation of the Zener reduces the apparent shunt capacitance as "seen" by signal and why utilized for Hpot Sense terminal. Hcur isn't affected by the Zener shunt capacitance, and so just back-to-back used.
One of our Lab Grade LCR Meters the TH2830 is rated for a Maximum Input Voltage at any terminal of +-42 DC Volts, so using 24V Zeners provides some safety margin. When doing a Short Cal you must bypass the DC obviously as the DC coupling capacitors provide DC isolation!!
After some quick tests we can verify this setup works very nicely with the TH2830, however the IM3536 is having an issue with Calibration which we'll investigate when time permits.
Edit: Problem solved, see #64 below.
Anyway, hope this is of interest to some folks.
Best,
-
Few more images.
Best,
-
After some quick tests we can verify this setup works very nicely with the TH2830, however the IM3536 is having an issue with Calibration which we'll investigate when time permits.
After some thought and more testing figured out the issue with the IM3536. Because it operates differently than the TH2830, the IM3536 requires a low impedance DC coupling on the Hpot terminal as well as the Hcur terminal. Using a 10uF for the Hcur works fine, however the 1uF on the Hpot terminal has too much impedance difference wrt to the Hcur and flags an error failure during Calibration.
Solution was to replace the 1uF with a 10uF on the Hpot terminal and problem solved :-+
BTW one of the issues with these cheap SMD fixtures with the "U" shaped bottom cover attached at the ends with just two bolts, the cover is too flexible and isn't mechanically secure as the ones with the cover attached on the end sides with 4 bolts. Why this matters is when measuring small capacitors (High Z) any physical change in the configuration and/or fixture (flex of the cover) can change the measurement. If the case flexes, then the interior fields change as the case/cover are the effective ground shield for the fixture interior and effect the measurement. These interior field effects are removed during Calibration but must remain stable during measurements.
Best,
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After some quick tests we can verify this setup works very nicely with the TH2830, however the IM3536 is having an issue with Calibration which we'll investigate when time permits.
After some thought and more testing figured out the issue with the IM3536. Because it operates differently than the TH2830, the IM3536 requires a low impedance DC coupling on the Hpot terminal as well as the Hcur terminal. Using a 10uF for the Hcur works fine, however the 1uF on the Hpot terminal has too much impedance difference wrt to the Hcur and flags an error failure during Calibration.
Solution was to replace the 1uF with a 10uF on the Hpot terminal and problem solved :-+
BTW one of the issues with these cheap SMD fixtures with the "U" shaped bottom cover attached at the ends with just two bolts, the cover is too flexible and isn't mechanically secure as the ones with the cover attached on the end sides with 4 bolts. Why this matters is when measuring small capacitors (High Z) any physical change in the configuration and/or fixture (flex of the cover) can change the measurement. If the case flexes, then the interior fields change as the case/cover are the effective ground shield for the fixture interior and effect the measurement. These interior field effects are removed during Calibration but must remain stable during measurements.
Best,
I arrived at this topic because I was looking for a SMD fixture for my super cheap Chinese component tester. Having read the thread it looks like the target resolution is zepto Farads with 4-wire sensors.
Rather than attempting to use split probes, which I suspect will be unreliable in practice, I propose coaxial probes. The sense wire would be old wire-wrap. The good-ole stuff is silver plated, single strand. It has a tough thin insulation. I think a better option than enamel wire.
The drawings show a probe tip drilled to allow wire wrap to be placed inside. The tip of the wire wrap would be soldered to just behind the face of the probe tip. This would place the sense wire very close to the smd component with very little opportunity for error. This would allow the cheap-as SMD fixture to be used by simply replacing the probes with the coaxial versions shown in the drawings.
I could make the coaxial probes but this modification would be wasted on me. I don't have the cheap-as smd fixture and I am waiting for the next firmware release for the Chinese component tester to measure down to yocto Farads. In the mean time it would be interesting to see someone make/try/test coaxial probes.
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That's an interesting idea, as it does move the connection point for the Force and Sense just inside the probe tip. This would remove most the impedance effects of the plunger, altho the resistance of plunger is very low as it's a solid chunk of plated brass. :-+
However, it doesn't help with the SMD DUT contact effects from the plunger tip to the DUT SMD end terminals. We've found this effect (DUT Terminals contact point) to be the most problematic in low Z SMD measurements and why we developed the Split-Kelvin SMD Fixture shown here:
https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/ (https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/)
Here the Force and Sense connections are made by the DUT end terminals, on the DUT side of the connection. This Split technique works very well down to milliohm Z levels with excellent repeatability as we've done extensive testing with thousands of measurements, also holds up for higher frequency measurements as we've just built another using shielded coax from the BNC connectors to the PCB connection as shown in the attached image.
We don't have the capability to machine a plunger like you've shown, would be interesting to see how well it works :)
Best,
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Is the clamp the Adafruit 2459?
Thanks,
Josh
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Got it off AliEx, there are variations. Get the one like shown in the image, note how the plunger is attached.
Best,
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I just measured your PCB, it looks like the $4 Adafruit 2459 should fit well.
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After some quick tests we can verify this setup works very nicely with the TH2830, however the IM3536 is having an issue with Calibration which we'll investigate when time permits.
After some thought and more testing figured out the issue with the IM3536. Because it operates differently than the TH2830, the IM3536 requires a low impedance DC coupling on the Hpot terminal as well as the Hcur terminal. Using a 10uF for the Hcur works fine, however the 1uF on the Hpot terminal has too much impedance difference wrt to the Hcur and flags an error failure during Calibration.
Solution was to replace the 1uF with a 10uF on the Hpot terminal and problem solved :-+
BTW one of the issues with these cheap SMD fixtures with the "U" shaped bottom cover attached at the ends with just two bolts, the cover is too flexible and isn't mechanically secure as the ones with the cover attached on the end sides with 4 bolts. Why this matters is when measuring small capacitors (High Z) any physical change in the configuration and/or fixture (flex of the cover) can change the measurement. If the case flexes, then the interior fields change as the case/cover are the effective ground shield for the fixture interior and effect the measurement. These interior field effects are removed during Calibration but must remain stable during measurements.
Best,
I arrived at this topic because I was looking for a SMD fixture for my super cheap Chinese component tester. Having read the thread it looks like the target resolution is zepto Farads with 4-wire sensors.
Rather than attempting to use split probes, which I suspect will be unreliable in practice, I propose coaxial probes. The sense wire would be old wire-wrap. The good-ole stuff is silver plated, single strand. It has a tough thin insulation. I think a better option than enamel wire.
The drawings show a probe tip drilled to allow wire wrap to be placed inside. The tip of the wire wrap would be soldered to just behind the face of the probe tip. This would place the sense wire very close to the smd component with very little opportunity for error. This would allow the cheap-as SMD fixture to be used by simply replacing the probes with the coaxial versions shown in the drawings.
I could make the coaxial probes but this modification would be wasted on me. I don't have the cheap-as smd fixture and I am waiting for the next firmware release for the Chinese component tester to measure down to yocto Farads. In the mean time it would be interesting to see someone make/try/test coaxial probes.
Let's see what you think of ST42 tweezers when I visit next weekend.
:popcorn:
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I just measured your PCB, it looks like the $4 Adafruit 2459 should fit well.
We've got a few of those shown in your image, they were Ok but the one like we've shown performed better. Note how the plunger attaches to the Lever Arm frame. Maybe get few of each and try them out, they are really cheap.
You need to mount the PCB on a BNC hosting fixture like shown. This was harvested from a cheap LCR Meter Kelvin Clip set also from AliEx. You want a rigid mechanical structure for the PCB to mount too. The PCB you have is an earlier version, doesn't have the "Guard" under the active DUT area, however that doesn't seem to matter much with lower Z measurements.
Think we also sent you some of the very early version PCBs (can't remember) which have the 4 BNCs directly mounted to the PCB, these worked well but we wanted a more rigid mechanical structure and prefer the latter version as shown.
Get yourself some low Z High Precision SMDs for checking things out, 2512 Resistors are a good start, and relatively easy to handle being large.
Best,
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We've got a few of those shown in your image, they were Ok but the one like we've shown performed better. Note how the plunger attaches to the Lever Arm frame. Maybe get few of each and try them out, they are really cheap.
Do you have any links to stuff you liked?
Get yourself some low Z High Precision SMDs for checking things out, 2512 Resistors are a good start, and relatively easy to handle being large.
I love 2512 resistors. That's what I usually use on my SMT guitar amp PCBs. Much less likely to go flying when I check them with a tweezers, and much easier to find when they do. 😉
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Do you have any links to stuff you liked?
Cheap Kelvin Clips for LCR Meter, toss everything except the case and BNCs, which we used.
https://www.aliexpress.us/item/3256804400699838.html?spm=a2g0o.order_list.order_list_main.90.503618020zePkv&gatewayAdapt=glo2usa (https://www.aliexpress.us/item/3256804400699838.html?spm=a2g0o.order_list.order_list_main.90.503618020zePkv&gatewayAdapt=glo2usa)
Preferred type Lever Arm & Plunger.
https://www.aliexpress.us/item/3256803387853373.html?spm=a2g0o.order_list.order_list_main.149.503618020zePkv&gatewayAdapt=glo2usa (https://www.aliexpress.us/item/3256803387853373.html?spm=a2g0o.order_list.order_list_main.149.503618020zePkv&gatewayAdapt=glo2usa)
Different type.
https://www.aliexpress.us/item/3256801104879812.html?spm=a2g0o.order_list.order_list_main.255.503618020zePkv&gatewayAdapt=glo2usa (https://www.aliexpress.us/item/3256801104879812.html?spm=a2g0o.order_list.order_list_main.255.503618020zePkv&gatewayAdapt=glo2usa)
You need to get a few of the plungers and go through them to find the better ones, being so cheap they are highly variable!! Try out both types, they both work, we just like the version shown slightly better.
Be sure and use an internal shield with the BNC case, especially if you don't use shielded wires to the PCB (like we did originally). We made one up with the shielded wires for higher frequency use, however the unshielded wires worked fine, OCD ;)
Note how you can flip over the plunger to use the standard supplied tip (black) for larger SDM DUTs.
Best,
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Thanks!
Your second link is the same as the first though.
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Fixed now!!
Best,
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That's an interesting idea, as it does move the connection point for the Force and Sense just inside the probe tip. This would remove most the impedance effects of the plunger, altho the resistance of plunger is very low as it's a solid chunk of plated brass. :-+
However, it doesn't help with the SMD DUT contact effects from the plunger tip to the DUT SMD end terminals. We've found this effect (DUT Terminals contact point) to be the most problematic in low Z SMD measurements and why we developed the Split-Kelvin SMD Fixture shown here:
https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/ (https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/)
Here the Force and Sense connections are made by the DUT end terminals, on the DUT side of the connection. This Split technique works very well down to milliohm Z levels with excellent repeatability as we've done extensive testing with thousands of measurements, also holds up for higher frequency measurements as we've just built another using shielded coax from the BNC connectors to the PCB connection as shown in the attached image.
We don't have the capability to machine a plunger like you've shown, would be interesting to see how well it works :)
Best,
Hi
From observation of your clamp-down design and information provided, it looks ideal for larger devices. I'd blatantly copy it.
My coaxial idea was intended to replace the existing probes for testing of smaller smd. The problems you found with contact points could potentially be solved using V faces that would cut through surface oxides to get to bare metal, in exactly the same way wire wrap connects to square posts. If the issue around DUT contact point variance could be solved, I suspect the coaxial probe would perform better than the standard probe. The sense signal would be shielded from noise and the parasitics between the DUT and the sense wire would be very small.
I do have the materials and capability to make prototype coaxial probes but I don't have the fixture, a quality LCR meter or the need to accurately measure component values. So it is likely the coaxial probes will remain a thought experiment.
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A larger design, but similar concept is the coaxial kelvin probes as used for battery testing etc.
I got some from Aliexpress but they are far too big for SMD, they are similar in design though.