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| Cheap SMD LCR Fixture, the Good, Bad and Ugly |
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| M Harris:
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.... |
| mawyatt:
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 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, |
| mawyatt:
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!! |
| hpw:
--- Quote from: mawyatt on September 28, 2023, 01:09:23 pm ---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 --- End quote --- 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. |
| mawyatt:
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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