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Cheap SMD LCR Fixture, the Good, Bad and Ugly
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joeqsmith:
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.
mawyatt:
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,
precaud:

--- Quote from: mawyatt on November 23, 2022, 08:10:55 pm ---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.

--- End quote ---

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.
mawyatt:

--- Quote from: precaud on November 26, 2022, 02:43:16 pm ---
--- Quote from: mawyatt on November 23, 2022, 08:10:55 pm ---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.

--- End quote ---

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.

--- End quote ---

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,
ExaLab:
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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