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Cheap SMD LCR Fixture, the Good, Bad and Ugly

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mawyatt:
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,

ExaLab:
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...

Kean:

--- Quote from: mawyatt on August 31, 2023, 03:53:23 pm ---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.

--- End quote ---

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

TERRA Operative:
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?

mawyatt:

--- Quote from: ExaLab on September 01, 2023, 09:50:43 am ---
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...

--- End quote ---

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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