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Cheap SMD LCR Fixture, the Good, Bad and Ugly
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KungFuJosh:
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. 😉
mawyatt:
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,
mawyatt:
Couple more.

Best,
hpw:

--- Quote from: mawyatt on September 28, 2023, 01:09:23 pm ---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.

--- End quote ---

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



mawyatt:
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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