Sensitive LCR Meter Measurements

[mawyatt]

Decided to post this after doing some evaluation measurements over here:

https://www.eevblog.com/forum/testgear/lcr-cal-test-pcb/

When doing sensitive High Z LCR measurements (for example, small C at low F) one must be aware of the impinging E fields.

Kelvin Clips like the TH26011CS and others generally do a good job but can fail at higher impedances often encountered during small capacitor measurements. If one studies the TH Kelvin Clip construction carefully, the entire length of the handle half is unshielded right to the clip tips. This allows these conductors to radiate and be susceptible to localized disturbances such as your hands, affecting any attempted quality measurements. Since most don't have "Calibrated Hands" this effect can't be totally eliminate by careful calibration unless one wants to glue their hands to the Kelvin Clip Handles

Tonghui specs the TH260011CS to 100KHz which seems reasonable, they have a version that specs higher but we don't have such so can't say what is different, altho likely the shields from the cable extends closer to the Kelvin Clip tips. There may be others from quality sources that also a specified to higher frequency use.

This is one reason we prefer the use of "dedicated" fixtures like the TH26048A and SMD types (TH26008) which attach directly to the LCR meter BNCs, since these have a shielded enclosure for the conductors with keeps most of the fields contained and repeatable.

The SMD Fixture types do have "exposed conductors" without shields in the SMD Plungers, altho the wiring to BNCs is all within the case shielding and the plungers are somewhat fixed just above the case ground shield so the E fields are sorta "bent" down towards the case cover shield. One can see the effects by moving your hand close to the plunger to observe the effects, however not nearly as sensitive as the TH Kelvin Clips mentioned.

There are also some nice SMD Tweezers cables like the TH26009B for use with LCR meters, these are rated to 15MHz and carry the shields right to the tweezer tips. 

There's also the issues at the opposite end of impedances like the low Z stuff, but we'll pause on that for the moment.

Anyway, we've known this for some time (shame on us for not posting earlier     ) but never considered posting until making the measurements mentioned, hopefully saving others from additional LCR measurement uncertainty.

Edit: When posting below we recalled an illustration that Hioki had which showed a conductive work base connected to the "Guard Terminal" on the LCR meter for sensitive measurements. We found our PCBite plate is conductive and connected it to the LCR Meter "Guard Terminal" as shown below. Results improved considerably wrt to TH260011CS Kelvin Clip hand location sensitivity with the Guarded Conductive Plate, altho close by the handle still showed high sensitivity as would be expected since the plate can't have much effect for the near field of the handle conductor within

Best,

[KungFuJosh]

So are we gonna add some shielding to our kelvin clips now? 😉

[mawyatt]

We're not!! Just going to be careful of use and understand the sensitivity of the measurements at hand

Later we might investigate Kelvin Clips with higher rated frequency use, but even then one is still stuck with the cable length effects, so best to just stay at low frequencies and be aware of the actual Kelvin configuration  limitations.

Honestly was surprised to find the Tonghui Clip handles totally unshielded, that was something we didn't know until evaluating your PCB with the 1pF test cap, and trying to determine the source on measurement uncertainty and noting how sensitive hand location was. I mean within ~100mm of the clips, and began to show effects as we would move our hand to write the measurement value only to see a different value on the IM3536!!

Operating on a conductive workbench plate would likely help, but we don't have such. Hioki has an illustration of using a conductive plate attached to the "Guard" terminal which we should probably do. Need to find such tho!! Edit: Found one, the PCbite base is conductive, added above!!

Best,

[KungFuJosh]

Did the plate help with the 1pF test?

[mawyatt]

Yep, sure did

See edit in OP.

Best,

[Sensorcat]

... the entire length of the handle half is unshielded right to the clip tips.

I'm not sure if I understand correctly: Does that mean the full path from the little box, where the big and the four small cable meet, to the tips, unshielded? If yes, that's quite disappointing! Raises the question if other manufacturers do better...

[mawyatt]

... the entire length of the handle half is unshielded right to the clip tips.

I'm not sure if I understand correctly: Does that mean the full path from the little box, where the big and the four small cable meet, to the tips, unshielded? If yes, that's quite disappointing! Raises the question if other manufacturers do better...

No, the 4 wires are shielded but at the Kelvin Clip handles where the wire enters the handle, the wires are not attached to any shield there, just at the box. The center conductor goes to the long metal "blade" that becomes the Kelvin Clip tip at the handle. So the entire length of there handle is not shielded and why your hand has such an influence on the measurement.

Best,