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| EC8010:
I stopped using leads like those shown when I discovered that the capacitance of their coaxial cables gets converted into 100nH of series inductance. Until I discovered that, I thought all electrolytic capacitors had an ESL of about 100nH. |O I've found the best way to measure components is to hang a die-cast aluminium box directly off the analyser's four BNCs and have the Kelvin clips on unscreened leads. I also got fancy and bolted a vertical steel bar down the middle of the back of the box. Why? If you cut two pieces of tin plate and fold a right angle on their ends, then a few neodymium magnets to hold the tin plate in place allows you to have a variable length screen between the two Kelvin clips. Provided the screen is adjusted to the right length, I find my stray capacitance is only about 5fF - allowing me to measure little capacitors accurately. My large box is 8" x 5" x 3" so I bolted it to the ends of a pair of 1/2" steel rods that go under the analyser to take the weight off the BNCs. |
| mawyatt:
Not sure "what leads" you are referring to? If it's those in post #21 & #25 that come out the side of the small fixture box, they are not coaxial and in fixed position within a sturdy small shielded enclosure that directly attaches to the LCR meter with the 4 BNCs. The H and L sides are shielded within the box as shown. Since these leads don't move when installing the fixture, nor when making measurements, their effects are somewhat removed by the proper Open/Short Calibration. The "trick" with this fixture is allowing the SMD DUT end terminals to make the actual Force and Sense connections for Hcur, Hpot and Lcur, Lpot respectively. This effectively removes the contact issues with SMD devices and has shown excellent repeatability with very small Z DUTs. This fixture works well up to 8MHz limit of our IM3536 we've found, even with low Z (or higher Z components). We developed this specialized fixture for low Z SMD components, and it easily outperforms our best Tonghui Kelvin Clips for SMD devices, especially with very low Z components. What will be interesting (if the custom PCBs ever arrive) is will driving the Guard section underneath the active PCB contact area (the IM3536 has Active Guard capability, which significantly improves Kelvin Clip use as previously mentioned) improves results, this area is vacant of any direct PCB conductor with the present custom PCB. Anyway, in our case this unique SMD fixture has produced excellent results and displaced the normal SMD LCR Fixture and Kelvin Clips for low Z SMD component measurements. Still use the regular SMD fixtures for higher Z DUTs and rarely employ Kelvin Clips for anything other than leaded components (which is what they are intended for). BTW curious as to your LCR Meter setups, please post some images :-+ Best, |
| EC8010:
--- Quote from: mawyatt on August 08, 2024, 05:43:21 pm ---Not sure "what leads" you are referring to? --- End quote --- I was thinking of the leads shown in the photograph in the first post. This is my large box. The Kelvin clips are Mueller. The Hameg measures R + jX at 68 frequencies from 20Hz to 200kHz (excluding mains frequency) and reports directly to a spreadsheet. Spreadsheet subtracts a S/C calibration run, or on high impedance stuff an O/C calibration run of 5fF in parallel with the instrument's noise floor. Spreadsheet then least squares fits a model to the curve, calculates manufacturer's uncertainties and adds error bars. So far, I've about six different models for capacitors and inductors. It's noticeable that tightening the two screws at the bottom to tightly close the lid reduces measured uncertainties due to hum. The two tin plate screens slide against one another to give variable length. |
| mawyatt:
That's a nice setup!! You've effectively eliminated most of the disturbances by placing the DUT inside the shielded enclosure. Only issue might be the variation in movement of the DUT and Kelvin Clips & Leads between the Open Cal and Changing DUTs since within the box you have parasitic capacitance in all directions to the shield. The movable shield is clever, can you maintain the DUT and Kelvin Clips leads positions well enough for pF measurements with good repeatability? Also understand your statement about a bar for support, that aluminum box looks pretty heavy hanging on those 4 BNCs!! Nice work :-+ Best, |
| EC8010:
Thank you. Yes, there's stray capacitance to the shield. Unavoidable. For the smaller box, I lined it with 10mm thick hard foam. No idea what the plastic is, but (being foam) it will have a nice low permittivity and prevent the DUT getting too close to the shield. I didn't have any more hard foam to use for the large box. Surprisingly, repeatability isn't a problem provided I'm careful not to let the movable shield get too close (<2mm) to the DUT and provided I arrange the DUT to hang (if its smallish) such that a little movement isn't a problem. The biggest problem is that the Mueller Kelvin clips don't always make a good contact - they have flat faces rather than corrugations to make their contact and their springs aren't all that powerful. The limiting factor for 1pF capacitances is actually the instrument itself. The impedance of 1pF at any frequency below 200kHz is quite high and that dominates the uncertainty at 1% (if I remember correctly). In general, the box allows the instrument to perform to its specifications. Which is nice. The box itself isn't heavy, but I often test big heavy lumps like transformers in there, perhaps weighing a kilo or two, which would definitely be unkind to the BNCs. Those M5 screws with penny washers go into the rods. |
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