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| Cheap SMD LCR Fixture, the Good, Bad and Ugly |
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| mawyatt:
That's an interesting idea, as it does move the connection point for the Force and Sense just inside the probe tip. This would remove most the impedance effects of the plunger, altho the resistance of plunger is very low as it's a solid chunk of plated brass. :-+ However, it doesn't help with the SMD DUT contact effects from the plunger tip to the DUT SMD end terminals. We've found this effect (DUT Terminals contact point) to be the most problematic in low Z SMD measurements and why we developed the Split-Kelvin SMD Fixture shown here: https://www.eevblog.com/forum/projects/different-type-lcr-smd-fixture/ Here the Force and Sense connections are made by the DUT end terminals, on the DUT side of the connection. This Split technique works very well down to milliohm Z levels with excellent repeatability as we've done extensive testing with thousands of measurements, also holds up for higher frequency measurements as we've just built another using shielded coax from the BNC connectors to the PCB connection as shown in the attached image. We don't have the capability to machine a plunger like you've shown, would be interesting to see how well it works :) Best, |
| KungFuJosh:
Is the clamp the Adafruit 2459? Thanks, Josh |
| mawyatt:
Got it off AliEx, there are variations. Get the one like shown in the image, note how the plunger is attached. Best, |
| KungFuJosh:
I just measured your PCB, it looks like the $4 Adafruit 2459 should fit well. |
| tautech:
--- Quote from: dazz1 on September 01, 2024, 09:13:48 am --- --- Quote from: mawyatt on August 07, 2024, 01:37:11 pm --- --- Quote from: mawyatt on August 07, 2024, 03:26:14 am ---After some quick tests we can verify this setup works very nicely with the TH2830, however the IM3536 is having an issue with Calibration which we'll investigate when time permits. --- End quote --- After some thought and more testing figured out the issue with the IM3536. Because it operates differently than the TH2830, the IM3536 requires a low impedance DC coupling on the Hpot terminal as well as the Hcur terminal. Using a 10uF for the Hcur works fine, however the 1uF on the Hpot terminal has too much impedance difference wrt to the Hcur and flags an error failure during Calibration. Solution was to replace the 1uF with a 10uF on the Hpot terminal and problem solved :-+ BTW one of the issues with these cheap SMD fixtures with the "U" shaped bottom cover attached at the ends with just two bolts, the cover is too flexible and isn't mechanically secure as the ones with the cover attached on the end sides with 4 bolts. Why this matters is when measuring small capacitors (High Z) any physical change in the configuration and/or fixture (flex of the cover) can change the measurement. If the case flexes, then the interior fields change as the case/cover are the effective ground shield for the fixture interior and effect the measurement. These interior field effects are removed during Calibration but must remain stable during measurements. Best, --- End quote --- I arrived at this topic because I was looking for a SMD fixture for my super cheap Chinese component tester. Having read the thread it looks like the target resolution is zepto Farads with 4-wire sensors. Rather than attempting to use split probes, which I suspect will be unreliable in practice, I propose coaxial probes. The sense wire would be old wire-wrap. The good-ole stuff is silver plated, single strand. It has a tough thin insulation. I think a better option than enamel wire. The drawings show a probe tip drilled to allow wire wrap to be placed inside. The tip of the wire wrap would be soldered to just behind the face of the probe tip. This would place the sense wire very close to the smd component with very little opportunity for error. This would allow the cheap-as SMD fixture to be used by simply replacing the probes with the coaxial versions shown in the drawings. I could make the coaxial probes but this modification would be wasted on me. I don't have the cheap-as smd fixture and I am waiting for the next firmware release for the Chinese component tester to measure down to yocto Farads. In the mean time it would be interesting to see someone make/try/test coaxial probes. --- End quote --- Let's see what you think of ST42 tweezers when I visit next weekend. :popcorn: |
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