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| Cheap SMD LCR Fixture, the Good, Bad and Ugly |
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| TERRA Operative:
I wouldn't mind a PCB or two but I'm in Japan. Have you released design files or Gerbers etc? |
| mawyatt:
Here's some additional work we've done with these cheap SMD LCR Fixtures. The DC Bias Adapter is a good addition and works well with all our Bench Type Lab Grade LCR Meter fixtures, however it is somewhat bulky, especially for use with SMD Fixtures. Since we often work with SMD components, we decided to modify one of our 2 SMD Fixtures to allow direct External DC DUT Bias, similar to what the DC Bias Adapter does. To make this more useful want all the components inside the Fixture case. Since this Bias function doesn't have to work with all the various LCR Attachments like Kelvin Clips, Tweezers, Leaded Component Fixture, and so on, and we aren't interested in working with very large value capacitors for example (this is a SMD fixture after all!!), keeping things simple and small was a priority. The SMD fixtures are structured so the Force and Sense signals are terminated at the plunger midpoint, thus there is always a Force and Sense connection, whether a DUT is installed or not. This simplifies the DC bias setup and LCR Meter protection scheme. We used a 10uF Mylar Film (250V) for the series Hcur decoupling and a 1uF Mylar Film (400V) for the Hpot decoupling. Meter protection is utilized on the Hcur and Hpot with back-to-back Zeners (24V) for Hcur and back-to-back diode isolated Zeners for Hpot. The diode isolation of the Zener reduces the apparent shunt capacitance as "seen" by signal and why utilized for Hpot Sense terminal. Hcur isn't affected by the Zener shunt capacitance, and so just back-to-back used. One of our Lab Grade LCR Meters the TH2830 is rated for a Maximum Input Voltage at any terminal of +-42 DC Volts, so using 24V Zeners provides some safety margin. When doing a Short Cal you must bypass the DC obviously as the DC coupling capacitors provide DC isolation!! 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. Edit: Problem solved, see #64 below. Anyway, hope this is of interest to some folks. Best, |
| mawyatt:
Few more images. Best, |
| mawyatt:
--- 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, |
| dazz1:
--- 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. |
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