P2102A USER: Hey Steve, I was super-excited to get my P2102A 2-port probe, but I am trying to figure out how to calibrate it for ultra-low impedance measurements. I seem to be getting bad results, can you help?
Steve Sandler: This is a great question, and I am happy to help solve this for you. A traditional Vector Network Analyzer (VNA) would require KNOWN Short-Open-Load (S-O-L) calibration for each port and a KNOWN THRU calibration. Since there is significant coupling in a probe, ISOLATION calibration is also required. The OMICRON Lab Bode 100 VNA does not support any of these calibrations, but we do have a few tricks to make this possible.
First, with the P2102A probe and J2102B common mode transformer connected in the traditional 2-port shunt-thru configuration, select the Shunt-Thru measurement from the Impedance Analysis tab and start a new measurement.
Figure 1 The Bode 100 measurement selection dialog. Select Shunt-Thru to perform the 2-port shunt-thru measurement.
Using the calibration board (P2100A-CAL) that comes with the probe, start a 2-port shunt-through measurement by setting the Source level to +13dBm, the Receiver 1 attenuator to 10dB, Receiver 2 attenuator to 20dB, and the Receiver bandwidth (RBW) to 30Hz, as shown below. You could set the Receiver bandwidth higher for the open and load calibrations, but it is easy to forget to reset it, so this really works best for me.
Note: the calibration procedure is independent of the probe head size chosen.
Figure 2 The Bode 100 dialog for configuring the AC sweep settings.
Perform the OPEN and LOAD calibrations using the calibration PCB or any other handy OPEN and LOAD calibrators. These are not critical for low impedance measurement.
Figure 3 The Bode 100 calibration dialog.
Reduce the Receiver 2 attenuator to 0dB.
Figure 4 Revised AC sweep dialog.
Place the probe on the ISOLATION pads (NOT the SHORT pad). Perform the SHORT calibration using the isolation pads.
Figure 5 P2100A-CAL calibration board that comes with the P2102A probe.
After completing all three calibrations, you can start making measurements using the probe.
Figure 6 Calibration dialog after completing the three calibrations.
First, as a check, I measure the post calibration results while still on the isolation pads. The impedance floor should be about 20uOhms and a few pH.
Figure 7 Isolation measurement after calibration.
Next, I measured the P2100A-CAL SHORT pad, and you can see the low frequency ground loop effect is still present, though greatly reduced by the J2102B. The SHORT pad resistance is 200uOhms and the SHORT pad inductance is about 200pH for this 1206 head.
Figure 8 Short measurement after calibration.
I also measure some parts whose impedance I have measured previously (a “known”). Preferably 1mOhm, 500uOhm, and 250uOhm resistors, whichever measured close to the nominal value.
Figure 9 1mOhm, 500uOhm, and 250uOhm sample measurements.
I also measured a mounted 1uF, 0805 chip capacitor. The capacitance measures 950nF and the ESL measures 211pH. This is slightly lower than the manufacturer’s datasheet reports, but certainly within 80-100pH.
This method is not perfect, but it does offer a simple and reasonably accurate calibration method that supports measuring sub-milliohms and sub-nH using the P2102A probe with the Bode 100 VNA. For other analyzers, the full 2-port calibration is still recommended.