Issue link: https://iconnect007.uberflip.com/i/1210212
FEBRUARY 2020 I DESIGN007 MAGAZINE 31 swapping of components, we can opt to use simple pressure mount; maybe, we can reduce the contact resistance and improve the consis- tency of our collected data by applying a dot of silver paste under the component terminals. If we decide to solder the component, we can improve the repeatability of the measurement by pushing down the parts on the pads dur- ing soldering. Then, we can make sure that the thickness of the solder layer between the com- ponent terminal and fixture pad is the possible minimum. In Figure 5, the two fixtures are shown with 1210-size ceramic capacitors soldered on them. The two setups in Figure 4 use slightly differ- ent settings. Though the 330-µF ceramic ca- pacitor has approximately 1 mOhm ESR, the impedance rises as we go toward lower fre- quencies. We also used cables with low-braid resistance; therefore, we did not need a com- mon-mode transformer. This allowed us to set the upper sweep limit to 100 MHz. Full two- port SOLT calibration was done with a Keysight mechanical cali- bration standard. The network analyzer gives us two-port S parameters. From the scattering matrix, we use one of the transfer parameters: S 21 or S 12 . They should be the same or very close to minor measurement er- rors. During the through calibra- tion, the 0 dB level of S 21 is set when Port 1 and Port 2 are di- rectly joined without a DUT. After calibration, from the measured S 21 value, we can calculate the Z DUT unknown complex impedance: In the next step, we take the imaginary part of the impedance and assume that it comes from capacitance or inductance. If the imaginary part of impedance comes from capacitance or induc- tance, we can use the following formulas, re- spectively: Here, w is the radian frequency, or 2 pF. You can apply both formulas simultaneously over the entire frequency and rearrange them for C and L. They will give the correct (pos- itive) capacitance and inductance values in their respective portion of the frequency range and will give negative results in the frequency ranges where the assumption about the nature of impedance is incorrect. Figure 6 shows the measurement result from the setups and DUT shown in Figures 4 and 5. We make use of the impedance analysis op- tion of the network analyzer, and we can set up the screen to show four simultaneous trac- es: impedance magnitude (upper left), ef- fective series resistance, or Rs (upper right); Figure 4: Measurement setup with Keysight E5061B network analyzer. The common-mode toroid transformer on the right photo is encapsulated in a small black plastic box. Figure 5: 1210 size MLCC parts soldered on the PCB fixtures (330-µF, 4V part on the left and 47-µF, 16V part on the right).