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30 DESIGN007 MAGAZINE I FEBRUARY 2020 Generic PCB fixtures, such as those shown in Figures 1 and 2, can be created from small, co- planar, 50-ohm traces that have exposed trace and ground next to each other on the same side of the fixture. The DUT can be connected between the trace and the ground shape, al- lowing us to use the two-port, shunt-through measurement topology. Having a sufficient- ly large solder-mask-free ground shape next to the trace allows us to accommodate a large number of different case styles and sizes with the same board. Having connectors at the ends of the through trace will allow for quick con- nections and disconnections, though we could also use permanently attached (soldered) ca- bles. Soldered connections would eliminate the need for separate cables with connectors at both ends, but would make the calibration a little bit more difficult. Figure 2 shows an unassembled panel of eight fixture boards that we can break away. Though the eight boards carry different labels, physically, they are the same. If we solder SMA female connectors to both ends, the fixtures will conveniently take cables with male connectors. The lines of the fixture are co-planar waveguide (CPW) over ground. The printed circuit material is FR-4, and the board thickness is 0.8 mm. The gold-plated nickel over copper is 35 µm (1 oz), and the line width is 1 mm with 0.254 mm separation to ground. As you can see from the measured TDR response of Figure 3, the coplanar traces are close to 50 ohms and have only a 175-ps delay, which means for a lot of measurements up to 10 MHz, a simple response through cali- bration is enough. If we want to start the sweep anywhere be- low a few times 10 kilohertz, and, at the same time, we also want to measure components that have low impedance at low frequencies, such as low-ESR high-capacitance parts, we run up against the cable-braid loop error [6] . De- pending on how we want to reduce the cable- braid loop error, the chosen solution may come with its own limitation at low or high frequen- cies. For the photo on the right in Figure 4, I used a homemade common-mode choke with an upper bandwidth of approximately 50 MHz. This setup data was collected in the 300 Hz to 30 MHz frequency range with a simple THRU calibration. Professional options for common- mode transformers for power-integrity mea- surement purposes are also available today [7] . The setup on the left in Figure 4 uses flexi- ble coaxial cables with low braid resistance [8] , which eliminates the need for a common-mode transformer, as long as the DUT impedance is not extremely low. With these fixtures, we also have the option of connecting the DUT in different ways. Me- chanically and electrically, we get the most ro- bust and most reliable connection if we sol- der or firmly clip the part to the fixture. If we want to re-use the fixture and speed up the Figure 2: Unassembled panel of RF experimenter boards from SV1AFN [5] . Figure 3: TDR response of a matched-terminated fixture shown in Figure 2.

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