New-Tech Europe Magazine | May 2016

frequency verifies the improvement between ZACS242-100W+ in which the resistors are positioned flat against the PCB, and ZACS363- 100W+ in which the resistors are positioned vertically, orthogonal to the PCB. Figure 3 shows a comparison of insertion loss versus frequency for the old and new designs. While insertion loss for ZACS242- 100W+ degrades above 2400 MHz, the modified design in ZACS362- 100W+ achieves low insertion loss up to 3600 MHz, amounting to a 50% expansion in operating bandwidth. Both models provide 100W power handling as splitters, although whereas ZACS242-100W+ handles up to 40W RF power as a combiner, new model ZACS362-100W+ can handle up to 5W as a combiner. In all other respects, ZACS362-100W+ provides comparable performance to that of ZACS242-100W+ up to 3600 MHz with high isolation (22 dB typ.), and low phase and amplitude unbalance (1° and 0.15 dB, respectively). Conclusion The design technique presented in this article takes advantage of a basic principle of parallel plate capacitance to minimize the capacitance of resistors in a splitter/combiner circuit,

Figure 1: Board layout of ZACS242-100W+ with 4 100W chip resistors.

resistors in Figure 1 sit face-down, with the entire conductive surface flat against the PCB. While we cannot reduce the size of the resistor or the area of the conductive surface itself, we can reorient the resistor to the PCB to minimize the overlapping area of the parallel plates. The splitter/combiner was thus rebuilt with the resistors oriented 90° to the PCB as shown in Figure 2. Reorienting the resistors this way effectively reduces the resistor capacitance by more than 10 fold, which in turn significantly reduces the overall insertion loss at higher frequencies. Test data for insertion loss swept over

100W+. Expanding Bandwidth by Minimizing Resistor Capacitance To support high power requirements at higher frequencies, a design goal was set to extend the frequency range of ZACS242-100W+ while maintaining low insertion loss and comparable performance overall. One way to achieve this would be to reduce the capacitance from the resistors. We know that capacitance is a function of the overlapping area of conductive surfaces on the bottom of the resistor and on the PCB. Note that the

Figure 2: 100W Chip resistor oriented with conductive surface perpendicular to the PCB.

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