Expanding Frequency Range in High-Power Splitter/Combiners by Minimizing Resistor Capacitance

Mini-Circuits’ ZACS242-100W+ is a coaxial high-power, 2-way 0° splitter/combiner capable of handling up to 100W RF input power as a splitter.  Its power handling capability makes this model a useful building block for signal distribution in high-power systems from 500 to 2400 MHz.  However, at frequencies above 2400 MHz, the component’s insertion loss performance degrades, hampering the splitter’s usability in higher-frequency applications.

This article will present a design modification of ZACS242-100W, which achieves a 50% bandwidth increase by reducing signal loss due to the capacitance of the chip resistors in the circuit.  The modification will be shown in a new model, ZACS362-100W+ which exhibits comparable power handling capability to that of its predecessor, but with low insertion loss up to 3600 MHz.  The same technique has been used to expand the frequency range of other high-power splitter models in Mini-Circuits line to support high-power applications at higher frequencies.

Bandwidth Constraint: Power Handling vs. Resistor Capacitance

The power handling capability of a power splitter is essentially determined by the power handling of the internal resistors.  The power handling of a resistor is proportional to its size; the higher the power, the larger the resistor.  Therefore, the resistors used in a 100W splitter/combiner will be relatively large. This is important to consider in regard to the effect of resistor capacitance on insertion loss.

ZACS242-100W+ utilizes four 100W chip resistors configured in as shown in Figure 1.  The conductive metallization the bottom of each resistor creates a capacitance to the PCB which can be expressed by the equation for the capacitance between two parallel plates:

In other words, assuming constant  and constant , the greater the area of conductive material on the bottom of the resistor overlapping the PCB, the greater the capacitance from the resistor.  The capacitance of the resistors shown in Figure 1 adds to the overall insertion loss through the circuit, especially at higher frequencies, and this is primarily what constrains the bandwidth in ZACS242-100W+.

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 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 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.

Figure 3: Comparison of insertion loss vs. frequency for old and new splitter/combiner designs.  Orienting the resistors vertically to the PCB achieves a 50% increase in rated operating bandwidth.

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, thereby significantly improving insertion loss performance at higher frequency.  The same technique has been used to expand the frequency of other high-power splitter/combiner designs in Mini-Circuits’ line, and new designs are now in development to offer similar high-power capability up to 6 GHz.