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