Copyright 2005 ISC Technologies
4.1.3 Splitter
The splitter consists of three splitters to provide a four-way divider. Each splitter is a ninety
degree hybrid and a 50-ohm load. A ninety degree hybrid is a length of wire line in this
case. The length is determined by the frequency band of the power amplifier. The wireline
is a form of semi-rigid 50-ohm coax that has two center conductors insulated from each
other.
Within the useable bandwidth, each hybrid splits the RF power into approximately two
equal parts that differ in phase by about ninety degrees. As the split and phase differ, some
power is dissipated in the load resistors terminating the fourth port of the hybrids. When
power is reflected from any other port, the bulk of this reflected power (nominally half) is
dissipated in the load. The remaining power is split equally between the other two ports.
The process of splitting and combining is very phase sensitive. Incorrect phasing will result
in power wastage, increased heat through power dissipation, reduced MTBFs. and reduced
output power. As a result, all cables between the splitter and the finals, and the combiner
must be exactly the same length. The necessary phase adjustments for manufacturing tol-
erances are provided by the tuning capacitors in the finals.
4.1.4 Final
All final modules are identical, only one module is described. The final module amplifies the
output of the splitter to about 65 watts and feeds this to the combiner.
The final is a common emitter class C amplifier. The transistor’s (Q1) input impedance is
matched to 50 ohms across the frequency bandwidth using micro striplines Z2, and Z3,
with capacitors C2, C3, C17, C5, and C6. Power (28 Vdc) is fed via a faston tab to the
collector of transistor Q1 through a dc-coupling network. Matching of the transistor’s (Q1)
output is provided by Z4, Z3, C8, C9, C10, and C18. In addition, C12 fine tunes the output.
4.1.5 Combiner
Like the splitter, the combiner consists of three ninety-degree combiners providing a four-
way combiner. Each combiner is a ninety degree hybrids and a 50-ohm load. A ninety de-
gree hybrid is a length of wireline in this case. The length is determined by the frequency
band of the power amplifier. The wireline is a form of semi-rigid 50-ohm coax that has two
center conductors insulated from each other.
Within the useable bandwidth, each hybrid combines the approximately equal RF feeds
(each feed differs in phase by about ninety degrees), into one. As the phase differs, some
power is dissipated in the load resistor terminating the fourth port of the hybrid. The resis-
tive load’s most important function in the combiner is to dissipate the bulk of any reflected
power (nominally half). The remainder of the reflected power is split equally between the
other two ports (typically).
4.1.6 Low Pass Filter and Directional Coupler
The low pass filter attenuates the harmonics generated by the power amplifier. The direc-
tional coupler samples, and detects output power and reflected power. These signals are
then passed to the alarm logic. The output of the combiner is fed through the low pass filter
(C1, C2, C3, C4, L1, L2, and L3) and the directional coupler to the power amplifier’s output
connector. The directional coupler consists of three micro striplines Z1, Z2, and Z3 with
loads and detector circuits at both coupled ports. The directional coupler provides about -33
dB of coupling at the center frequency. Schottkey diodes (D1 and D2) are used for de-
tection of forward and reflected RF power.
