Motorola MTR2000 T5544 Instruction / Field Service Manual page 308

VHF External Double Circulator Module
5
FUNCTIONAL THEORY OF OPERATION
The following theory of operation describes the operation of the Dual Circulator Assembly at a functional level.
The information is presented to give the service technician a basic understanding of the functions performed by
the double circulator in order to facilitate maintenance and troubleshooting to the module level. Refer to Figure 4
for a block and interconnect diagram of the Double Circulator assembly.
Functional Operation
Transmit RF
From Power
Amplifier Module
Figure 4. Functional Block and Interconnect Diagram for External Double Circulator Module
8
The double circulator assembly accepts transmit RF output power from
the Power Amplifier module and provides 45 dB of isolation between the
Power Amplifier module and the transmit antenna. The assembly consists
of two circulators in a single case, each with a 50Ω load. Each circulator
allows forward RF energy to pass through to the output, while routing any
reflected RF energy to the corresponding 50Ω load.
Most of the reflected energy is absorbed by the 50Ω load (heat sink)
mounted) connected to the second circulator. A thermistor mounted on
the head sink provides a variable resistance signal proportional to the heat
sink temperature. This signal is routed to the Station Control Module via
the Temp Sense cable and backplane. If the heat sink temperature exceeds
a preset threshold, the Station Control Module enables PA cutback mode.
Double Circulator
Circulator Circulator
50 Ohm
Load
50 Ohm Thermistor
Load 20K @25°C
T°
Heat Sink
Low Pass Filter
RF Output
To Transmit
Antenna
To Station
Backplane
68P81096E49-C
09/30/05