Service manual

CIRCUIT DESCRIPTION
6-12
Month 2000
Part No. 001-2004-601
line on the input and Q503 has a half-wave on the out-
put. These T-lines are used to drive the 60W amplifiers
out of phase. The output of the combiner is fed from
WO513 directly to the forward/reverse power detect
board.
The Wilkinson splitter and combiner provide the
capability to split the drive input and combine the
final outputs while maintaining isolation between the
two final amplifiers. The combiner consists of two
quarter-wave transmission lines and a balancing resis
-
tor. During normal operation, a signal of relatively
equal phase and amplitude is present on both ends of
the balancing resistor. Therefore, no current flows and
no power is dissipated in the balance resistor. If one
final failed, the other final of a pair would continue to
function.
6.3.4 POWER DETECTORS
The supply current is monitored through a resis-
tor that creates a current output level indicative of the
power output. The outputs of U503, U504 and U505
are monitored by the repeater software through the RF
Interface Board. If a final amplifier fails, the software
will reduce the output power to prevent over-driving
the remaining final amplifier.
6.3.5 THERMAL SENSOR
Thermal protection is provided by temperature
sensor U507. The operating range of the sensor is
from -30° C to 100° C (-22° F to 212° F). Amplifier
U506A sends the output of U507 through WO509 to
the RF Interface Board. The RFIB reduces the power
amplifier to half power (via the MPC) if the tempera
-
ture reading is too high and turns the fan on and off
(not via the MPC). The fan is turned on at approxi
-
mately 50°C and off again at 42°C.
6.3.6 FORWARD/REVERSE POWER DETECT,
CIRCULATOR, LOW-PASS FILTER
The power amplifier output is directly coupled to
the forward/reverse power detect board via a jumper.
The output then enters the circulator and exits to the
low-pass filter board and the antenna jack for a mini
-
mum power output of 110W at the default setting. If
an antenna is not connected, the circulator connects
the output power to R685.
Forward and reverse power are electromagneti-
cally coupled from the input and reflected ports of the
circulator. R663/R680 calibrate the forward and
reverse sense levels. The sensed levels are coupled to
the RF Interface Board and software.
6.4 RF INTERFACE BOARD
The RF Interface Board (RFIB) connects the
Receiver, Exciter and Power Amplifier to the back
-
plane and power supply (see Figure 6-7).
The input and output connectors for the RF Inter-
face Board are defined as follows.
Figure 6-6 110W POWER AMPLIFIER BLOCK DIAGRAM
POWER CONTROL
RF DETECT
PRE-DRIVER CURRENT
DRIVER CURRENT
FINAL 1 CURRENT
FINAL 2 CURRENT
TEMPERATURE SENSOR
U501
PRE-DRIVER
Q501
DRIVER
RF IN
U504
Q502
FINAL 1
FINAL 2
Q503
U505
AMP TEMP SENSE
U507U506A
U502
U503
U651A/B
FORWARD
POWER
U652A/B
REVERSE
(REFLECTED)
POWER
LPF RF OUTPUT
RF LOAD