CIRCUIT DESCRIPTION
6-9
September 1994
Part No. 001-2008-300
6.3.2 DRIVER (Q501)
The output of U501 passes through several sec-
tions of 50 ohm microstrip and matching capacitors to
the emitter of Q501. Driver Q501 is a common base
amplifier with a normal output of approximately 22W.
Supply voltage is RF bypassed by various capacitors
and microstrip. C519, C520, C521 and microstrip
match the output of the driver to 50 ohms at J501.
A501 couples driver output to the input splitter of the
finals.
6.3.3 FINAL AMPLIFIERS (Q502, Q503)
Q502 and Q503 are combined 60W amplifiers.
The 22W RF input from J501 on the output of driver
Q501 is applied to WO514 through a coaxial cable
and connector. A 50 ohm microstrip connects the RF
to a 70.7 ohm Wilkinson splitter and then to the emit-
ter of each common-base amplifier. The 60W outputs
on the collectors of the amplifiers are combined using
a Wilkinson combiner. A502 has a half-wave trans-
mission line on the input and Q503 has a half-wave on
the output. These transmission 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 resistor. During normal operation, sig-
nals 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 bal-
ance resistor. If one final failed, half the drive power
would be dissipated by the resistor and the other final
would continue to function.
6.3.4 POWER DETECTORS (U504A, U502B)
Electromagnetic coupling is used to sample the
output of each final amplifier. The RF is then fed to a
rectifier to create a voltage indicative of the power
output. The outputs of U504A and U502B are moni-
tored by the repeater software through the RF Inter-
face Board. If a final amplifier fails, the software will
reduce the output power to prevent overdriving the
remaining final amplifier.
6.3.5 THERMAL SENSOR (U503)
Thermal protection is provided by temperature
sensor U503. The operating range of the sensor is
from -30
°
C to 100
°
C (-22
°
F to 212
°
F). CR505 is
used to reference U503 above ground to allow the sen-
sor to read below 0
°
C. Amplifier U502A sends the
output of U503 through WO509 to the RF Interface
Board. The RF Interface Board 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.
Figure 6-5 POWER AMPLIFIER BLOCK DIAGRAM
RF IN
POWER CONTROL
DRIVER
THERMAL SENSOR
FINAL 2 POWER
FINAL 1 POWER
COMBINER
RF OUTPUT
LOW-PASS
FILTER
SENSE
POWER
PRE-DRIVER
AMP /
POWER
SENSE
U501
Q501
Q502
Q503
U502A
U503
THERMAL
SENSOR
AMP
RF LOAD
U651A/B
U652A/B
CIRCULATOR
2
2
SPLITTER
FORWARD
POWER
REVERSE
(REFLECTED)
POWER
FORWARD/REVERSE
POWER DETECTOR
U502B
U504A
