Ericsson 19D902783G1, Руководство по обслуживанию, Страница: 50 / 92

DESCRIPTION
The UHF Power Amplifier Assembly is a wide band RF
power amplifier operating over the 380-400, 410-430, 403-
425, 425-450, 450-470, 470-494, and 492-512 MHz range
without tuning. Its main function is to amplify the 10 mW FM
signal from the Transmitter Synthesizer to the rated RF output
at the antenna port. The output of the Power Amplifier Assem-
bly is adjustable from rated power to 3dB lower at the PA
output J104.
The assembly consists of a printed wiring board (A1) and
associated components, including a power module and three
RF power transistors, mounted to the heat sink assembly. The
printed wiring board (A1) contains both the power amplifier
circuitry and the power control circuitry.
Unfiltered supply voltage, A+, for the power amplifier
circuits enters the assembly via feedthrough capacitor, C1.
Power cable W4 routes the A+ from C1 to J103 on the PWB.
Filtered A+ voltage for the power control circuit enters the
assembly via control cable W13 which connects to the PWB at
J201.
The Power Control circuitry sets the output power level by
adjusting the PA Power Set level. It keeps the output power
constant despite variations in input power, power amplifier
gain, or temperature through the use of a feedback control loop
in the PA assembly.
CIRCUIT ANALYSIS
POWER AMPLIFIER
The power amplifier section of the PA Board consists of
an Exciter, a Small Signal Gain Stage, a Low Level Amplifier,
a Driver, and the Power Amplifier Finals. All these gain stages
have an input and output impedance of 50 ohms. Figure 1 is a
block diagram showing the signal flow within the Power Am-
plifier Assembly.
Exciter (U101)
The Exciter stage uses a broadband silicon monolithic
microwave integrated circuit (MMIC) amplifier. The signal
from transmitter synthesizer, typically 10 dBm (10 mW), is
input to the Exciter through a 10 dB resistive pad (R1, R2, and
R31). The Exciter amplifies the resulting 0 dBm (1 mW)
signal to 12 dBm (16 mW).
The MMIC requires a 5 volt supply source. The 8 volt
regulator (U100) provides the 5 volts to the MMIC via a
dropping resistor R30.
Small Signal Gain Stage
The Small Signal Gain Stage consists of Q7 and its
associated bias and matching circuitry. Collector voltage is
fed through R39, R40, and L23. Resistor R33 sets the quies-
cent bias of the part. The transistor input impedance is
matched to the 50 ohm output of the Exciter by C59, C61,
C62, and C63. L24 provides the necessary output matching.
The stage provides 14 dB of gain to amplify the signal from
the Exciter to 26 dBm (400 mW).
Low Level Amplifier (U102)
The Low Level Amplifier (LLA) stage uses a 50 ohm
thick film RF Power Module to amplify and control of the
output power. Internally, the module is a three stage ampli-
fier. The power control circuitry controls the gain of the first
and second stages by varying the collector voltage level of
Q203. The third stage gain remains constant with A+ provid-
ing the DC supply voltage.
The signal from the Small Signal Gain stage, typically
26 dBm (400 mW), is input into the LLA. Under typical
Power Set conditions, the LLA amplifies the signal to a
typical output level of 40.5 dBm (11.2 W).
Driver (Q1)
The driver is a 6 dB RF amplifier consisting of transistor
Q1 and its associated circuitry. The signal from the LLA,
typically 40.5 dBm (11.2 W), is amplified to 46.5 dBm (45.0
W). The transistor input is matched to 50 ohms by C65, C66,
C27, C67, and a piece of printed transmission line. The drive
signal is then split with a printed in-phase Wilkenson splitter,
providing equal power to each of the final devices.
Power Amplifier Finals (Q2, Q3)
Each of the Power Amplifier Final devices is capable of
producing 5 to 6 dB of gain. The output signal from the
Splitter is impedance matched to each of the finals. Under
optimum conditions each final amplifies the input signal to
between 50 and 70 watts output power (depending on band
split). The outputs are then impedance matched to the input
of the Combiner. The Combiner is a printed in-phase Wilkin-
son type which combines (sums) the output power of the
finals. This produces an output power of approximately
100W, (depending on band split) which is coupled to the
directional coupler (part of A1 PWB) and on to the antenna
circuits. In addition, the directional coupler samples both
forward and reverse power and sends this sample to the
Power Control circuitry.
Copyright © July 1992, Ericsson GE Mobile Communications Inc.
Table 1 - General Specifications
ITEM SPECIFICATION
FREQUENCY 450 MHz - 470 MHz (G3)
403 MHz - 425 MHz (G6)
425 MHz - 450 MHz (G7)
380 MHz - 400 MHz (G8)
470 MHz - 494 MHz (G9)
492 MHz - 512 MHz (G10)
410 MHz - 430 MHz (G11)
OUTPUT POWER (RF) 65 watts - 130 watts (G3)
55 watts - 110 watts (G6, G7, G9, G10 & G11)
45 watts - 90 watts (G8)
INPUT POWER (RF) 10 mW min. into
≤
2:1 VSWR.
TEMPERATURE RANGE -30
°
C TO +60
°
C (Ambient air)
SUPPLY VOLTAGE 13.4 Vdc
CURRENT 29 Amps max. (26 A typical @ rated power, 13.4V) (G3)
29 Amps max. (21 A typical @rated power , 13.4V)(G6, G7, G9, G10 & G11)
29 Amps max. (20 A typical @ rated power, 13.4V) (G8)
DUTY CYCLE Continuous
STABILITY Stable into 3:1 VSWR; all temp.,voltage,freq. 55 watts - 110 watts (G3)
or 45 watts - 90 watts (G6, G7, G9, G10 & G11) or 45watts - 90 watts (G8)
RUGGEDNESS AT HIGH VSWR No damage into open or shorted load.
Figure 1 - Block Diagram
LBI-38674F
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