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Theory of Operation
Synthesizer Detailed Functional Description
power delivered into the next stage. By controlling the
drive power to U6501 and the following stages in the
power amplifier lineup, ALC loop is able to regulate the
output power of the transmitter.
The antenna switch is switched synchronously with the
keyed 9.1 voltage. In the transmit mode, this 9.1 voltage
is high—turning on diodes CR6502, CR6503 and CR1.
When CR6502 is turned on, it forms a low impedance to
the RF transmit path and allows the signal to pass to the
antenna via harmonic filter hybrid H6502. Diode CR1 in
H6502 forms a low impedance that is transformed up to
an open circuit through a quarter wavelength transmis-
sion line. This prevents transmitter power from being
delivered into the receiver. Diode CR6503 is also turned
on in transmit mode further isolating the receiver from
transmitter energy. The transmitter power from the
antenna switch passes to the antenna via harmonic filter
hybrid module H6502. H6502 incorporates a low pass
filter, which attenuates harmonic energy out of the
transmitter to specifications level.
In receive mode, diodes CR6502, CR6403 and CR1 are all
off. The off capacitance of CR6502 is tuned by L6512 to
form a high impedance network looking into the trans-
mitter. Therefore, received signal energy from the
antenna is channeled to the input of the receiver with
minimal loss maximizing sensitivity.
A forward power detector utilizing a directional coupler
is included in the harmonic filter hybrid. This forward-
power detector is a microstrip printed circuit, which
couples a small amount of the forward energy to diode
CR2 where it is rectified. This rectified signal (VFOR-
WARD) forms the forward voltage which the power
control circuit holds constant. Holding this voltage con-
stant (which is proportional to the RF rectified energy
appearing across the diode) ensures the forward-power
out of the radio is held to a constant value.
Thermistor R6519 senses temperature in a location near
Q6505. This signal is fed back into the power control cir-
cuit to protect Q6505 against over-temperature condi-
tions.
Resistor R6520, in series with the A+ line supply, feeds
voltage to Q6505. The voltage across R6520 is monitored
and the difference voltage is channeled to the power
control circuit. The power control circuit monitors the
voltage drop across the resistor, which is determined by
the magnitude of the collector current in Q6505. It uses
this as a limiting mechanism whereby the power control
circuit limits the magnitude of current that can be drawn
by Q6505. This protects the device from over dissipa-
tion.
Bias voltage reverse polarity protection for the transmit-
ter is provided by diode CR6508. Under reverse polarity
conditions to the radio, this diode conducts and protects
the radio from damage. This diode also provides tran-
sient over-voltage protection by breaking down when
the supply voltage to the radio exceeds 24 VDC.
8-4
GTX LTR/Privacy Plus 800 MHz Mobile Service Manual
Power Control Circuitry
The power control circuitry consists of three mecha-
nisms:
Power Set
The control loop compares the power output level to
the pre-set value. The VFORWARD voltage from the
forward power detector is amplified by U701 pins 5 -7,
and compared to D/A U702-2 & 4. Should a difference
be detected, an error current is generated into the loop
integrator U702-8 & 9, until equilibrium is obtained.
The use of 2 D/A outputs for power set enables fine
tuning where pin 2 is coarse tune through resistor R703
and pin 4 is fine tune through resistor R706.
Current Limit
Current to the power module U6501 (or RF final device
Q6505) is monitored using R6520 and compared to a
pre-set limit D/A U702-11 and U701-(1-3). When the
limit is exceeded, an error current is generated through
diode CR721 to the loop integrator, which lessens the
PA_CNTRL line and reduces the power amplifier out-
put.
Thermal Protection
When the power module U6501 (or RF final device
Q6505) overheats, the resistance of thermistor R6519
drops. Because of the voltage divider formed by R721,
R725, and R6519, this resistance drop is sensed and
amplified through U701-(12-14). This causes a current
error through diode CR721 to flow into the loop inte-
grator and reduce transmitted output power.
When controller U101-45, detects a voltage propor-
tional to 85 degrees Centigrade through R726-727 it
reduces transmit power by 50 percent during subse-
quent Tx key-ups.
Synthesizer Detailed Functional
Description
The complete synthesizer subsystem consists of refer-
ence oscillator U5700, Fractional-N Synthesizer IC
U5702, two on-board voltage controlled oscillators,
buffer stage U5701, and two discrete buffer amplifiers
Q5707, and Q5705.
Reference oscillator U5700 contains a temperature
compensated crystal which operates at a frequency of
16.8 MHz. The output of the oscillator U5700-10 is
applied to U5702-14 via R5715 and C5733.
There are two varactor tuned VCOs. The oscillator fre-
quency is controlled by the voltage applied via L5701
and L5702. This control voltage ranges from 2.5 to 10.5
VDC. A lower control voltage produces a lower fre-
6880905Z99-O
June, 2000
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