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CIRCUIT DESCRIPTION
4-12
Part No. 001-3422-003
4.4.5 TRANSMITTER KEY-UP CONTROL
Q121, Q122 and Q123 act as switches which turn on with the RX_EN line. When the line goes low Q121 is
turned off, which turns Q122 on, turning Q123 on. This applies 13.6V to U111 before the TX_EN line goes high.
U111A/B provide the key-up and key-down conditioning circuit. C116 and R117 provide a ramp-up and
ramp-down of the 9V transmit supply during key-up and key-down which reduces load pull of the VCO during
key-up. The conditioning provides a stable 5.5V output by balancing the 5.5V reference with the 5.5V regulated
supply.
The output on U111B, pin 7 is applied to comparator U111D, pin 12, the non-inverting input. The output of
U111D, pin 14 is applied to the b ase of current source Q124. The output of Q124 is on the emitter and is applied
back to the inverting input of comparator U111D, pin 13. A decrease or increase on U111D, pin 13 causes a
correction by U111D to stabilize the 9V transmit output. R125/R126 establish the reference voltage on U111D,
pin 13. C123 provides RF bypass, C124 provides RF decoupling and C125 stabilizes the output. The 9V transmit
voltage is then distributed to the circuits.
4.5 VOLTAGE REGULATORS
4.5.1 +9.6 AND +5.5V REGULATED
The +5V applied on J201, pin 5 is applied to the base of Q131 turning the transistor on. This causes the
collector to go low and applies a low to the control line of U141, pin 1 and R131 is a pull-up resistor. The 13.6V
from J201, pin 2 is on U141, pin 6 to produce a +9.6V reference output on U141, pin 4. C145 stabilizes the
voltage and C146 provides RF decoupling. C144 provides RF bypass and C118 provides RF decoupling. C137 is
a bypass capacitor for U131.
The low from the collector of Q131 is also applied to the control line of U131, pin 1. C136 decouples RF and
R131 is a pull-up resistor. The 13.6V from J201, pin 2 is on U131, pin 6 to produce a +5.5V output on U131, pin
4. C135 stabilizes the voltage and C136 provides RF decoupling. C137 is a bypass capacitor for U131.