User Manual
CIRCUIT DESCRIPTION
4-8
June 1997
Part No. 001-3412-002
Q531 decreases the input voltage to U521 to lower the
power. The control voltage is isolated from RF by fer-
rite bead EP532 and C531 decouples RF.
The forward/reverse power voltages from
U581A/B are also applied to U913 for Diagnostic out-
puts on J201, pin 14.
The low-pass filter consists of C552, L551,
C553, L552, C554, L553, C555, L554 and C556. The
filter attenuates spurious frequencies occurring above
the transmit frequency band. The transmit signal is
then fed through the antenna switch to antenna jack
J501.
4.4.4 ANTENNA SWITCH (CR561, CR562)
The antenna switching circuit switches the
antenna to the receiver in the receive mode and the
transmitter in the transmit mode. In the transmit
mode, +9V is applied to L555 and current flows
through diode CR561, L561, diode CR562, and R561.
When a diode is forward biased, it presents a low
impedance to the RF signal; conversely, when it is
reverse biased (or not conducting), it presents a high
impedance (small capacitance). Therefore, when
CR561 is forward biased, the transmit signal has a
low-impedance path to the antenna through coupling
capacitor C562.
C583, L561, and C564 form a discrete quarter-
wave line. When CR561 is forward biased, this quar-
ter-wave line is effectively AC grounded on one end
by C564. When a quarter-wave line is grounded on
one end, the other end presents a high impedance to
the quarter-wave frequency. This blocks the transmit
signal from the receiver. C561/C562 matches the
antenna to 50 ohms in transmit and receive.
4.4.5 TRANSMIT 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 con-
ditioning 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 the non-
inverting input of comparator U111D, pin 12. The
output of U111D, pin 14 is applied to the base of cur-
rent source Q124. The output of Q124 is on the emit-
ter and is applied back to the inverting input of com-
parator U111D, pin 13. A decrease or increase at
U111D, pin 13 causes a correction by U111D to stabi-
lize the 9V transmit output. R125/R126 establishes
the reference voltage on U111D, pin 13. C123 pro-
vides 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 +5-16V 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 U131, pin 2 and R132 provides supply
voltage isolation. The 13.3V from J201, pin 2 is on
U131, pin 6 to produce a +5.5V reference output on
U131, pin 4. C132 stabilizes the voltage and C131/
C133 provide RF decoupling. C134 provides RF
bypass and C136 provides RF decoupling. C135
helps to stabilize the voltage when the +5.5V supply
first turned on.
The low from the collector of Q131 is also
applied to the control line of U141, pin 2. The 13.3V
from J201, pin 2 is on U141, pin 6 to produce a +9.6V
output on U141, pin 4. C144 provides RF bypass and
C146 provides RF decoupling. C145 helps to stabilize
the voltage when the +9.6V supply first turned on.