Motorola GP328 Plus Service Manual page 61

Transmitter
4.1.2 Antenna Switch
The antenna switch circuit consists of two PIN diodes (D3521 and D3551), a pi network (C3531,
L3551 and C3550), and two current limiting resistors (R3571, R3572, R3573 ). In the transmit mode,
B+ at PCIC (U3502) pin 23 will go low and turn on Q3561 where a B+ bias is applied to the antenna
switch circuit to bias the diodes "on". The shunt diode (D3551) shorts out the receiver port, and the pi
network, which operates as a quarter wave transmission line, transforms the low impedance of the
shunt diode to a high impedance at the input of the harmonic filter. In the receive mode, the diodes
are both off, and hence, there exists a low attenuation path between the antenna and receiver ports.
4.1.3 Harmonic Filter
The harmonic filter consists of C3532 to C3536, L3531 and L3532. This network forms a low-pass
filter to attenuate harmonic energy of the transmitter to specifications level. The harmonic filter
insertion loss should be less than 1.2dB.
4.1.4 Antenna Matching Network
A matching network which is made up of L3538 and C3537 is used to match the antenna's
impedance to the harmonic filter. This will optimize the performance of the transmitter and receiver
into an antenna.
4.1.5 Power Control Integrated Circuit (PCIC)
The transmitter uses the Power Control IC (PCIC), U3502 to control the power output of the radio by
maintaining the radio current drain. The current to the final stage of the power module is supplied
through R3519 (0.1ohms), which provides a voltage proportional to the current drain. This voltage is
then fedback to the Automatic Level Control (ALC) within the PCIC to keep the whole loop stable.
The PCIC has internal digital to analog converters (DACs) which provide the reference voltage of the
control loop. The voltage level is controlled by the microprocessor through the data line of the PCIC.
There are resistors and integrators within the PCIC, and external capacitors (C3562, C3563 and
C3565) in controlling the transmitter rising and falling time. These are necessary in reducing the
power splatter into adjacent channels.
U3503 and its associated circuitry acts as a temperature cut back circuitry. This circuitry provides the
necessary voltage to the PCIC to cut the transmitter power when the radio temperature gets too high.
5A-5