Service manual

2-35
The 900 MHz transmitter contains the following basic circuits:
• power amplifier
• antenna switch
• harmonic filter
• power control integrated circuit (PCIC).
2.21.1 Power Amplifier
The power amplifier consists of two devices:
• 5185130C65 driver IC (U101) and
• 4813828A09 LDMOS PA (Q101).
The 30C65 driver IC contains a 2 stage amplification with a supply voltage of 7.5V.
This RF driver IC is capable of supplying an output power of 0.3W (pin 6 and 7) with an input signal of
2.5mW (4dBm) (pin16). The current drain would typically be 200mA while operating in the frequency
range of 896-941 MHz.
The 28A09 LDMOS PA is capable of supplying an output power of 4.5W with an input signal of 0.3W.
The current drain would typically be 1100mA while operating in the frequency range of 896-941 MHz.
The power out can be varied by changing the biasing voltage and the drive level from the driver IC.
2.21.2 Antenna Switch
The antenna switch circuit consists of two PIN diodes (CR101 and CR102), a pi network (C115, L109
and C138), and three current limiting resistors (R102, R103, R106). In the transmit mode, B+ at PCIC
(U102) pin32 will go high, applying a B+ bias to the antenna switch circuit to bias the diodes “on”. The
shunt diode (CR102) 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.
2.21.3 Harmonic Filter
The harmonic filter consists of L104, L105, C114, C115, C124,C125, and C126. It has been optimized
for efficiency of the power amplifier. This type of filter has the advantage that it can give a greater
attenuation in the stop-band for a given ripple level. The harmonic filter insertion loss is typically
0.9 dB, and less than 1.2dB.
2.21.4 Power Control Integrated Circuit (PCIC)
The transmitter uses the Power Control IC (PCIC), U102 to regulate the power output of the radio.
The current to the final stage of the power module is supplied through R101, which provides a voltage
proportional to the current drain. This voltage is then fed back to the Automatic Level Control (ALC)
within the PCIC to regulate the output power of the transmitter.
The PCIC has internal digital to analog converters (DACs) which provide the reference voltage of the
control loop. The reference voltage level is programmable through the SPI line of the PCIC.
There are resistors and integrators within the PCIC, and external capacitors (C156, C157, and C158)
in controlling the transmitter rising and falling time. These are necessary in reducing the power
splatter into adjacent channels.
U103 and its associated components are part of the temperature cut back circuitry. It senses the
printed circuit board temperature around the transmitter circuits and output a DC voltage to the PCIC.
If the DC voltage produced exceeds the set threshold in the PCIC, the transmitter output power will be
reduced so as to reduce the transmitter temperature.