Service manual

2-20
disconnecting the transmitter from the antenna while L103 and C147 form a series resonant circuit
effectively connect the receiver to the antenna.
2.13.3 Harmonic Filter
The harmonic filter consists of components C103, C106, C103, C107,C110, C111, C114, C115 and
inductors L100, L101, and L102 which are a part of the SH100 assembly. The harmonic filter for
lowband is pole zero design. This feature gives greater attenuation in low frequencies where the
harmonic energy of the transmitter is the greatest and less attenuation in high frequencies where
there is less harmonic energy. The harmonic filter insertion loss is typically less than 0.8 dB.
2.13.4 Antenna Matching Transformer
The antenna matching transformer (T100) matches the antenna impedance with the harmonic filter to
optimize the performance of the transmitter and receiver.
2.13.5 Power Control Integrated Circuit (PCIC)
The transmitter uses the PCIC (U102) to regulate the power output of the radio. To accomplish this,
the voltage across R102 is sensed. This voltage drop is directly proportional to the current drawn in
the final stage of the transmitter. This voltage is compared to a programmable reference inside the
PCIC and the voltage on PCIC pin 4 adjusted. Pin 4 connects to the PA driver IC (U101) pin 1 via
resistor R100 and varies RF output power of the driver. This controls the current drain of the final
stage and sets the output power.
2.13.6 Temperature Cut Back Circuit
Temperature sensor VR101 and associated components are part of a temperature cut back circuit.
This circuit senses the printed circuit board temperature around the transmitter circuits and outputs a
DC voltage to the PCIC. If the DC voltage produced exceeds the set threshold of the PCIC, the
transmitter output power decreases to reduce the transmitter temperature.
2.13.7 Electrostatic Discharge (ESD) Protection Circuit
The LDMOS PA device (Q100) is very sensitive to static discharge. To protect the device from ESD, a
protection circuit consisting of single high-speed Schottky Diode (D104) is connected from the
Antenna Nut (J102) to ground. This diode effectively shorts ESD energy to ground, but looks like an
open circuit to normal RF energy. The diode turns on when the voltage at the antenna nut exceeds
150V.
2.14 Low Band Receiver
The low band receiver consists of a front end, back end, and automatic gain control circuits. A block
diagram of the receiver is shown in Figure 2-16. Detailed descriptions of these stages are contained
in the paragraphs that follow.