Service manual

2-13
2.9.3 Harmonic Filter
The harmonic filter consists of components 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 is typically less than 1.2dB.
2.9.4 Antenna Matching Network
A matching network, made up of L3538 and C3537, is used to match the antenna impedance to the
harmonic filter. This optimizes the performance of the transmitter and receiver into an antenna.
2.9.5 Power Control Integrated Circuit (PCIC)
The transmitter uses 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 (0.1
ohms), which provides a voltage proportional to the current drain. The voltage is then fed back to the
automatic level control (ALC) within the PCIC to provide loop stability.
The PCIC also contains internal digital-to-analog converters (DACs) that provide the reference
voltage for the control loop. The voltage level is controlled by the microprocessor through the data line
of the PCIC.
The resistors and integrators within the PCIC, and external capacitors (C3562, C3563, and C3565)
control the transmitter rise and fall times. These are necessary to reduce the power splatter into
adjacent channels.
U3503 and its associated components act as a temperature cut back circuit. This provides the
necessary voltage to the PCIC to cut the transmitter power if the radio temperature gets too high.
2.10 VHF Receiver
The VHF receiver consists of a front end, back end, and automatic gain control circuits. A block
diagram of the VHF receiver is shown in Figure 2-11. Detailed descriptions of these features are
contained in the paragraphs that follow.