Motorola GP580 Service Information - Page 12

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2-2

The LDMOS PA is capable of supplying an output power of 7W with an input signal of 0.3W. The

current drain is typically around 1800mA while operating in the frequency range of 136-174MHz.

The power output is varied by changing the bias voltage.

2.2

Antenna Switch

The antenna switch circuit consists of two pin diodes, D3521 and D3551, a pi network (C3531,

L3551 and C3550), and three current limiting resistors (R3571, R3572, R3573). In the transmit

mode, B+ at PCIC (U3502) pin 23 goes low to 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, creating a low attenuation path between the antenna and receiver

ports.

2.3

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 is typically less than 1.2dB.

2.4

Antenna Matching Network

A matching network made up of L3538 and C3537/C3539 is used to match the antenna impedance

to the harmonic filter. This optimizes the performance of the transmitter and receiver into the

antenna.

2.5

Power Control Integrated Circuit (PCIC)

The transmitter uses the 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,

which provides a voltage proportional to the current drain. This voltage is then fedback 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 of 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.

THEORY OF OPERATION