Motorola GP540 Service Information - Page 19

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Frequency Generation Circuit

Battery Saver

In the receive mode, U3801 pin 19 is low or grounded. This activates the receive VCO by enabling

the receive oscillator and the receive buffer of U3801. The RF signal at U3801 pin 8 is routed

through a matching network. The resulting LO RF INJECTION signal is applied to the mixer at

T3302.

During the transmit condition, when PTT is pressed, 3.2 volts is applied to U3801 pin 19. This

activates the transmit VCO by enabling the transmit oscillator and the transmit buffer of U3801. The

RF signal at U3801 pin 10 is injected into the input of the PA module (U3501 pin16). This RF signal

is the TX RF INJECTION. Also in transmit mode, the audio signal to be frequency modulated onto

the carrier is received through U3701, pin 41.

When a high impedance is applied to U3801 pin19, the VCO is operating in battery saver mode. In

this case, both the receive and transmit oscillators as well as the receive, transmit and prescaler

buffer are turned off.

Table 2-1 Level Shifter Logic

Desired

AUX 4

Mode

n.u.

AUX 3

High (@3.2V)

Low

Hi-Z/Float

Hi-Z/Float (@1.6V)

(@1.6V)

TRB

2-9