Motorola Professional GP680 Service Information - Page 17

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Frequency Generation Circuitry

Battery Saver

In the receive mode, U241 pin 19 is low or grounded. This activates the receive VCO by enabling the

receive oscillator and the receive buffer of U241. The RF signal at U241 pin 8 is run through a

matching network. The resulting RF signal is the LO RF INJECTION and it is applied to the mixer at

T302.

During the transmit condition, when PTT is depressed, five volts is applied to U241 pin 19. This

activates the transmit VCO by enabling the transmit oscillator and the transmit buffer of U241. The

RF signal at U241 pin 10 is injected into the input of the PA module (U101 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 the U201 pin 41.

When a high impedance is applied to U241 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

Low

High

Low

AUX 3

High (@3.2V)

High (@4.8V)

Low

Hi-Z/Float (@2.5V)

TRB

2-9