Motorola GP340 ATEX, Service Information

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2-8 THEORY OF OPERATION
4.2 Voltage Controlled Oscillator (VCO)
(Refer to Figure 2-5 and the Voltage Controlled Oscillator schematic diagram)
The VCOBIC (U241) in conjunction with the Fractional-N synthesizer (U201) generates RF in both
the receive and the transmit modes of operation. The TRB line (U241 pin 19) determines which
oscillator and buffer will be enabled. A sample of the RF signal from the enabled oscillator is routed
from U241 pin 12, through a low pass filter, to the prescaler input (U201 pin 32). After frequency
comparison in the synthesizer, a resultant CONTROL VOLTAGE is received at the VCO. This
voltage is a DC voltage between 3.5V and 9.5V when the PLL is locked on frequency.
The VCOBIC(U241) is operated at 4.54 V (VSF) and Fractional-N synthesizer (U201) at 3.3V. This
difference in operating voltage requires a level shifter consisting of Q260 and Q261 on the TRB line.
The operation logic is shown in Table 2-1.
Figure 2-5 VCO Block Diagram