The VCOB IC (U241), shown in Figure 2-5, 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 are enabled. A sample of the RF signal from the enabled
oscillator is routed from U241, pin 12, through a low pass Þlter, 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 VCOB IC 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.
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