Motorola GP540 Service Information - Page 18
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2-8
4.2
Voltage Controlled Oscillator (VCO)
(Refer to Figure 2-5 and the UHF Voltage Controlled Oscillator schematic diagram)
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 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 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.
Rx-SW
Tx-SW
(U201 Pin28)
Steer Line
Voltage
(VCTRL)
RX VCO
RX Tank
Circuit
TX VCO
TX Tank
Circuit
Pin 20
Pin7
Pin13
Switching Network
Vcc-Superfilter
Pin3
Collector/RF in
Pin4
RX
Pin5
Active Bias
Pin6
TX
Active Bias
Pin16
Pin15
Vsens
Circuit
Pin18
Vcc-Logic
(U201 Pin28)
Figure 2-5 UHF VCO Block Diagram
Level Shifter
Prescaler Out
TRB_IN
Pin 19
TX/RX/BS
U241
VCOBIC
Rx
Tx
Pin2
Pin1
Pins 9,11,17
Tx-I adjust
Rx-I adjust
THEORY OF OPERATION
5V
AUX3 (U201 Pin2)
AUX4 (U201 Pin3)
Network
U201 Pin 32
Pin 12
LO RF INJECTION
Presc
Matching
Network
RX
Pin8
(U201 Pin28)
Pin14
VCC Buffers
TX
TX RF Injection
Pin10
Attenuator
Low Pass
Filter