Motorola GP series, Detailed Service Manual

Page: 126 / 494

1-14 THEORY OF OPERATION
7.2 Voltage Controlled Oscillator (VCO)
( Refer to Figure 2-6 and the UHF Voltage Controlled Oscillator schematic diagram)
The VCOB IC (U241), shown in Figure 2-6, 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.
Figure 1-8 UHF VCO Block Diagram
Presc
RX
TX
Matching
Network
Low Pass
Filter
Attenuator
Pin8
Pin14
Pin10
Level Shifter
Network
5V
(U201 Pin28)
VCC Buffers
TX RF Injection
U201 Pin 32
AUX4 (U201 Pin3)
AUX3 (U201 Pin2)
Prescaler Out
Pin 12
Pin 19 Pin 20
TX/RX/BS
Switching Network
U241
VCOBIC
Rx
Active Bias
Tx
Active Bias
Pin2
Rx-I adjust
Pin1
Tx-I adjust
Pins 9,11,17
Pin18
Vsens
Circuit
Pin15
Pin16
RX VCO
Circuit
TX VCO
Circuit
RX Tank
TX Tank
Pin7
Vcc-Superfilter
Collector/RF in
Pin4
Pin5
Pin6
RX
TX
(U201 Pin28)
Rx-SW
Tx-SW
Vcc-Logic
(U201 Pin28)
Steer Line
Voltage
(VCTRL)
Pin13
Pin3
TRB_IN
LO RF INJECTION