Voltage Controlled Oscillator (Vco) - Motorola GP Series Service Manual

2-8
4.2

Voltage Controlled Oscillator (VCO)

( Refer to Figure 2-5 and the 800 MHz Voltage Controlled Oscillator schematic diagram)
The VCOBIC (U250) in conjunction with the Fractional-N synthesizer (U201) generates RF in both
the receive and the transmit modes of operation. The TRB line (U250 pin 19) determines which
oscillator and buffer will be enabled. A sample of the RF signal from the enabled oscillator is routed
from U250 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 2.0V (low frequency) and 11.0V (high frequency) when the PLL is
locked on frequency.
The VCOBIC(U250) 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 Q200 and Q252 on the TRB line.
Rx-SW
Tx-SW
(U201 Pin 28)
VSF
Steer Line
Voltage
(VCTRL)
RX VCO
RX
Tank Circuit
TX TX VCO
Circuit
Tank
The operation logic is shown in Table 2-1.
Pin 20
Pin 7
Pin 13
Switching Network
Vcc-Superfilter
Pin 3
Collector/RF in
Pin 4
Pin 5 RX
Rx Active
Pin 6
Pin 16
TX
Tx Active
Pin 15
Vsens
Circuit
Pin 18
Vcc-Logic
VSF
(U201 Pin 28)
Figure 2-5 UHF VCO Block Diagram
Level Shifter
Network
TRB_IN
Pin 19
TX/RX/BS
Presc
U250
VCOBIC
RX
Bias
TX
Bias
Pin 2 Pin 1 Pins 9,11,17
Rx-I adjust
Tx-I adjust
THEORY OF OPERATION
5V
AUX3 (U201 Pin 2)
AUX4 (U201 Pin 3)
Pin 12 Prescaler Out
LO RF INJECTION
Pin 8
Injection
Amplifier
VSF
Pin 14
(U201 Pin 28)
VCC Buffers
Pin 10
TX RF Injection
Buffer
Amplifier
U201 Pin 32