Vco - Voltage Controlled Oscillator - Motorola GP328 Plus Service Manual

Frequency Generation Circuitry
6.2

VCO - Voltage Controlled Oscillator

(Refer to UHF Voltage Controlled Oscillator Schematic Diagram on page 5B-24)
Rx-SW
Tx-SW
(U201 Pin28)
VSF
Steer Line
Voltage
(VCTRL)
RX VCO
RX Tank
Circuit
TX VCO
TX Tank
Circuit
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.
Pin 20
Pin7
TX/RX/BS
Pin13
Switching Network
Vcc-Superfilter
Pin3
Collector/RF in
Pin4
RX
Active Bias
Pin5
Pin6
TX
Active Bias
Pin16
Pin15
Vsens
Circuit
Pin18
Vcc-Logic
VSF
(U201 Pin28)
Figure 5-5: VCO Block Diagram
Level Shifter
Network
Prescaler Out
TRB_IN
Pin 19 Pin 12
Presc
U241
VCOBIC
RX
Rx
Tx
TX
Pin2 Pin1 Pins 9,11,17
Tx-I adjust
Rx-I adjust
5V
AUX3 (U201 Pin2)
AUX4 (U201 Pin3)
U201 Pin 32
LO RF INJECTION
Matching
Network
Pin8
VSF
(U201 Pin28)
Pin14
VCC Buffers
TX RF Injection
Pin10
Attenuator
5B-11
Low Pass
Filter