Voltage Controlled Oscillator (Vco) - Motorola GM328 Service Manual

2-8
4.3

Voltage Controlled Oscillator (VCO)

The voltage controlled oscillator (VCO), shown in Figure 2-4, consists of a VCO buffer IC (VCOBIC,
U5301), TX and RX tank circuits, external RX buffer stages, and modulation circuits.
The VCOBIC together with Fractional-N synthesizer (U5201) generates the required frequencies in
both transmit and receive modes. The TRB line (U5301, pin 19) determines which tank circuits and
internal buffers are to be enabled. A high level on TRB enables TX tank and TX output (pin 10), and
a low enables RX tank and RX output (pin 8). A sample of the signal from the enabled output is
routed from U5301, pin 12 (PRESC_OUT), via a low pass filter of U5201, pin 32 (PREIN).
A steering line voltage (VCTRL) between 3.0V and 10.0V at varactor diode CR5311 tunes the full
TX frequency range (TXINJ) from 450 MHz to 520 MHz, and at varactor diodes CR5301, CR5302,
and CR5303 tunes the full RX frequency range (RXINJ) from 405 MHz to 475 MHz. The tank circuits
uses the Hartley configuration for wider bandwidth. For the RX tank circuit, external transistor
Q5301 is used in conjunction with the internal transistor for better side-band noise.
Rx-SW
Tx-SW
(U5201 Pin28)
Steer Line
Voltage
(VCTRL)
RX VCO
Circuit
RX Tank
Q4301
TX VCO
TX Tank
Circuit
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
(U5201 Pin28)
Figure 2-4 UHF VCO Block Diagram
UHF (450-520 MHz) Frequency Synthesis
AUX3 (U5201 Pin2)
Prescaler Out
TRB IN
Pin 19 Pin 12
Presc
U5301
VCOBIC
RX
Rx
Tx
TX
Pin2 Pin1 Pins 9,11,17
Tx-I adjust
Rx-I adjust
U5201 Pin 32
LO RF INJECTION
Matching
Network
Q4332
Pin8
(U5201 Pin28)
Pin14
VCC Buffers
TX RF Injection
Pin10
Attenuator
Low Pass
Filter