Voltage Controlled Oscillator (Vco) - Motorola GM328 Service Manual

2-8
4.3

Voltage Controlled Oscillator (VCO)

The Voltage Controlled Oscillator (VCO) consists of the VCO/Buffer IC (VCOBIC, U5301), the TX
and RX tank circuits, the external RX buffer stages, and the modulation circuitry.
Rx-SW
Tx-SW
(U5201 Pin 28)
Steer Line
Voltage
(VCTRL)
RX VCO
Circuit
RX Tank
Q5301
TX VCO
TX Tank
Circuit
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, to pin 32 of U5201 (PREIN).
A steering line voltage (VCTRL) between 3.0V and 10.0V at varactor diode CR5311 will tune the full
TX frequency range (TXINJ) from 450 MHz to 527 MHz, and at varactor diodes CR5301, CR5302
and CR5303 will tune the full RX frequency range (RXINJ) from 405 MHz to 482 MHz. The tank
circuits uses the Hartley configuration for wider bandwidth. For the RX tank circuit, an external
transistor Q5301 is used in conjunction with the internal transistor for better side-band noise.
Similarly, an external transistor Q5311 is used in the 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 Pin 28)
Figure 2-4 UHF VCO Block Diagram
UHF (450-527 MHz) Frequency Synthesis
AUX3 (U5201 Pin 2)
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
Q5332
Pin8
(U5201 Pin28)
Pin14
VCC Buffers
TX RF Injection
Pin10
Attenuator
Low Pass
Filter