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, U4301), the TX
and RX tank circuits, the external RX buffer stages, and the modulation circuitry.
Rx-SW
Tx-SW
(U4201 Pin 28)
Steer Line
Voltage
(VCTRL)
RX VCO
Circuit
RX Tank
Q4301
TX VCO
TX Tank
Circuit
The VCOBIC together with Fractional-N synthesizer (U4201) generates the required frequencies in
both transmit and receive modes. The TRB line (U4301 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 U4301 pin 12 (PRESC_OUT), via a low pass filter, to pin 32 of U4201 (PREIN).
A steering line voltage (VCTRL) between 3.0V and 10.0V at varactor diode CR4311 will tune the full
TX frequency range (TXINJ) from 403 MHz to 470 MHz, and at varactor diodes CR4301, CR4302
and CR4303 will tune the full RX frequency range (RXINJ) from 358 MHz to 425 MHz. The tank
circuits uses the Hartley configuration for wider bandwidth. For the RX tank circuit, an external
transistor Q4301 is used in conjunction with the internal transistor for better side-band noise.
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
(U4201 Pin 28)
Figure 2-4 UHF VCO Block Diagram
UHF (403-470 MHz) Frequency Synthesis
AUX3 (U4201 Pin 2)
Prescaler Out
TRB IN
Pin 19 Pin 12
Presc
U4301
VCOBIC
RX
Rx
Tx
TX
Pin2 Pin1 Pins 9,11,17
Tx-I adjust
Rx-I adjust
U4201 Pin 32
LO RF INJECTION
Matching
Network
Q4332
Pin8
(U4201 Pin28)
Pin14
VCC Buffers
TX RF Injection
Pin10
Attenuator
Low Pass
Filter