Voltage Controlled Oscillator (Vc0) - Motorola CDM1550 LS+ Detailed Service Manual

Theory of Operation 2-23
2.6.3 Voltage Controlled Oscillator (VCO)
The Voltage Controlled Oscillator (VCO) consists of the VCO/Buffer IC (VCOBIC, U3301), the TX
and RX tank circuits, the external RX VCO and buffer stages, and the modulation circuits.
AUX3 (U3201 Pin2)
TRB IN Prescaler Out
Pin 20 Pin 19
~-SW~~~~P~in~7------~----~~~~--~~
TX/RXIBS
Tx-SW Pin13 Switching Network
~~----~~~----~
U3301
VCOBIC
Pin 12
.----~
U3201 Pin 32
LO RF INJECTION
Pin2 Pin1 Pins9,11,17
Rx-1
adju st Tx-l adjust
(U3211 Pin1)
Figure 2-10. 200 MHz VCO Block Diagram
The VCOBIC together with the Fractionai-N synthesizer (U3201) generates the required frequencies
in both the transmit and receive modes. The TRB line (U3301, pin 19) determines which tank
circuits and internal buffers are to be enabled. A high level on TRB enables the TX tank and TX
output (pin 1 0), and a low enables the RX tank and RX output (pin 8). A sample of the signal from
the enabled RF output is routed from U3301, pin 12 (PRESC_OUT), via a low pass filter, to U3201,
pin 32 (PREIN).
A steering line voltage (VCTRL) between 2.5 volts and 11 volts at varactor diode 03361 tune the full
TX frequency range (TXINJ) from 217 MHz to 222 MHz, and varactor diode 03341 tunes the full RX
frequency range (RXINJ) from 262 MHz to 267 MHz. The RX tank circuit uses a Hartley
configuration for wider bandwidth. For the RX tank circuit, an external transistor Q3304 is used for
better side-band noise.