Voltage Controlled Oscillator (Vco) - Motorola CM200 Service Manual

2-6
4.3

Voltage Controlled Oscillator (VCO)

The Voltage Controlled Oscillator (VCO) consists of the VCO/Buffer IC (VCOBIC, U201), the TX and
RX tank circuits, the external RX buffer stages, and the modulation circuitry.
Rx-SW
Tx-SW
(U200 Pin 28)
Steer Line
Voltage
(VCTRL)
RX VCO
Circuit
RX Tank
TX VCO
TX Tank
Circuit
The VCOBIC together with the LVFRAC-N (U200) generate the required frequencies in both
transmit and receive modes. The TRB line (U201, pin 19) determines which VCO and buffer is
enabled (high being TX output at pin 10, low being RX output at pin 8). A sample of the signal from
the enabled output is routed from U201, pin 12 (PRESC_OUT), via a low pass filter to U200, pin 32
(PREIN).
A steering line voltage between 3.0 V and 10.0 V at varactor D204 tunes the TX VCO through the
frequency range of 465-495 MHz, and at D203 tunes the RX VCO through the frequency range of
420.15-450.15 MHz.
The external RX amplifier is used to increase the output from U201, pin 8 from 3-4 dBm to the
required 10 dBm for proper mixer operation. In TX mode, the modulation signal from the LVFRAC-N
(U200, pin 41) is applied to the VCO by way of the modulation circuit D205, R212, R211, C2073.
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
(U200 Pin 28)
Figure 2-4 UHF VCO Block Diagram
AUX3 (U200 Pin 2)
Prescaler Out
TRB IN
Pin 19 Pin 12
Presc
U201
VCOBIC
RX
Rx
Tx
TX
Pin2 Pin1 Pins 9,11,17
Tx-I adjust
Rx-I adjust
THEORY OF OPERATION
U200 Pin 32
LO RF INJECTION
Q200
Buffers
Pin8
(U200 Pin28)
Pin14
VCC Buffers
TX RF Injection
Pin10
Attenuator
Low Pass
Filter