Motorola cm200 Commercial Series, Service Manual

Page: 266 / 352

2-6 THEORY OF OPERATION
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.
Figure 2-4 UHF VCO Block Diagram
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.
Presc
RX
TX
Buffers
Q200
Low Pass
Filter
Attenuator
Pin8
Pin14
Pin10
(U200 Pin28)
VCC Buffers
TX RF Injection
U200 Pin 32
AUX3 (U200 Pin 2)
Prescaler Out
Pin 12 Pin 19
Pin 20
TX/RX/BS
Switching Network
U201
VCOBIC
Rx
Active Bias
Tx
Active Bias
Pin2
Rx-I adjust
Pin1
Tx-I adjust
Pins 9,11,17
Pin18
Vsens
Circuit
Pin15
Pin16
RX VCO
Circuit
TX VCO
Circuit
RX Tank
TX Tank
Pin7
Vcc-Superfilter
Collector/RF in
Pin4
Pin5
Pin6
RX
TX
(U200 Pin 28)
Rx-SW
Tx-SW
Vcc-Logic
(U200 Pin 28)
Steer Line
Voltage
(VCTRL)
Pin13
Pin3
TRB IN
LO RF INJECTION