Voltage Controlled Oscillator (Vco) - Motorola CM140 Service Information

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 amplifier, 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 3.0V and 10.0V at varactor D204 tunes the TX VCO through the
frequency range of 146-174MHz, and at D203 tunes the RX VCO through the frequency range of
190-219MHz.
The external RX amplifier is used to increase the output from U201, pin 8 from 3-4 dBm to the
required 15dBm 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
Pin13
Switching Network
Vcc-Superfilter
Pin3
Collector/RF in
Pin4
RX
Pin5 Active Bias
Pin6
TX
Active Bias
Pin16
Pin15
Vsens
Circuit
Pin18
Vcc-Logic
(U200 Pin 28)
Figure 2-4 VHF VCO Block Diagram
AUX3 (U200 Pin 2)
Prescaler Out
TRB IN
Pin 19
TX/RX/BS
U201
VCOBIC
Rx
Tx
Pin2 Pin1 Pins 9,11,17
Tx-I adjust
Rx-I adjust
THEORY OF OPERATION
U200 Pin 32
Pin 12
LO RF INJECTION
Presc
Q200
Buffer
RX
Pin8
(U200 Pin28)
Pin14
VCC Buffers
TX
TX RF Injection
Pin10
Attenuator
Low Pass
Filter