Voltage Controlled Oscillator (Vco) - Motorola GM Series Service Information

VHF (136-174MHz) Frequency Synthesis
A voltage of 5V applied to the super filter input (U3201 pin 30) supplies an output voltage of 4.5 VDC
(VSF) at pin 28. It supplies the VCO, VCO modulation bias circuit (via R3363) and the synthesizer
charge pump resistor network (R3251, R3252). The synthesizer supply voltage is provided by the
5V regulator U3211.
In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin
VCP (U3201-47), a voltage of 13 VDC is being generated by the positive voltage multiplier circuitry
(D3201, C3202, C3203). This voltage multiplier is basically a diode capacitor network driven by two
(1.05MHz) 180 degrees out of phase signals (U3201-14 and -15).
Output LOCK (U3201-4) provides information about the lock status of the synthesizer loop. A high
level at this output indicates a stable loop. IC U3201 provides the 16.8 MHz reference frequency at
pin 19.
The serial interface (SRL) is connected to the microprocessor via the data line DATA (U3201-7),
clock line CLK (U3201-8), and chip enable line CSX (U3201-9).
4.3

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 buffer stages, and the modulation circuitry.
Rx-SW
Tx-SW
(U3201 Pin28)
Steer Line
Voltage
(VCTRL)
RX VCO
Circuit
RX Tank
Q3304
TX VCO
TX Tank
Circuit
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
(U3211 Pin1)
Figure 2-1 VHF VCO Block Diagram
AUX3 (U3201 Pin2)
Prescaler Out
TRB IN
Pin 19 Pin 12
Presc
U3301
VCOBIC
RX
Rx
Tx
TX
Pin2 Pin1 Pins 9,11,17
Tx-I adjust
Rx-I adjust
U3201 Pin 32
LO RF INJECTION
Matching
Network
Q3301
Pin8
(U3211 Pin1)
Pin14
VCC Buffers
TX RF Injection
Pin10
Attenuator
2-7
Low Pass
Filter