Voltage Controlled Oscillator (Vco) - Motorola CDM Series Detailed Service Manual

UHF Band 1 (403–470 MHz) 25–40 W LDMOS Frequency Synthesis
In order to generate a high voltage to supply the phase detector (charge pump) output stage at
pin 47 VCP (U4201-47), a voltage of 13 VDC is being generated by the positive voltage multiplier
circuitry (D4201, C4202, C4203). This voltage multiplier is basically a diode capacitor network
driven by two (1.05MHz) 180 degrees out of phase signals (U4201-14 and -15).
Output LOCK (U4201-4) provides information about the lock status of the synthesizer loop. A high
level at this output indicates a stable loop. IC U4201 provides the 16.8 MHz reference frequency at
pin 19.
The serial interface (SRL) is connected to the µP via the data line DATA (U4201-7), clock line CLK
(U4201-8), and chip enable line CSX (U4201-9).
4.3

Voltage Controlled Oscillator (VCO)

The Voltage Controlled Oscillator (VCO) consists of the VCO/Buffer IC (VCOBIC, U4301), the TX
and RX tank circuits, the external RX buffer stages, and the modulation circuitry.
Rx-SW
Tx-SW
(U4201 Pin 28)
Steer Line
Voltage
(VCTRL)
RX VCO
Circuit
RX Tank
Q4301
TX VCO
TX Tank
Circuit
The VCOBIC together with Fractional-N synthesizer (U4201) generates the required frequencies in
both transmit and receive modes. The TRB line (U4301 pin 19) determines which tank circuits and
internal buffers are to be enabled. A high level on TRB enables TX tank and TX output (pin 10), and
Pin 20
Pin7
TX/RX/BS
Pin13 Switching Network
Vcc-Superfilter
Pin3
Collector/RF in
Pin4
RX
Rx
Active Bias
Pin5
Pin6
TX
Tx
Active Bias
Pin16
Pin15
Vsens
Circuit
Pin18
Vcc-Logic
(U4201 Pin 28)
Figure 12-4 UHF VCO Block Diagram
AUX3 (U4201 Pin 2)
Prescaler Out
TRB IN
Pin 19 Pin 12
Presc
U4301
VCOBIC
RX
Pin8
Pin14
TX
Pin10
Pin2 Pin1 Pins 9,11,17
Tx-I adjust
Rx-I adjust
Section 12: 2-7
U4201 Pin 32
LO RF INJECTION
Matching
Low-Pass
Network
Filter
Q4332
(U4201 Pin28)
VCC Buffers
TX RF Injection
Attenuator
6881091C63-F