Uhf (403-470 Mhz) Frequency Synthesis; Reference Oscillator; Fractional-N Synthesizer; Voltage Controlled Oscillator (Vco) - Motorola GM328 Service Manual

UHF (403-470 MHz) Frequency Synthesis

4.2

Fractional-N Synthesizer

The LVFRAC-N synthesizer IC (U4201) consists of a pre-scaler, a programmable loop divider,
control divider logic, a phase detector, a charge pump, an A/D converter for low frequency digital
modulation, a balance attenuator to balance the high frequency analogue modulation and low
frequency digital modulation, a 13V positive voltage multiplier, a serial interface for control, and
finally a super filter for the regulated 5 volts.
DATA (U0101 PIN 100)
CLOCK (U0101 PIN 1)
CSX (U0101 PIN 2)
MOD IN (U0221 PIN 40)
+5V (U4211 PIN 1)
5, 20, 34, 36
+5V (U4211 PIN 1)
REFERENCE
OSCILLATOR
VOLTAGE
MULTIPLIER
A voltage of 5V applied to the super filter input (U4201 pin 30) supplies an output voltage of 4.5
VDC(VSF) at pin 28. It supplies the VCO, VCO modulation bias circuit (via R4322) and the
synthesizer charge pump resistor network (R4251, R4252). The synthesizer supply voltage is
provided by the 5V regulator U4211.
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 microprocessor via the data line DATA (U4201-7),
clock line CLK (U4201-8), and chip enable line CSX (U4201-9).
7
DATA
8
CLK
9
CEX
10
MODIN
13, 30
VCC, DC5V
VDD, DC5V
23
XTAL1
U4201
LOW VOLTAGE
24
XTAL2
FRACTIONAL-N
25
SYNTHESIZER
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14
15
PRESCALER IN
Figure 2-3 UHF Synthesizer Block Diagram
4
LOCK
19
FREFOUT
6, 22, 33, 44
GND
43
IOUT
45
IADAPT
41
MODOUT
3
AUX4
1 (NU)
AUX2
TRB
2
AUX3
FILTERED 5V
28
SFOUT
BIAS1
40
BIAS2
AUX1
39
48
BWSELECT
LOCK (U0101 PIN 56)
FREF (U0221 PIN 34)
STEERING
LINE
2-POLE
LOOP
FILTER
VCO Bias
LO RF INJECTION
VOLTAGE
CONTROLLED
OSCILLATOR
TX RF INJECTION
(1ST STAGE OF PA)
To IF
Section
2-7