Motorola CDM Series Detailed Service Manual page 502

Section 9: 2-6
In applications where less frequency stability is required the oscillator inside U4201 is used along
with an external crystal Y4261, varactor diode D4261, C4261, C4262 and R4262. In this case,
Y4262, R4263, C4235 and C4251 are not used. When Y4262 is used, Y4261, D4261, C4261,
C4262 and R4262 are not used, and C4263 is increased to 0.1 uF.
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 balanced attenuator to balance high frequency analog 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.
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
VCP (U5701-32), 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
signals (1.05MHz) 180 degrees out of phase (U4201-14 and -15).
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
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.
6881091C63-F
UHF Band 1 (403–470 MHz) 25–40 W Bipolar Frequency Synthesis
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 9-3 UHF Synthesizer Block Diagram
4
LOCK
LOCK (U0101 PIN 56)
19
FREFOUT
FREF (U0221 PIN 34)
6, 22, 33, 44
GND
43 2-POLE
IOUT
LOOP
45
IADAPT
FILTER
41
MODOUT
3
AUX4
1 (NU)
AUX2
TRB
2
AUX3
FILTERED 5V
28
SFOUT
BIAS1
40
BIAS2
AUX1
39
48
BWSELECT
To IF
Section
STEERING
LINE
VCO Bias
LO RF INJECTION
VOLTAGE
CONTROLLED
OSCILLATOR
TX RF INJECTION
(1ST STAGE OF PA)