Reference Oscillator; Fractional-N Synthesizer - Motorola CDM Series Detailed Service Manual

Section 4: 2-6
4.1

Reference Oscillator

The reference oscillator (Y3263) contains a temperature compensated crystal oscillator with a
frequency of 16.8 MHz. An analog-to-digital (A/D) converter internal to U3201 (LVFRAC-N) and
controlled by the µP via serial interface (SRL) sets the voltage at the warp output of U3201 (pin 25)
to set the frequency of the oscillator. The output of the oscillator (U3263 pin 3) is applied to pin 23
(XTAL1) of U3201 via R3263 and C3235.
In applications where less frequency stability is required, the oscillator inside U3201 is used along
with an external crystal Y3261, varactor diode D3261, C3261, C3262 and R3262. In this case,
Y3263, R3263, C3235 and C3251 are not used. When Y3263 is used, Y3261, D3261, C3261,
C3262 and R3262 are not used, and C3263 is increased to 0.1 uF.
4.2

Fractional-N Synthesizer

The LVFRAC-N synthesizer IC (U3201) 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 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.
DATA (U0101 PIN 100)
CLOCK (U0101 PIN 1)
CSX (U0101 PIN 2)
MOD IN (U0221 PIN 40)
+5V (U3211 PIN 1)
5, 20, 34, 36
+5V (U3211 PIN 1)
REFERENCE
OSCILLATOR
VOLTAGE
MULTIPLIER
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 47 VCP (U3201-47), a voltage of 13 VDC is being generated by the positive voltage multiplier
6881091C63-F
7
DATA
8
CLK
9
CEX
10
MODIN
13, 30
VCC, DC5V
VDD, DC5V
23
XTAL1
U3201
LOW VOLTAGE
24
XTAL2
FRACTIONAL-N
25
SYNTHESIZER
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14
15
PRESCALER IN
Figure 4-3 VHF Synthesizer Block Diagram
VHF (136–174 MHz) 1–25W Frequency Synthesis
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
AUX2
TRB
2
AUX3
FILTERED 5V
28
SFOUT
BIAS1
40
BIAS2
AUX1
39
48
BW SELECT
STEERING
LINE
LO RF INJECTION
VOLTAGE
CONTROLLED
OSCILLATOR
TX RF INJECTION
(1ST STAGE OF PA)
TO IF SECTION