Frequency Synthesis; Fractional-N Synthesizer - Motorola GM328 Service Manual

Frequency Synthesis

4.0 Frequency Synthesis
The frequency synthesizer subsystem, shown in Figure 2-3, consists of the reference oscillator
crystal (Y1201), the Low Voltage Fractional-N synthesizer (LVFRAC-N, U1201), and the receive and
transmit VCOs and buffers (Q1303 through Q1308 and associated components).
DATA (U0101 PIN 100)
CLOCK (U0101 PIN 1)
CSX (U0101 PIN 2)
MOD IN (U0221 PIN 40)
9,3V (U641 PIN 5)
5, 13, 20, 34, 36
+5V (U3211 PIN 1)
REFERENCE
OSCILLATOR
VOLTAGE
MULTIPLIER
Q1202
BUFFER
4.1

Fractional-N Synthesizer

The LVFRAC-N synthesizer IC (U1201) consists of a reference oscillator, pre-scaler, a
programmable loop divider, control divider logic, phase detector, a charge pump, A/D converter for
low frequency digital modulation, a balance attenuator to balance the 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 9.3 volt supply.
Regulated 9.3 volts DC applied to the super filter input (U1201, pin 30) delivers a very low noise
output voltage of 8.3 volts DC (VSF) at pin 28. External device Q1201 allows greater current
sourcing capability. The VSF source supplies the receive and transmit VCOs and first buffer stages.
The synthesizer IC supply voltage is provided by a dedicated 5V regulator (U1250) to minimize
power supply noise.
In order to generate a high voltage to supply the phase detector (charge pump) output stage at pin
VCP (U1201 pin 47), a capacitive voltage multiplier circuit (CR1202 and C1209) generates a voltage
7
DATA
8
CLK
U1201
9
CEX
LOW VOLTAGE
10
MODIN
FRACTIONAL-N
30
SYNTHESIZER
SFIN
VDD, DC5V
23
XTAL1
24
XTAL2
25
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14
15
PRESCALER IN
Figure 2-3 Low Band Synthesizer Block Diagram
4
LOCK
19
FREFOUT
6, 22, 33, 44
GND
43
IOUT
45
IADAPT
41
MODOUT
3
AUX4
N.C.
1
AUX2
N.C.
2
AUX3
FILTERED 8,3V
28
SFOUT
BIAS1
40
AUX1 BIAS2
39
48
BWSELECT
LOCK (U0101 PIN 56)
FREF (U0221 PIN 34)
STEERING
LINE
2-POLE
LOOP
FILTER
VCTRL
LO RF INJECTION
VOLTAGE
CONTROLLED
OSCILLATOR
TX RF INJECTION
(1ST STAGE OF PA)
2-7