Synthesizer - Motorola GP328 Plus Service Manual

5D-10
6.1

Synthesizer

(Refer to 330-400MHz Synthesizer Schematic Diagram on page 5D-24)
The Fractional-N Synthesizer uses a 16.8MHz crystal (FL201) to provide a reference for the system.
The LVFractN IC (U201) further divides this to 2.1MHz, 2.225MHz, and 2.4MHz as reference
frequencies. Together with C206, C207, C208, R204 and CR203 , they build up the reference
oscillator which is capable of 2.5ppm stability over temperatures of -30 to 85°C. It also provides
16.8MHz at pin 19 of U201 to be used by ASFIC and LVZIF.
The loop filter which consist of C231, C232, C233, R231, R232 and R233 provides the necessary dc
steering voltage for the VCO and determines the amount of noise and spur passing through .
In achieving fast locking for the synthesizer, an internal adapt charge pump provides higher current at
pin 45 of U201 to put synthesizer within the lock range. The required frequency is then locked by
normal mode charge pump at pin 43 .
Both the normal and adapt charge pumps get their supply from the capacitive voltage multiplier which
is made up of C258, C259, C228, triple diode CR201 and level shifters U210 and U211. Two 3.3V
square waves ( 180 deg out of phase) are first shifted to 5V, then along with regulated 5V , put
through arrays of diodes and capacitors to build up 13.3V at pin 47 of U201.
DATA (U409 PIN 100)
CLOCK (U409 PIN 1)
CSX (U409 PIN 2)
MOD IN (U404 PIN 40)
+5V (U247 PIN 4)
5, 20, 34, 36
(U248 PIN 5)
REFERENCE
OSCILLATOR
VOLTAGE
MULTIPLIER
7
DATA
8
CLK
9
CEX
U251
LOW VOLTAGE
10
MODIN
FRACTIONAL-N
13, 30
SYNTHESIZER
V , DC5V
CC
V , 3.3V
DD
23
XTAL1
24
XTAL2
25
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14
15
PRESCALER IN
Figure 5-4: Synthesizer Block Diagram
4
LOCK
19
FREFOUT
6, 22, 23, 24
GND
43
IOUT
45
IADAPT
41
MODOUT
3
AUX4
DUAL
TRANSIS
2
AUX3
TORS
FILTERED 5V
28
SFOUT
BIAS1
40
BIAS2
AUX1
39
48
DUAL
R5
TRANSIS-
5V
TORS
Frequency Generation Circuitry
LOCK (U409 PIN 56)
FREF (U201 PIN 21 & U404 PIN 34)
STEERING
LINE
2-POLE
LOOP
FILTER
LO RF INJECTION
VOLTAGE
CONTROLLED
OSCILLATOR
TX RF INJECTION
(1ST STAGE OF PA)