Synthesizer - Motorola GP Series Detailed Service Manual

Frequency Generation Circuit
5.1

Synthesizer

(Refer to Figure 2-5 and the UHF Synthesizer schematic diagram)
The Fractional-N synthesizer, shown in Figure 2-5, 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 to be used 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 for use by the ASFIC and IF.
The loop filter consists of components C231, C232, C233, R231, R232 and R233. This circuit
provides the necessary dc steering voltage for the VCO and determines the amount of noise and
spur passing through.
To achieve fast locking for the synthesizer, an internal adapt charge pump provides higher current at
pin 45 of U201 to put the synthesizer within 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
made up of C258, C259, C228, triple diode CR201, and level shifters U210 and U211. Two 3.3V
square waves, 180 degrees 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 1-5 UHF 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
TRANSISTORS
5V
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)
R5
1-9