Synthesizer - Motorola GP Series Service Manual

Frequency Generation Circuit
4.1

Synthesizer

( Refer to Figure 2-4 and the 800 MHz Synthesizer schematic diagram)
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 C235, C236, C237, R211 and CR203, they comprise 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.
Some models are equipped with a packaged 1.5ppm reference oscillator, Y200. On these models
components C235, C236, C237, CR203, FL201, and R211 are not placed. Components C238,
C239, C241, R212, R213, R214 and Y200 are placed instead.
The loop filter which consists of C220, C225, C226, R204, R209 and R210 provides the necessary
dc steering voltage for the VCO and provides filtering of noise and spurs from U201.
In achieving fast locking for the synthesizer, an internal adapt charge pump provides higher current
at pin 45 of U201 to put the synthesizer within the lock range. The required frequency is then locked
by the normal mode charge pump at pin 43.
Both the normal and adapt charge pumps get their supply from the capacitive multiplier which is
made up of D201, D202, C244, C245, C246, C247, R200, R218, C208, C243, R219, and R220.
Two 3.3 V square waves (180 degrees out of phase) are applied to R219 and R220. These square
waves switch alternate sets of diodes from D201 and D202, which in turn charge C244, C245, C246,
and C247 in a bucket brigade fashion. The resulting output voltage that is applied to pin 47 of U201
is typically 12.8V and allows the steering line voltage (VCO control voltage) to reach 11V.
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
12.8V
Voltage
Multiplier
7
DATA
8
CLK
9
CEX
10
MODIN
13,30
V , DC5V
CC
U251
V , 3.3V
Low Voltage
DD
Fractional-N
23
XTAL1

Synthesizer

24
XTAL2
25
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14
3.3Vp-p
3.3Vp-p
Figure 2-4 800 MHz Synthesizer Block Diagram
4
LOCK
19
FREFOUT
6,22,23,24
GND
43
IOUT
45
IADAPT
41
MODOUT
3
AUX4
2
AUX3
28
SFOUT
40
BIAS1
39
BIAS2
AUX1
15 48
Transistors
5V
Prescaler In
LOCK (U409 Pin 56)
FREF (U201 Pin 21 & U404 Pin 34)
Steering
2-Pole
Line
Loop Filter
11.0V
Dual
Transistors Voltage
Controlled
Filtered 5V
Oscillator
Dual
R405
2-7
LO RF
Injection
TX RF
Injection
(First Stage of PA)