VHF Theory of Operation: Synthesizer 6-9
6.5 Synthesizer
(Refer to the VHF Synthesizer Schematic Diagram on page 9-342.)
The Fractional-N Synthesizer uses a 16.8MHz crystal (Y3761) to provide a reference for the system.
The LVFractN IC (U3701) further divides this to 2.1MHz, 2.225 MHz, and 2.4 MHz as reference
frequencies. Together with C3761, C3762, C3763, R3761 and D3761, they build up the reference
oscillator which is capable of 2.5ppm stability over temperatures of -30 to 85°C. It also provides 16.8
MHz at pin 19 of U3701 to be used by ASFIC.
The loop filter which consist of C3721, C3722, R3721, R3722 and R3723 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 U3701 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 C3701 to C3704 and triple diodes D3701, D3702. Two 3.3 V square waves (180
deg out of phase) are first multiplied by four and then shifted, along with regulated 5 V, to build up
13.5 V at pin 47 of U3701.
Figure 6-5. Synthesizer Block Diagram
DATA
CLK
CEX
MODIN
VCC, DC5V
XTAL1
XTAL2
WARP
PREIN
VCP
REFERENCE
OSCILLATOR
VOLTAGE
MULTIPLIER
VOLTAGE
CONTROLLED
OSCILLATOR
2-POLE
LOOP
FILTER
DATA (U409 PIN 100)
CLOCK (U409 PIN 1)
CSX (U409 PIN 2)
MOD IN (U404 PIN 40)
+5V (U3711 PIN 4)
7
8
9
10
13, 30
23
24
25
32
47
VMULT2 VMULT1
BIAS1
SFOUT
AUX3
AUX4
IADAPT
IOUT
GND
FREFOUT
LOCK
4
19
6, 22, 23, 24
43
45
3
2
28
14
15
40
FILTERED 5V
STEERING
LINE
LOCK (U409 PIN 56)
PRESCALER IN
LO RF INJECTION
TX RF INJECTION
(1ST STAGE OF PA
FREF (U3220 PIN 21 & U404 PIN 34)
39
BIAS2
41
DUAL
TSTRS
48
5V
R5
5, 20, 34, 36
(U3201 PIN 5)
AUX1
VDD, 3.3V
MODOUT
U3701
LOW VOLTAGE
FRACTIONAL-N
SYNTHESIZER
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