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VHF Frequency Generation Circuitry
4.1
Fractional-N Synthesizer
The Fractional-N synthesizer, shown in Figure 2-4, uses a 16.8 MHz crystal (Y201) to provide the
reference frequency for the system. External components C201-C203, R202 and D201 are also part
of the temperature-compensated oscillator circuit. The dc voltage applied to varactor D201 from
U201 pin 25 is determined by a temperature-compensation algorithm within U201, and is specific to
each crystal Y201, based on a unique code assigned to the crystal that identifies its temperature
characteristics. Stability is better than 2.5 ppm over temperatures of -30 to 60 °C. Software-
programmable electronic frequency adjustment is achieved by an internal DAC which provides a
frequency adjustment voltage from U201 pin 25 to varactor D201.
The synthesizer IC U201 further divides the 16.8 MHz signal to 2.1 MHz, 2.225 MHz, or 2.4 MHz for
use as reference frequencies. It also provides a buffered 16.8 MHz signal at U201 pin 19 for use by
the ASFICcmp.
To achieve fast locking of the synthesizer, an internal adapt charge pump provides higher current at
U201 pin 45 to quickly force the synthesizer within lock range. The required frequency is then locked
by the normal mode charge pump at pin 43. A loop filter (C243-C245 and R243-R245) removes
noise and spurs from the steering voltage applied to the VCO varactors, with additional filtering
located in the VCO circuit.
Both the normal and adapt charge pumps get their supply from the capacitive voltage multiplier
made up of C221-C224 and D220-D221. Two 3V square waves from U201 pins 14-15 provide the
drive signals for the voltage multiplier, which generates 12.1V at U201 pin 47. This voltage is filtered
by C225-C228.
One of the auxiliary outputs of the synthesizer IC (AUX3, U201 pin 2) provides the TRB signal which
determines the operating mode of the VCO, either receive or transmit.
DATA (U401 Pin 100)
CLOCK (U401 Pin 1)
SYNTH_CS (U401 Pin 47)
MOD IN (U451 Pin 40)
+5V (U310 Pin 5)
+3V (U330 Pin 5)
Reference
Oscillator
Voltage
Multiplier
7
DATA
8
CLK
9
CEX
10
MODIN
13,30
V
, 5V
CC
5,20,34,36
V
, 3V
Low Voltage
DD
Fractional-N
23
XTAL1
Synthesizer
25
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14
Figure 2-4 VHF Synthesizer Block Diagram
4
LOCK
19
FREFOUT
6,22,23,24
GND
43
IOUT
45
IADAPT
U201
41
MODOUT
2
AUX3
28
SFOUT
40
BIAS1
39
BIAS2
15
Prescaler In
LOCK (U401 Pin 56)
FREF (U451 Pin 34)
Steering
2-Pole
Line
Loop Filter
VCO
Mod
TRB
Voltage
Controlled
Filtered 5V
Oscillator
(First Stage of PA)
2-7
LO RF
Injection
TX RF
Injection