Section 12: 2-6 UHF Band 1 (403–470 MHz) 25–40 W LDMOS Frequency Synthesis
6881091C63-F
4.1 Reference Oscillator
The reference oscillator (Y4262) contains a temperature compensated crystal oscillator with a
frequency of 16.8 MHz. An analog-to-digital (A/D) converter internal to U4201 (LVFRAC-N) and
controlled by the µP via serial interface (SRL) sets the voltage at the warp output of U4201 pin 25 to
set the frequency of the oscillator. The output of the oscillator (pin 3 of Y4262) is applied to pin 23
(XTAL1) of U4201 via a RC series combination.
In applications where less frequency stability is required the oscillator inside U4201 is used along
with an external crystal Y4261, varactor diode D4261, C4261, C4262 and R4262. In this case,
Y4262, R4263, C4235 and C4251 are not used. When Y4262 is used, Y4261, D4261, C4261,
C4262 and R4262 are not used, and C4263 is increased to 0.1 uF.
4.2 Fractional-N Synthesizer
The LVFRAC-N synthesizer IC (U4201) consists of a pre-scaler, a programmable loop divider,
control divider logic, a phase detector, a charge pump, an A/D converter for low frequency digital
modulation, a balance attenuator to balance high frequency analog modulation and low frequency
digital modulation, a 13V positive voltage multiplier, a serial interface for control, and finally a super
filter for the regulated 5 volts.
Figure 12-3 UHF Synthesizer Block Diagram
A voltage of 5V applied to the super filter input (U4201 pin 30) supplies an output voltage of 4.5
VDC(VSF) at pin 28. It supplies the VCO, VCO modulation bias circuit (via R4322) and the
synthesizer charge pump resistor network (R4251, R4252). The synthesizer supply voltage is
provided by the 5V regulator U4211.
DATA
CLK
CEX
MODIN
VCC, DC5V
XTAL1
XTAL2
WARP
PREIN
VCP
REFERENCE
OSCILLATOR
VOLTAGE
MULTIPLIER
DATA (U0101 PIN 100)
CLOCK (U0101 PIN 1)
CSX (U0101 PIN 2)
MOD IN (U0221 PIN 40)
+5V (U4211 PIN 1)
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, 33, 44
43
45
3
2
28
14
15
40
FILTERED 5V
STEERING
LOCK (U0101 PIN 56)
PRESCALER IN
FREF (U0221 PIN 34)
39
BIAS2
41
48
5, 20, 34, 36
+5V (U4211 PIN 1)
AUX1
VDD, DC5V
MODOUT
U4201
LOW VOLTAGE
FRACTIONAL-N
SYNTHESIZER
AUX2
1 (NU)
BWSELECT
VCO Bias
TRB
To IF
Section
TX RF INJECTION
(1ST STAGE OF PA)
LO RF INJECTION
VOLTAGE
CONTROLLED
OSCILLATOR
LINE
2-POLE
LOOP
FILTER
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