
2-10
Voltage Controlled Oscillator (VCO)
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 the lock range. The required frequency is then locked by
normal mode charge pump at U201, 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 U201-47.
Figure 2-7.
UHF Synthesizer Block Diagram
2.8
Voltage Controlled Oscillator (VCO)
The VCOB IC (U241), shown in Figure 2-8, in conjunction with the Fractional-N synthesizer (U201)
generates RF in both the receive and the transmit modes of operation. The TRB line (U241-19)
determines which oscillator and buffer are enabled. A sample of the RF signal from the enabled
oscillator is routed from U241-12, through a low pass filter, to the prescaler input (U201-32). After
frequency comparison in the synthesizer, a resultant DC control voltage is received at the VCO. When
the PLL is locked on frequency, this voltage can vary between 3.5 V and 9.5V.
The VCOB IC (U241) is operated at 4.54 V (VSF) and Fractional-N synthesizer (U201) at 3.3V. This
difference in operating voltage requires a level shifter consisting of Q260 and Q261 on the TRB line.
DATA
CLK
CEX
MODIN
V
CC
,
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 (U247 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
TxRF INJECTION
(1ST STAGE OF PA)
FREF (U201 PIN 21 & U404 PIN 34)
39
BIAS2
41
DUAL
TRANSIS
DUAL
TRANSIS
-
48
5V
R5
5, 20, 34, 36
(U248 PIN 5)
AUX1
V
DD
,
3.3V
MODOUT
U201
LOW VOLTAGE
FRACTIONAL-N
SYNTHESIZER
TORS
TORS