Voltage Control Oscillator (Vco) - Motorola PRO3150 Service Manual

Voltage Control Oscillator (VCO)

In achieving fast locking for the synthesizer, an internal adapt charge pump provides higher current at
pin 45 of U3701 to put the synthesizer within 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 made
up of C3701 to C3704 and triple diodes D3701and D3702. Two 3.3V square waves (180 degrees out
of phase) are first multiplied by four and then shifted, along with regulated 5V, to build up 13.5V at pin
47 of U3701.
DATA (U409 PIN 100)
CLOCK (U409 PIN 1)
CSX (U409 PIN 2)
MOD IN (U404 PIN 40)
+5V (U3711 PIN 4)
5, 20, 34, 36
(U3201 PIN 5)
REFERENCE
OSCILLATOR
VOLTAGE
MULTIPLIER

2.12 Voltage Control Oscillator (VCO)

The VCOB IC (U3801), shown in Figure 2-13, in conjunction with the Fractional-N synthesizer
(U3701), generates RF in both the receive and the transmit modes of operation. The TRB line (U3801,
pin 19) determines which oscillator and buffer will be enabled. A sample of the RF signal from the
enabled oscillator is routed from U3801, pin 12, through a low pass filter, to the prescaler input
(U3701, pin 32). After frequency comparison in the synthesizer, a resultant control voltage is received
7
DATA
8
CLK
9
CEX
U3701
LOW VOLTAGE
10
MODIN
FRACTIONAL-N
13, 30
SYNTHESIZER
VCC, DC5V
VDD, 3.3V
23
XTAL1
24
XTAL2
25
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14
15
PRESCALER IN
Figure 2-12.
4
LOCK
19
FREFOUT
6, 22, 23, 24
GND
43
IOUT
45
IADAPT
41
MODOUT
3
AUX4
2
AUX3
FILTERED 5V
28
SFOUT
BIAS1
40
BIAS2
AUX1
39
48
DUAL
5V
TSTRS
VHF Synthesizer Block Diagram
LOCK (U409 PIN 56)
FREF (U3220 PIN 21 & U404 PIN 34)
STEERING
LINE
2-POLE
LOOP
FILTER
LO RF INJECTION
VOLTAGE
CONTROLLED
OSCILLATOR
(1ST STAGE OF PA)
R5
2-17
x
T RF INJECTION