Voltage Control Oscillator (Vco) - Motorola Professional Radio Service Manual

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
U3701, pin 45 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 D3701 and 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
U3701, pin 47.
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 2-14, in conjunction with the FRACTN synthesizer (U3701)
generates RF in both the receive and transmit modes of operation. The TRB line (U3801, pin 19)
determines which oscillator and buffer are 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 at the VCO.
This voltage is a DC voltage typically between 3.5V and 9.5V when the PLL is locked on frequency.
The RF section of the VCOB IC (U3801) is operated at 4.54 V (VSF), while the control section of the
VCOBIC and FRACN synthesizer (U3701) is operated at 3.3V. The operation logic is shown in Table
2-6.
In the receive mode, U3801, pin 19 is low or grounded. This activates the receive VCO by enabling
the receive oscillator and the receive buffer of U3801. The RF signal at U3801, pin 8 is routed through
a matching network. The resulting LO RF INJECTION signal is applied to the mixer at T3302.
7
DATA
8
CLK
9
CEX
10
MODIN
13,30
V , DC5V
CC U3701
V , 3.3V
Low Voltage
DD
Fractional-N
23
XTAL1
Synthesizer
24
XTAL2
25
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14
Figure 2-13: VHF Synthesizer Block Diagram
4
LOCK
19
FREFOUT
6,22,23,24
GND
43
IOUT
45
IADAPT
41
MODOUT
3
AUX4
2
AUX3
28
SFOUT
40
BIAS1
39
BIAS2
AUX1
48
15
Transistors
5V
Prescaler In
LOCK (U409 Pin 56)
FREF (U3220 Pin 21 & U404 Pin 34)
Steering
2-Pole
Line
Loop Filter
Voltage
Controlled
Filtered 5V
Oscillator
Dual
R405
2-17
LO RF
Injection
TX RF
Injection
(First Stage of PA)