Voltage Control Oscillator (Vco); Figure 2-8:Uhf Synthesizer Block Diagram - Motorola Professional Radio Service Manual

2-10
To achieve fast locking for the synthesizer, an internal adapt charge pump provides higher current at
U201 (pin 45) to put the synthesizer within lock range. The required frequency is then locked by
normal mode charge pump at U201 (pin 47).
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 (pin 47).
DATA (U409 Pin 100)
CLOCK (U409 Pin 1)
CSX (U409 Pin 2)
MOD IN (U404 Pin 40)
+5V (U247 Pin 4)
5,20,34,36
(U248 Pin 5)
Reference
Oscillator
Voltage
Multiplier
2.8

Voltage Control Oscillator (VCO)

The VCOB IC (U241), shown in Figure 2-9, in conjunction with the Fractional-N synthesizer (U201)
generates RF in both the receive and the transmit modes of operation. The TRB line (U241 pin 19)
determines which oscillator and buffer are enabled. A sample of the RF signal from the enabled
oscillator is routed from U241 (pin 12), through a low pass filter, to the prescaler input (U201 pin 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 3.5V and 9.5V.
The VCOB IC is operated at 4.54V (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. The level
shifter logic is shown in Table 2-5.
In the receive mode, U241 (pin 19) is low or grounded. This activates the receive VCO by enabling
the receive oscillator and the receive buffer of U241. The RF signal at U241 (pin 8) is run through a
matching network. The resulting LO RF INJECTION signal is applied to the mixer at T302.
When PTT is pressed during the transmit condition, five volts is applied to U241 (pin 19). This
activates the transmit VCO by enabling the U241 transmit oscillator and buffer. The TX RF
INJECTION signal at U241 (pin 10) is injected into the input of the PA module (U101 pin 16). Also in
7
DATA
8
CLK
9
CEX
10
MODIN
13,30
V , DC5V
CC
U201
V , 3.3V
Low Voltage
DD
Fractional-N
23
XTAL1
Synthesizer
24
XTAL2
25
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14
Figure 2-8: UHF 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
15 48
Transistors
5V
Prescaler In
LOCK (U409 Pin 56)
FREF (U201 Pin 21 & U404 Pin 34)
Steering
2-Pole
Line
Loop Filter
Dual
Transistors Voltage
Controlled
Filtered 5V
Oscillator
Dual
R405
LO RF
Injection
TX RF
Injection
(First Stage of PA)