Voltage Controlled Oscillator (Vco); Figure 3-13. 700 Mhz Synthesizer Block Diagram - Motorola HT1250-LS+ Service Manual

Theory of Operation
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
To achieve fast locking for the synthesizer, an internal adapt charge pump provides higher current at
U201 pin 45 to quickly force 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 C211-14 and CR201-2. Two 3.3V square waves
from U201 pins 14-15 provide the drive signals for the voltage multiplier, which generates 12.8V at
U201 pin 47. This voltage is filtered by C223-5.
One of the auxiliary outputs of the synthesizer IC (pin 48) controls transistor switch Q201 which
outputs a switched R5 voltage source, present during receive mode. Another auxiliary output (pin 2)
controls transistor switch Q202 which outputs a switched AU3-5V source for TX buffer stage Q601 in
the VCO circuit.
3.10.3

Voltage Controlled Oscillator (VCO)

The VCOBIC (U600), shown in Figure 3-14, in conjunction with the Fractional-N synthesizer (U201)
generates RF in both the receive and the transmit modes of operation. The TRB line (U600 pin 19)
determines which oscillator and buffer are enabled, and is controlled by the AUX4 signal from the
synthesizer (U201 pin 3). Transistor Q603 level-shifts the synthesizer's 3.3V logic level to the 5V
operating level of the VCOBIC. A sample of the RF signal from the enabled oscillator is routed from
U600 pin 12 through a low pass filter (L203, C234 in the synthesizer circuit), to the prescaler input of
the synthesizer IC (U201 pin 32). After frequency comparison in the synthesizer, a resultant DC
control voltage is used to steer the VCO frequency. When the PLL is locked on frequency, this voltage
can vary between 2 to 9 volts in the RX mode or 3 to 11 volts in the TX mode.
In the receive mode, the TRB line (U600 pin 19) is low. This activates the receive VCO and the
receive buffer of U600, which operate within the range of 819.35 to 837.35 MHz. The RX VCO
frequency is determined by ceramic tank resonator FL600, varactors CR600 and CR601, and
associated components. Feedback to maintain oscillation is provided by divider C607-8 and
associated components. The buffered RF signal at U600 pin 8 is further amplified by Q600 and
applied as RX_LO to the low-pass injection filter in the receiver front end circuit.
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
25
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14

Figure 3-13. 700 MHz 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 (U404 Pin 34)
Steering
2-Pole
Line
Loop Filter
Dual
Transistors Voltage
Controlled
Filtered 5V
Oscillator
Dual
R405
3-15
LO RF
Injection
TX RF
Injection
(First Stage of PA)