Voltage Controlled Oscillator (Vco); Figure 4-4. Vhf Synthesizer Block Diagram - Motorola EP450 Service Manual

4-6
One of the auxiliary outputs of the synthesizer IC (AUX3, U201 pin 2) provides the TRB signal which
determines the operating mode of the VCO, either receive or transmit.
DATA (U401 Pin 100)
CLOCK (U401 Pin 1)
SYNTH_CS (U401 Pin 47)
MOD IN (U451 Pin 40)
+5V (U310 Pin 5)
+3V (U330 Pin 5)
Reference
Oscillator
Voltage
Multiplier
4.4.2

Voltage Controlled Oscillator (VCO)

The VCOBIC (U251), shown in
generates RF in both the receive and the transmit modes of operation. The TRB line (U251 pin 19)
determines which oscillator and buffer are enabled. A sample of the RF signal from the enabled
oscillator is routed from U251 pin 12 through a low pass filter, 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 3 V and 9 V. L251 and C251 further attenuate noise and spurs on the steering line voltage.
In the receive mode, the TRB line (U251 pin 19) is low. This activates the receive VCO and the
receive buffer of U251, which operate within the range of 180.85 to 206.85 MHz. The VCO frequency
is determined by tank inductor L254, C253-C257, and varactor D251. The buffered RF signal at U251
pin 8 is further amplified by Q280 and applied as RX_INJ to the low-pass injection filter in the receiver
front end circuit.
In the transmit mode, U251-19 is driven high by U201 pin 2, enabling the transmit VCO and buffer.
The 136-162 MHz RF signal from U251 pin 10 is applied as TX_INJ to the input of the transmitter
circuit via matching network C290-C291 and L291. TX VCO frequency is determined by L264, C263-
June, 2005
136-162 MHz VHF Theory Of Operation: VHF Frequency Generation Circuitry
7
DATA
8
CLK
9
CEX
10
MODIN
13,30
V , 5V
CC U201
5,20,34,36
V , 3V Low Voltage
DD
Fractional-N
23
XTAL1
Synthesizer
25
WARP
32
PREIN
47
VCP
VMULT2 VMULT1
14

Figure 4-4. VHF Synthesizer Block Diagram

Figure 4-5, in conjunction with the Fractional-N synthesizer (U201)
4
LOCK
19
FREFOUT
6,22,23,24
GND
43
IOUT
45
IADAPT
41
MODOUT
2
AUX3
Filtered 5V
28
SFOUT
40
BIAS1
39
BIAS2
15
Prescaler In
LOCK (U401 Pin 56)
FREF (U451 Pin 34)
Steering
2-Pole
Line
Loop Filter
LO RF
Injection
VCO
Mod
TRB
Voltage
Controlled
Oscillator
TX RF
Injection
(First Stage of PA)
HKLN4216C