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2-8
4.2
Voltage Controlled Oscillator (VCO)
(Refer to Figure 2-4 and the VHF Voltage Controlled Oscillator schematic diagram)
The VCOB IC (U3801), shown in Figure 2-5, 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 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.
Rx-SW
Tx-SW
(U3701 Pin28)
Steer Line
Voltage
(VCTRL)
RX VCO
RX Tank
Circuit
TX VCO
TX Tank
Circuit
Pin 20
Pin7
Pin13
Switching Network
Vcc-Superfilter
Pin3
Collector/RF in
Pin4
RX
Pin5
Active Bias
Pin6
TX
Active Bias
Pin16
Pin15
Vsens
Circuit
Pin18
Vcc-Logic
(U3701 Pin28)
Figure 2-5 VHF VCO Block Diagram
AUX3 (U3701 Pin2)
Prescaler Out
TRB_IN
Pin 19
TX/RX/BS
Presc
U3801
VCOBIC
Rx
Tx
Pin2
Pin1
Pins 9,11,17
Tx-I adjust
Rx-I adjust
THEORY OF OPERATION
U3701 Pin 32
Pin 12
LO RF INJECTION
Matching
Network
RX
Pin8
(3701 Pin28)
Pin14
VCC Buffers
TX
TX RF Injection
Pin10
Attenuator
Low Pass
Filter