Frequency Generation Circuitry 6B-11
5.2 VCO - Voltage Controlled Oscillator
(Refer to
VHF Voltage Controlled Oscillator Schematic Diagram
on page 6B-23)
The VCOBIC (U3801) 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 will be 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.
Figure 6-5: VCO Block Diagram
Presc
RX
TX
Matching
Network
Low Pass
Filter
Attenuator
Pin8
Pin14
Pin10
(3701 Pin28)
VCC Buffers
TX RF Injection
U3701 Pin 32
AUX3 (U3701 Pin2)
Prescaler Out
Pin 12Pin 19
Pin 20
TX/RX/BS
Switching Network
U3801
VCOBIC
Rx
Active Bias
Tx
Active Bias
Pin2
Rx-I adjust
Pin1
Tx-I adjust
Pins 9,11,17
Pin18
Vsens
Circuit
Pin15
Pin16
RX VCO
Circuit
TX VCO
Circuit
RX Tank
TX Tank
Pin7
Vcc-Superfilter
Collector/RF in
Pin4
Pin5
Pin6
RX
TX
(U3701 Pin28)
Rx-SW
Tx-SW
Vcc-Logic
(U3701 Pin28)
Steer Line
Voltage
(VCTRL)
Pin13
Pin3
TRB_IN
LO RF INJECTION
VSF
VSF
VSF
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