Section 5: 2-8 UHF Band 1 (403–470 MHz) 1–25 W Frequency Synthesis
6881091C63-F
4.3 Voltage Controlled Oscillator (VCO)
The Voltage Controlled Oscillator (VCO) consists of the VCO/Buffer IC (VCOBIC, U4301), the TX
and RX tank circuits, the external RX buffer stages, and the modulation circuitry.
Figure 5-4 UHF VCO Block Diagram
The VCOBIC together with Fractional-N synthesizer (U4201) generates the required frequencies in
both transmit and receive modes. The TRB line (U4301 pin 19) determines which tank circuits and
internal buffers are to be enabled. A high level on TRB enables TX tank and TX output (pin 10), and
a low enables RX tank and RX output (pin 8). A sample of the signal from the enabled output is
routed from U4301 pin 12 (PRESC_OUT), via a low-pass filter, to pin 32 of U4201 (PREIN).
A steering line voltage (VCTRL) between 3.0 V and 10.0 V at varactor diode CR4311 will tune the
full TX frequency range (TXINJ) from 403 MHz to 470 MHz, and at varactor diodes CR4301,
CR4302 and CR4303 will tune the full RX frequency range (RXINJ) from 358 MHz to 425 MHz. The
tank circuits uses the Hartley configuration for wider bandwidth. For the RX tank circuit, an external
transistor Q4301 is used in conjunction with the internal transistor for better side-band noise.
Presc
RX
TX
Matching
Network
Low-Pass
Filter
Attenuator
Pin8
Pin14
Pin10
(U4201 Pin28)
VCC Buffers
TX RF Injection
U4201 Pin 32
AUX3 (U4201 Pin 2)
Prescaler Out
Pin 12Pin 19
Pin 20
TX/RX/BS
Switching Network
U4301
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
(U4201 Pin 28)
Rx-SW
Tx-SW
Vcc-Logic
(U4201 Pin 28)
Steer Line
Voltage
(VCTRL)
Pin13
Pin3
TRB IN
LO RF INJECTION
Q4301
Q4332
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