Voltage-Controlled Oscillator (Vco); Figure 6-6. Vco Block Diagram - Motorola HT Series Service Manual

6-10
6.6

Voltage-Controlled Oscillator (VCO)

(Refer to the VHF Voltage-Controlled Oscillator Schematic Diagram on page 9-95.)
Rx-SW
Tx-SW
(U3701 Pin28)
VSF
Steer Line
Voltage
(VCTRL)
RX VCO
RX Tank
Circuit
TX VCO
TX Tank
Circuit
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 3.5 V and 9.5 V when the PLL is locked on frequency.
Pin 20
Pin7
TX/RX/BS
Pin13
Switching Network
Vcc-Superfilter
Pin3
Collector/RF in
Pin4
RX
Active Bias
Pin5
Pin6
TX
Active Bias
Pin16
Pin15
Vsens
Circuit
Pin18
Vcc-Logic
VSF
(U3701 Pin28)

Figure 6-6. VCO Block Diagram

VHF Theory of Operation: Voltage-Controlled Oscillator (VCO)
AUX3 (U3701 Pin2)
Prescaler Out
TRB_IN
Pin 19 Pin 12
Presc
U3801
VCOBIC
RX
Rx
Tx
TX
Pin2 Pin1 Pins 9,11,17
Tx-I adjust
Rx-I adjust
U3701 Pin 32
LO RF INJECTION
Matching
Network
Pin8
VSF
(3701 Pin28)
Pin14
VCC Buffers
TX RF Injection
Pin10
Attenuator
Low Pass
Filter