Service manual

2-18
During the transmit condition, when PTT is pressed, 3.2 volts is applied to U3801, pin 19. This
activates the transmit VCO by enabling the transmit oscillator and the transmit buffer of U3801. The
RF signal at U3801, pin 10 is injected into the input of the PA module (U3501, pin16). This RF signal
is the TX RF INJECTION. Also in transmit mode, the audio signal to be frequency modulated onto the
carrier is received through U3701, pin 41.
When a high impedance is applied to U3801, pin19, the VCO is operating in battery saver mode. In
this case, both the receive and transmit oscillators as well as the receive transmit and prescaler buffer
are turned off.
Figure 2-14: VHF VCO Block Diagram
Table 2-6 VCO Control Logic
Desired Mode AUX 4 AUX 3 TRB
Tx Not Used High (@3.2V) High (@3.2V)
Rx Not Used Low Low
Battery Saver Not Used Hi-Z/Float
(@1.6V)
Hi-Z/Float (@1.6V)
Presc
RX
TX
Matching
Network
Low Pass
Filter
Attenuator
Pin 8
Pin 14
Pin 10
(U3701 Pin 28)
VCC Buffers
TX RF Injection
U3701 Pin 32
AUX3 (U3701 Pin 2)
Prescaler Out
Pin 12
Pin 19
Pin 20
TX/RX/BS
Switching Network
U3801
VCOBIC
Rx Active
Bias
Tx Active
Bias
Pin 2
Rx-I adjust
Pin 1
Tx-I adjust
Pins 9,11,17
Pin 18
Vsens
Circuit
Pin 15
Pin 16
TX VCO
Circuit
TX
Tank
RX VCO
Circuit
RX
Tank
Pin 7
Vcc-Superfilter
Collector/RF in
Pin 4
Pin 5
Pin 6
RX
TX
(U3701 Pin 28)
Rx-SW
Tx-SW
Vcc-Logic
(U3701 Pin 28)
Steer Line
Voltage
(VCTRL)
Pin 13
Pin 3
TRB_IN
LO RF INJECTION