3-10 Theory of Operation
vary between 3.5V and 9.5V. An additional loop filter pole (R3828, C3831 and C3843, further
attenuate noise and spurs on the steering line voltage.
In the receive mode, the TRB line (U3801 pin 19) is low. This activates the receive VCO and the
receive buffer of U3801, which operate within the range of 261.85 to 266.85 MHz. The VCO frequency
is determined by tank inductor L3822, C3830 and varactor D3821. The buffered RF signal at U3801
pin 8 is further amplified by Q3802 and applied as RX_INJ to the low-pass injection filter in the
receiver front end circuit.
In the transmit mode, U3801-19 is driven high (4.4V) by U3701 pin 2, enabling the transmit VCO and
buffer. The 217-222 MHz RF signal from U3801 pin 10 is applied as TX_INJ to the input of the
transmitter driver IC (U3501 pin 16) via a resistive attenuator pad. TX VCO frequency is determined
by L3833, C3870 and varactor D3831. Audio from the synthesizer IC is applied as VCO_MOD to
varactor D3832 which modulates the transmit VCO.
Figure 3-9. 200 MHz VCO Block Diagram
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
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