Figure 3-9. 200 Mhz Vco Block Diagram - Motorola HT1250-LS+ Service Manual

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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.
Rx-SW
Tx-SW
(U3701 Pin 28)
Steer Line
Voltage
(VCTRL)
RX RX VCO
Tank Circuit
TX VCO
TX
Tank Circuit
Pin 20
Pin 7
Pin 13 Switching Network
Pin 3 Vcc-Superfilter
Collector/RF in
Pin 4
RX
Pin 5
Rx Active
Pin 6
Pin 16 TX
Tx Active
Pin 15
Vsens
Circuit
Pin 18
Vcc-Logic
(U3701 Pin 28)

Figure 3-9. 200 MHz VCO Block Diagram

AUX3 (U3701 Pin 2)
TRB_IN
Pin 19
TX/RX/BS
Presc
U3801
VCOBIC
RX
Bias
TX
Bias
Pin 2 Pin 1 Pins 9,11,17
Rx-I adjust Tx-I adjust
Theory of Operation
Prescaler Out
Pin 12
U3701 Pin 32
LO RF INJECTION
Pin 8
Matching
Low Pass
Network
Pin 14
(U3701 Pin 28)
VCC Buffers
Pin 10
TX RF Injection
Attenuator
Filter