Motorola APX 1000 Detailed Service Manual page 723

Theory of Operation: Main Board
To select the UHF TX VCO, pin D5 (GPO2_TEST2) must be at a high logic level and pin E5
(GPO1_TEST1) must be at a low logic level. The output of the TX VCO is then fed to pin 4 (RF1) of
switch U3001. The output of U3001 is then split into two signals. One to the Prescaler buffer input
and the other to the prebuffer. The output of the prebuffer is then fed to pin 1 (RFC) of U738. The
output of U738, pin 4 (RF1) then goes to the transmit injection buffer (comprised of Q774 and
surrounding circuitry). The output of the transmit buffer, then goes to the transmit section via the
TX_INJ transmission line.
UHF2: The voltage-controlled oscillators are varactor tuned. The voltage (0.3V–10.6V) varies as it is
being applied to varactors D3102 to D3105 and D3135 to D3138 of their respective Voltage-
Controlled Oscillator (VCO). The capacitance of the varactors also varies, thereby changing the
output frequency of the VCOs. Both the VCOs are used to cover the entire 70 MHz bandwidth of the
UHF range 2, 450 MHz to 520 MHz band.
• TX VCO covers the transmit frequencies from 450 MHz to 520 MHz
• RX VCO covers the receive Local Oscillator (LO) frequencies from 559.65 MHz to 629.65 MHz
To select the UHF2 RX VCO, pin E5 (GPO1_TEST1) must be at a high logic level and pin D5
(GPO2_TEST2) must be at a low logic level. The output of the RX VCO is then fed to pin 5 (RF2) of
switch U3238. The output of U3238 is then split into two signals. One to the Prescaler buffer input
and the other to the prebuffer. The output of the prebuffer is then fed to pin 1 (RFC) of U738. The
output of U738, pin 5 (RF2) then goes to the attenuator (comprised of R703, R704 and R707) and
then fed to the receiver section via the RX_LO transmission line.
To select the UHF2 TX VCO, pin D5 (GPO2_TEST2) must be at a high logic level and pin E5
(GPO1_TEST1) must be at a low logic level. The output of the TX VCO is then fed to pin 4 (RF1) of
switch U3238. The output of U3238 is then split into two signals. One to the Prescaler buffer input
and the other to the prebuffer. The output of the prebuffer is then fed to pin 1 (RFC) of U738. The
output of U738, pin 4 (RF1) then goes to the transmit injection buffer (comprised of Q774 and
surrounding circuitry). The output of the transmit buffer, then goes to the transmit section via the
TX_INJ transmission line.
700/800 MHz: The voltage-controlled oscillators are varactor tuned. The voltage (0.3V–10.6V) varies
as it is being applied to varactors VR3010 (700MHz), VR3011(700/800 MHz) and
VR3048 (800 MHz) of their respective Voltage-Controlled Oscillator (VCO). The capacitance of the
varactors also varies, thereby changing the output frequency of the VCOs. Three VCOs are used to
cover the entire 700/800 MHz band.
• VCO1 covers the RX 800 MHz band and the TX 700 MHz (764–776 MHz) band
• VCO2 covers the TX 700/800 MHz (794–824 MHz) band
• VCO3 covers the TX 800 MHz (850–870 MHz) band and the RX 700 MHz band
Three VCOs are selected using GPO1, GPO2 and GPO8 respectively.
To select the 700 RX VCO, pin D6 (GPO8) must be at a high logic level and pin E5 (GPO1) and pin
D5 (GPO2) must be at a low logic level. The output of the 700 RX VCO is then split into two signals.
One to the Prescaler buffer input and the other to the prebuffer input. The output of the prebuffer is
then fed to pin 1 (RFC) of U738. The output of U738, pin 5 (RF2) then goes to the pi attenuator
(comprised of R703, R704 and R707) and then fed to the receiver section via the RX_LO
transmission line.
To select the 800 RX VCO, pin E5 (GPO1) must be at a high logic level and pin D5 (GPO2) and pin
D6 (GPO8) must be at a low logic level. The output of the 800 RX VCO is then split into two signals.
One to the Prescaler buffer input and the other to the prebuffer input. The output of the prebuffer is
then fed to pin 1 (RFC) of U738. The output of U738, pin 5 (RF2) then goes to the pi attenuator
(comprised of R703, R704 and R707) and then fed to the receiver section via the RX_LO
transmission line.
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