Motorola Astro APX 3000, Detailed Service Manual

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3-18 Theory of Operation: Main Board
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 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.
VHF: The voltage-controlled oscillators are varactor tuned. The voltage (0.3V–10.6V) varies as it is
being applied to varactors D3206 to D3209 and D3246 to D3249 of their respective Voltage-
Controlled Oscillator (VCO). The capacitance of the varactors also varies, thereby changing
theoutput frequency of the VCOs. Both the VCOs are used to cover the entire 38 MHz bandwidth of
the VHF, 136 MHz to 174 MHz band.
• TX VCO covers the transmit frequencies from 136 MHz to 174 MHz
• RX VCO covers the receive Local Oscillator (LO) frequencies from 245.65 MHz to 283.65 MHz
The TX VCO and the RX VCO are selected using GPO1 and GPO2 respectively.To select the VHF
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 VHF 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 (VR3010M22, VR3003M22, VR3012M22 and VR3013M22
for 700MHz), (VR3011M22, VR3004M22, VR3014M22 and VR3015M22 for 700/800MHz) and
(VR3048M22, VR3017M22, VR3020M22 and VR3021M22 for 800MHz) 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.