Motorola APX 2000, Detailed Service Manual

Page: 48 / 1096

3-22 Theory of Operation: Main Board
To select the 700 TX 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 700 TX 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 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.
To select the 700/800 MHz TX VCO, pin D5 (GPO2) must be at a high logic level and pin E5 (GPO1)
and pin D6 (GPO8) must be at a low logic level. The output of the 700/800 MHz TX 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 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.
To select the 800 TX 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 800 TX 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 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.
900 MHz: The voltage-controlled oscillators are varactor tuned. The voltage (0.3V–10.6V) varies as
it is being applied to varactors VR3006, VR3008, VR3009, VR3010 and VR3011, VR3013, VR3014,
VR3048 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 bandwidth of the 900 MHz, 896–901 MHz and 935–940 MHz for transmitter, 935–940 MHz
for receiver.
• TX VCO covers the transmit frequencies from 896 MHz to 941 MHz
• RX VCO covers the receive Local Oscillator (LO) frequencies from 825.35 MHz to 830.35 MHz
The TX VCO and the RX VCO are selected using GPO1 and GPO2 respectively.
To select the 900 MHz RX VCO, pin D5 (GPO1_TEST2) must be at a high logic level and pin E5
(GPO2_TEST1) must be at a low logic level. The output of the RX VCO 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, with pin A5 (RFSTEPA_GPO6) at a high logic level,
output pin 5 (RF2) is selected and then goes to the attenuator (comprised of R703, R704 and R707)
and finally fed to the receiver section via the RX_LO transmission line.
To select the 900 MHz TX VCO, pin E5 (GPO2_TEST1) must be at a high logic level and pin D5
(GPO1_TEST2) must be at a low logic level. The output of the TX VCO 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, with pin A5 (RFSTEPA_GPO6) at a low logic level, pin 4
(RF1) is selected and 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.