Motorola APX 6000, Service Manual

Page: 49 / 438

Theory of Operation: Transceiver Board 3-23
UHF2 : The TX and RX VCOs used for UHF2 are contained in Y705 and Y704. To select UHF RX
VCO (450–520 MHz), pin 3 (SEL1) must be at a low logic level and pin 5 (SEL 2) at a high logic level,
on Y704. The UHF RX VCO output of Y704, pin 1 (POUT), is then fed to pin 4 (RF1) of switch U709.
The output of U709, pin 1 (RFC), is then split into two signals. One to the prescalar buffer input and
the other to the pre-buffer. The output of the prebuffer is then fed to pin 1 (RFC) of U710. The output
of U710, pin 5 (RF2) then goes to the attenuator (comprised of R712, R713 and R714) and then fed
to the receiver section via the TL_RX_LO transmission line.
To select the UHF TX VCO (450–520 MHz), pin 3 (SEL1) must be at a low logic level and pin 5
(SEL2) at a high logic level, on Y705. The UHF TX VCO output of Y705, pin 1 (POUT), is then fed to
pin 5 (RF2) of switch U709. The output of U709, pin 1 (RFC), is then split into two signals. One to the
prescalar buffer input and the other to the pre-buffer. The output of the prebuffer is then fed to pin 1
(RFC) of U710. The output of U710, pin 4 (RF1) then goes to the transmit injection buffer (comprised
of Q842 and surrounding circuitry).
700–800 MHz: The TX and RX VCOs used for 700–800 MHz are contained in Y706 and Y707. To
select the 700 RX VCO, pin 5 (SEL2) must be at a high logic level and pin 3 (SEL1) at a low logic
level, on Y706. The 700 RX VCO output of Y706, pin 1 (POUT), is then fed to pin 5 (RF2) of switch
U709. The output of U709, pin 1 (RFC), is then split into two signals. One to the prescalar buffer
input and the other to the pre-buffer. The output of the prebuffer is then fed to pin 1 (RFC) of U710.
The output of U710, pin 5 (RF2) then goes to the attenuator (comprised of R712, R713 and R714)
and then fed to the receiver section via the TL_RX_LO transmission line.
To select the 800 RX VCO, pin 3 (SEL1) must be at a high logic level and pin 5 (SEL2) at a low logic
level, on Y706. The 800 RX VCO output of Y706, pin 1 (POUT), is then fed to pin 5 (RF2) of switch
U709. The output of U709, pin 1 (RFC), is then split into two signals. One to the prescalar buffer
input and the other to the pre-buffer. The output of the prebuffer is then fed to pin 1 (RFC) of U710.
The output of U710 (RF2) then goes to the attenuator (comprised of R712, R713 and R714) and
then fed to the receiver section via the TL_RX_LO transmission line.
To select the 700 TX VCO, pin 3 (SEL1) must be at a high logic level and pin 5 (SEL2) at a low logic
level, on Y706. The 700 TX VCO output of Y706, pin 1 (POUT), is then fed to pin 5 (RF2) of switch
U709. The output of U709, pin 1 (RFC), is then split into two signals. One to the prescalar buffer
input and the other to the pre-buffer. The output of the prebuffer is then fed to pin 1 (RFC) of U710.
The output of U710, pin 4 (RF1) then goes to the transmit injection buffer (comprised of Q842 and
surrounding circuitry). The output of the transmit buffer, then goes to the transmit section via the
TL_TX_LO line.
To select the 800 TX VCO, pin 5 (SEL2) must be at a high logic level and pin 3 (SEL1) at a low logic
level, on Y706. The 800 TX VCO output of Y706, pin 1 (POUT), is then fed to pin 5 (RF2) of switch
U709. The output of U709, pin 1 (RFC), is then split into two signals. One to the prescalar buffer
input and the other to the pre-buffer. The output of the prebuffer is then fed to pin 1 (RFC) of U710.
The output of U710, pin 4 (RF1) then goes then goes to the transmit injection buffer (comprised of
Q842 and surrounding circuitry). The output of the transmit injection buffer, then goes to the transmit
section via the TL_TX_LO line.
To select the 7/800 TX VCO, pin 5 (SEL2) must be at a high logic level and pin 3 (SEL1) at a low
logic level, on Y707. The 7/800 talk around VCO output of Y707, pin 1 (POUT), is then fed to pin 8
(RF3) of switch U709. The output of U709, pin 1 (RFC), is then split into two signals. One to the
prescalar buffer input and the other to the pre-buffer. The output of the prebuffer is then fed to pin 1
(RFC) of U710. The output of U710, pin 4 (RF1) then goes then goes to the transmit injection buffer
(comprised of Q842 and surrounding circuitry). The output of the transmit injection buffer, then goes
to the transmit section via the TL_TX_LO line