Motorola APX 5000 Service Manual page 48

3-22
3.1.4.9 Buffers and VCOs
Q774 and surrounding circuitry is the prescalar buffer that takes the output of the VCOs and feeds
the prescalar input to the Trident IC, pin G1 (M_PRSC).
Q713 and surrounding circuitry is a buffer that provides the correct drive level to the receiver section
(via the transmission line TL_RX_LO) and to the input to the TX buffer (Q842 and surrounding
circuitry). Q712 and surrounding circuitry provide the bias to the buffer. The buffer formed by Q713
and its associated circuitry is called a "pre-buffer" at this stage.
R712, R713 and R714 help provide some extra isolation to the receiver.
Q842 and surrounding circuitry is the transmit injection buffer. The transmit injection buffer provide
the correct drive level to the transmitter section (via the transmission line TL_TX_LO). Q703 and
surrounding circuitry provide the bias to the transmit injection buffer.
VHF: The TX and RX VCOs used for the VHF band are contained in Y704 and Y707 respectively. To
select the VHF TX VCO, pin 3 (SEL1) must be at a high logic level and pin 5 (SEL2) at a low logic
level, on Y704. The VHF TX output of Y704, pin 1 (POUT) goes to pin 4 (RF1) of 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 VHF TX signal then
goes to the divide-by-two circuitry in the Trident IC, pin J2 (ESC_IN). Resistors R703, R704 and
capacitors C705, C748 and C747 provide the correct bias for the divide-by-two output. The output of
the divide-by-two circuitry, pin K1 (ESCD2_OUT) then goes through a 'second harmonic filter'
(comprised of C800, C801, C803 and L802) and attenuator (comprised of R709, R710 and R711).
The output of the attenuator is then fed to the transmit buffer (Q842 and surrounding circuitry). The
output of the transmit injection buffer, then goes to the transmit section via the TL_TX_LO
transmission line.
To select the VHF RX VCO, pin 3 (SEL1) must be at a high logic level and pin 5 (SEL2) at a low logic
level, on Y707. The output of the VHF RX VCO, pin 1 (POUT) of Y707 goes to pin 8 (RF3) of U709.
The output of U709, pin 1 (RFC) is split into two signals, one to the prescalar buffer (Q774) and the
other to the pre-buffer. The output of the pre-buffer 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.
UHF1: The TX and RX VCOs used for UHF1 are contained in Y705 and Y704. To select the UHF RX
VCO from (380–450 MHz), pin 3 (SEL1) must be at a high logic level and pin 5 (SEL 2) at a low logic
level, on Y704. To select UHF RX VCO (450–470 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 (380–445 MHz), pin 3 (SEL1) must be at a high logic level and pin 5
(SEL2) at a low logic level, on Y705. To select the UHF TX VCO (445–470 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).
Theory of Operation: Transceiver Board