Directional Coupler - Motorola APX 7000 Detailed Service Manual

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3-18
3.1.3.1 Driver Amplifier
The driver amplifier IC (VHF/UHF1/UHF2/700–800MHz: U1002, U902, U1500 and U1602) contains
two LDMOS FET amplifier stages and two internal resistor bias networks. Pin 16 is the RF input.
Modulated RF from the FGU, at a level of +3 dBm ±2 dB, is coupled through a blocking capacitor to
the gate of FET-1. An LC inter-stage matching network connects the first stage output VD1 to the
second stage input G2. The RF output from the drain of FET-2 is pin 6 (RFOUT1). Gain control is
provided by a voltage applied to pin 1 (VCNTRL). Typical output power is about +27 dBm (500 mW)
with VCNTRL at 5.5 V.
VHF: L901 and C904 are the components for the inter-stage matching network. Components C907,
C910, L904, C913, C914, L905 and C916 provide the match to the final device Q901. C907 also
serves as a DC block.
UHF1: L1601 and C1604 is the inter-stage matching network of the driver amplifier IC, C1607,
C1610, L1604, C1613, C1614, L1605 and C1616 serves as matching circuit of the driver IC to the
final device of Q1601. Capacitor C1607 also works as DC block to the circuit.
UHF2: L1501 and C1505 is the inter-stage matching network of the driver amplifier IC, U1500,
L1502, C1508, C1509, L1503, C1510, L1504 and C1511 serves as matching circuit of the driver IC
to the final device of Q1500. Capacitor C1509 also works as DC block to the circuit.
700–800 MHz: L1002, C1002 and C1004 are the inter-stage matching network. Components C1013
and C1023 match the output impedance to the input of the final device (Q1001); capacitor C1013
also serves as the DC block.
3.1.3.2 Power Amplifier Transistor
The power amplifier transistors, Q1013 and Q901 and Q1601 are LDMOS FETs housed in a
high-power, surface-mount, ring package. To prevent thermal damage, it is essential that the heat
sink of the power module be held in place against the radio chassis. All FETs are matched using a
lowpass topology. Drain bias is applied through L1007 for the 700–800 MHz, L906 for the VHF,
L1606 for UHF1 and L1505 for UHF2. Gain is dynamically controlled by adjusting the gate bias. The
gate is insulated from the drain and source so that gate bias current is essentially zero.
VHF: The output match consists of elements L907, C920, C921, L908, C922, L909, C924, L910,
C923 and C928. Gate bias is applied through R903, R904, R905, C915 and C917.
UHF1: C1620, C1621, C1622, L1609, C1624, L1610, C1623, C1628 and C1625 are the elements of
the output matching network apart from a transmission-line structure. The Gate biasing is applied
through a biasing network consist of R1603, R1604, R1605, C1615 and C1617.
UHF2: C1516, C1527, C1517, C1518, C1519, L1507, C1520 and C1521 are the elements of the
output matching network apart from a transmission-line structure. The Gate biasing is applied
through a biasing network consist of R1500, R1502, R1503, R1504, C1522 and C1523.
700–800 MHz: The input impedance-matching network is C1013 and C1023. A transmission-line
structure and C1019, C1020, L009 and C1021 form the output-matching network. Gate bias applied
through R1003, R1004, C1015 and R1005.

3.1.3.3 Directional Coupler

A directional coupler senses the transmitter forward and reverse power as control signals in the
transmitter's automatic level control (ALC) loop. Isolated ports are terminated with external resistors.
VHF/UHF1/UHF2/700–800 MHz: The directional coupler consists of three embedded transmission
lines.
Theory of Operation: Transceiver Board

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