Directional Coupler - Motorola APX 5000 Service Manual

Theory of Operation: Transceiver Board
3.1.3.1 Driver Amplifier
The driver amplifier IC (VHF – U902, UHF1 – U1602, UHF2 – U1500 and 7/800 – U1002) contains
one LDMOS FET amplifier stages and an 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 serve 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 C1504 is the inter-stage matching network of the driver amplifier IC. C1508,
C1509, C1510, C1511, L1502, L1503, and L1504 serve as matching circuit of the driver IC to the
final device of Q1500. Capacitor C1510 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, Q901, Q1013, Q1500 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 using the RF board screw. All
FETs are matched using a lowpass topology. Drain bias is applied through L906 for the VHF, L1606
for UHF1, L1505 for UHF2 and L1007 for the 7/800 MHz. 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 consists of R1603, R1604, R1605, C1615 and C1617.
UHF2: C1516, C1517, C1518, C1520, C1521, C1527, and L1507 are the elements of the output
matching network apart from a transmission-line structure. The Gate biasing is applied through a
biasing network consists of R1502, R1503, R1504 and C1522.
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.
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