Motorola APX 1000 Detailed Service Manual page 716

3-12
3.1.3.1 Driver Amplifier
The driver amplifier IC (VHF – U902, UHF1/UHF2 – U1602, 700/800 MHz – 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 is the inter-stage matching network. Components C907, C910, C913, C914,
C916 L904, L905 match the output impedance to the input of the final device (Q901). Capacitor
C907 also serves as the DC block.
UHF1/UHF2: 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.
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 (VHF – Q901, UHF1/UHF2 – Q1601, 700/800 MHz – Q1001) are
Silicon N-channel MOSFETs housed in a high-power, surface-mount, PMM 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 VHF, L1606 for UHF1/UHF2 and L1606 for 700/800 MHz band. 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: C927, C920, C921, C922, C924, C928, C923, C925, L907, L908, L909 and L910 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 R903, R904, R905, C915 and C917.
UHF1/UHF2: 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.
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 is used only for receiving mode. Only forward power is feed into the directional
coupler without sensing any reverse power.
3.1.3.4 Harmonic Filter
The harmonic filter is a high-power, low-loss, low-pass filter. Its purpose is to suppress transmitter
harmonics. The filter also improves receiver out-of-band rejection. Shield SH8 must be in place to
achieve the required stop band rejection.
VHF: The harmonic filter apply discrete components as the circuit line up. The pass band is up to
174 MHz while the stop band is above 270 MHz.
UHF1/UHF2: The harmonic filter apply discrete components as the circuit line up. The pass band is
up to 520 MHz while the stop band is above 1200 MHz.
Theory of Operation: Main Board