Motorola APX 2000, Detailed Service Manual

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Theory of Operation: Main Board 3-13
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 L1007 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, L907 C920, C921, L908, C922, L909, C924, L910, C923, C928 and C925 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.
700/800/900 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/UHF2/700/800/900 MHz: The directional coupler consists of three embedded
transmission lines.
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 550 MHz.
UHF1: 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.
UHF2: The harmonic filter apply discrete components as the circuit line up. The pass band is from
450–520 MHz while the stop band is above 900 MHz.
700/800 MHz: The harmonic filter uses both discrete components and transmission lines. The pass
band is up to 870 MHz, and the stop band is above 1500 MHz.
900 MHz: The harmonic filter uses both discrete components and transmission lines. The pass band
is from 896–941 MHz, and the stop band is above 1500 MHz.
3.1.3.5 Antenna Switch
The antenna switch consists of discrete components which comprise three ports, transmit, receive
and output path. During transmit mode, the antenna switch will be turned ON via Filter_Raw B+ and
input logic gate from Ant_Swi. When the conditions are fulfilled, the antenna switch will be turned ON
and the signal will route to the output port as it will notice high impedance on the receive path. The
same concept is applied during the receive mode where the switch is in OFF state, which provides
high RF impedance on the transmit mode.