Directional Coupler; Harmonic Filter - Motorola APX 3000 Detailed Service Manual

Theory of Operation: Main Board
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.
VHF: C927, L907 C920, C921, L908, C922, L909, C924, L910, C923, C928 and C925 are
theelements 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.
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, (UHF1/UHF2 – Q1601, VHF – Q901 and 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 L1606 for UHF1/ UHF2, L906 for VHF and L1007 for the 700/800 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
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.
VHF: C927, L907 C920, C921, L908, C922, L909, C924, L910, C923, C928 and C925 are
theelements 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.
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.
UHF1/ UHF2/ VHF/ 700–800 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.
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
from450–520 MHz while the stop band is above 900 MHz.
VHF: The harmonic filter apply discrete components as the circuit line up. The pass band is up to174
MHz while the stop band is above 550 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.
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