Motorola ASTRO XTL 1500 Detailed Service Manual page 92

Theory of Operation: Transmitter
Antenna Switch
The antenna switch functions in two modes, which are determined by the presence of K9.1V. The
K9.1V switch bias is applied via L6700, L6702, and C6700. When K9.1V is present, the switch is in
TX mode. D6701 and D6702 are forward biased forming a low-loss path from the RFPA final stage to
the harmonic filter and a 20 dB isolation path between the RFPA final stage and the RX front-end.
Isolation is achieved via a quarter-wave transmission line between D6701 and D6702. C6703
resonates out the D6702 on inductance improving the isolation.
When K9.1V is absent, the switch is in RX mode. D6701 and D6702 are reverse biased forming a
low-loss path from the harmonic filter to the RX front-end and a 20 dB isolation path from the
harmonic filter to the RFPA final stage. Isolation is achieved via the D6701 off resistance. L6703
resonates out the D6701 off capacitance improving the isolation.
Harmonic Filter
L6720-2, C6720-1, and two open-stub transmission lines form the seven-element, low-pass
harmonic filter. The filter attenuates harmonics generated by the RFPA when the antenna switch is in
TX mode and provides extra selectivity when the antenna switch is in RX mode.
Power Detector
The power detector consists of two asymmetric, coupled transmission lines and detection circuitry
that detects forward and reverse power. C6730-1, D6730, L6730, R6730-3, and R6735-6 form the
forward-power detector (VFORWARD), which is used for power leveling. C6732-3, D6731, L6731,
R6737-9, and R6733-4 form the reverse-power detector (VREVERSE). C6734-5 provides additional
harmonic attenuation. R6740 provides an electrostatic discharge path to protect the RFPA final stage
device.
3.12.4.1.3 Power Control
The power control section is comprised of a control loop to level forward power, and protection
mechanisms to reduce power to a safe level for the given environmental conditions (see
on page 3-44).
6815854H01-A
3-43
Figure 3-33
June 15, 2005