Power Control - Motorola ASTRO XTL 1500 Detailed Service Manual

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 L3700, L3731-2, and C3702. When K9.1V is present, the switch is in
TX mode. D3701-2 and D3704 are forward biased forming a low-loss path from the RFPA final stage
to the harmonic filter and a 60 dB isolation path between the RFPA final stage and the RX front-end.
Isolation is achieved via a quarter-wave transmission line between D3701 and D3702. D3701-4
serves as an ESD protection circuit against ESD discharge on the antenna connector.
When K9.1V is absent, the switch is in RX mode. D3701 and D3702 are reverse biased forming a
low-loss path from the harmonic filter to the RX front-end and a 60 dB isolation path from the
harmonic filter to the RFPA final stage. Isolation is achieved via the D3701 off resistance.
Harmonic Filter
L3720-7, C3720-3, form the twelve-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. C3730-1, D3730, L3730, R3730-3, and R3735-6 form the
forward-power detector (VFORWARD), which is used for power leveling. C3732-3, D3731, L3731,
R3737-9, and R3733-4 form the reverse-power detector (VREVERSE). L3737 provides an
electrostatic discharge path to protect the RFPA final stage device.

3.12.1.3 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 Figure 3-24).
6815854H01-A
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June 15, 2005