Motorola APX 1000 Detailed Service Manual page 717

Theory of Operation: Main Board
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.
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.
3.1.3.6 Reverse Power Protection
The radio, while in receive mode is constantly monitoring the input power from the antenna. This
power is sensed by the directional coupler and channeled into an RF detector U1106. The matching
network between the coupler and the detector consists of R1107, L1102, C1105 and C1107. Once
the input RF level exceeds a certain limit, the detector trips a logic circuit which enables attenuation
to protect the RF front end. This is to protect the front end from receiving large signal damage.
3.1.3.7 Transmitter Current Control
In TX mode, the transmitter Automatic Level Control (ALC) section enables the transmitter and
controls the TX current. Current control is used in all transmit mode. Several other functions included
in the TX ALC section of the radio are thermal cutback of power, current cutback of power, and
reverse power detection with means to disable the receiver in the event of high reverse power at the
antenna port.
3.1.3.7.1 Thermal Cutback
Temperature is sensed by IC U1238 and is located next to RFPA finals. Comparators, U1254
establish a temperature trip points. The combined logic of the comparator and temperature sense IC,
U1238, will trigger the transistor Q1264 and attenuate the power by reducing the voltage feed into
the smaller picker by 3 dB. There is only 1 stage of thermal cutback in the circuitry.
3.1.3.7.2 Current Control Mode
To enable the TX section, two control signals (DIG_DMCS and DIG_GPIO49) are required to trigger
the Tx circuitry. Once these signals are received, the TXH control logic is enabled and voltage will be
supplied to the driver amplifier.
In Current Control Mode, the sensing resistor R1229 is used to sense the small voltage drop across
the resistor. This voltage (Delta Voltage) is then feed into the differential operational amplifier U1202
to obtain the difference in voltage level. The output of the differential amplifier will then be filtered by
a second order low past filter before being amplified by the integrator op-amp.
3-13