Motorola APX 1000 Detailed Service Manual page 41

Theory of Operation: Main Board
provided by cascaded difference amplifiers, U1129 and U1130. A fixed threshold is provided by
voltage divider, R1169 and R1170. SPDT switch, U1144, changes the current limit threshold based
on the type of battery present. This threshold is based on the conversion characteristic of the current
shunt monitor circuit of U1101. The output of the current shunt monitor is fed to the first stage of the
difference amplifier. The setpoint voltage for the log amp is fed through the second stage of the
difference amplifier. When the current shunt monitor voltage exceeds the fixed threshold, the first
stage produces an output greater than 0V. Once the output of the first stage (U1129) is greater than
zero, this value is subtracted from the setpoint voltage to the log amp in the second stage (U1130),
resulting in a progressive cutback of power in the event PA current continues to climb above the
threshold.
3.1.3.7.2 Thermal Cutback
Thermal cutback works only in voltage control mode, which is the primary mode of TX power control.
Temperature is sensed by IC U1103 and is located next to RFPA finals. Comparators, U1113 and
U1114, establish two temperature trip points. The combined logic of the comparators and logic gate,
U1121, together with the truth table of the digital attenuator IC, U1112, determine the amount of
attenuation of the RF feedback to the log amp. Rated TX power is achieved with the attenuator at its
maximum attenuation of 7 dB. The first temperature threshold will subtract 3 dB of TX power and the
next (highest) trip point an additional 3 dB for 6 dB total.
3.1.3.7.3 Current Control Mode
In TDMA mode, excessive transient adjacent channel splatter caused by the RFPA, when it is under
a greater degree of compression than is expected at nominal supply voltage, is mitigated through a
system of saturation detection and switching of the TX power control mode from the voltage control
loop to the current control loop. Comparator U1131 compares the log amp output in voltage control
mode to a threshold voltage from DAC "C", which sets the threshold for handover to current control
mode and establishes the upper supply rail for the output of current loop integrator, U1104. This
threshold is tuned in the factory to correspond to transmitter rated power which means that the
threshold for handover occurs at a level corresponding to rated power minus 6% which is set by
divider R1162 and R1163. When PA saturation exceeds this threshold and the MAKO T/R control
signal (DIG_GPIO49) and DIG_DMCS combine logically to signal the end of the data portion of the
TX slot, D-flip flop U1132 latches which in turn switches (via U1133 SPDT switch) the output of the
current loop integrator, U1104, which sends the control voltage to either the VHF, UHF1/UHF2,
700/800 MHz or 900 MHz band PA.
Integrator, U1104, acts as a PI (Proportional Integral) controller in current mode. Current feedback
from the PA is obtained through current shunt resistor R1103 and sent to the current shunt monitor/
current to voltage converter, U1101. U1101 has a known gain characteristic for the current-to-voltage
conversion. The MAKO ramp is passed through a RC low pass filter for smoothing and on to the
current control integrator where it is proportioned with the voltage output of the current shunt monitor.
The integrator supply voltage comes from DAC "C" of the Quad DAC, U1125. By way of tuning the
DAC "C" value to match TX rated (tuned) power, this approach fixes the upper limit of the control
voltage in current control mode to correspond to the tuned power level, further reducing excess
adjacent channel splatter.
3.1.3.7.4 PA Offset, Control Voltage Gain Scaling
An offset voltage is applied to the control voltage at a summing junction made up of R1174 and
R1154 as a means of further improving transient adjacent channel splatter in the freq band by
pre-biasing the PA. The output of DAC "A" is summed with either the output of the log amp or the
integrator through R1174. The value for DAC "A" has been established and fixed in the firmware. is
sent to a non-inverting, gain-scaling amplifier, U901 (VHF), U1601 (UHF1/UHF2) or
U1001 (700/800/900 MHz) and then to the PA driver and final.
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