Motorola APX 5000 Service Manual page 41

Theory of Operation: Transceiver Board
3.1.3.7.1 Voltage Control Mode
The heart of the voltage control loop is a logarithmic amplifier based power control IC, U1105. Quad
DAC, U1125, receives the power tuning values via the SPI bus and converts them into a voltage at
"VOUTB." Resistors, R1121 and R1122, form a voltage divider to set the full-scale value of the DAC,
in this case approximately 1.4V. This power set voltage is then fed to the power control IC through
the current cutback op amp, U1130, and then into a lesser-of-two voltage decision circuit, consisting
of U1126 and U1127. This circuit, used exclusively in voltage control mode, provides the important
function of combining the MAKO ramp output with the DAC power set voltage to permit power
leveling since the MAKO DAC max amplitude cannot be controlled during TDMA mode. In all other
TX modes, the MAKO output is a fixed voltage, approximately 1.5V, which is always higher than the
DAC control voltage. The lesser-of-two circuit will then select the smaller input, the set voltage from
U1125, resulting in immediate TX turn on in analog or ASTRO mode. IN TDMA mode, the MAKO line
is a piecewise linear ramp whose timing is in accordance with Phase 2 requirements. At t = 0, the
ramp line is smaller than the TX set voltage so the MAKO ramp will control the TX power level,
resulting in a slower ramping up of the TX power. This continues until the MAKO ramp output
reaches the level of the power tuning DAC (which is always lower than the MAKO ramp maximum)
which causes control of TX power to be turned over to the power tuning DAC.
The output of the selector circuit passes through a 2nd order low pass filter (U1142) and then to the
log amp, U1105. The low pass filter performs the dual function of improving transient ACPR by
transforming a linear ramp with corners into a smooth second order waveform and by acting as a
reconstruction filter for the DAC. The log amp converts RF power fed back from the TX PA into a
current which is summed with the current from the conversion of the setpoint voltage from DAC
U1125. Any imbalance between the RF input level and the level corresponding to the setpoint
voltage is corrected at the VAPC output of the log amp which in turn drives the control voltage input
of the RFPA. The setpoint voltage effectively nulls the error in the loop caused by changes in the RF
level fed back to the log amp. RF from the RFPA is coupled through a directional coupler embedded
in the PC board and passed through a LC equalizer and then to digital attenuator, U1112, which is
used to implement thermal cutback in the event of an over-temperature condition.
Current protection and limiting in voltage control mode is 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 both 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.
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