Motorola APX 1000 Detailed Service Manual page 718

3-14
In order for the Automatic Logic Control (ALC) to function, 2 inputs are needed to be supplied to the
Integrator operational amplifier. First is the voltage from the sensing resistor and the second voltage
from the 10-bit DAC IC and MAKO. In order to trigger the voltage from the 10-bit DAC IC, three
control signals are required, ANA_CTRL_VSET, DIG_SPI_CLK, and DIG_DATA_SPI_MOSI. Once
these control signals are received, the 10-Bit DAC IC will provide an output based on the DAC
programmed. The output of the 10-Bit DAC IC ranges from 0V to 5V. The DAC scalar (U1253) will
then scale the voltage via a summing amplifier to enable a full voltage scaling. This voltage is known
as Vdac. The smaller picker (U1269) circuitry will then compare the input voltage from Vdac and
MAKO voltage (0-5V). The smaller picker will output the smaller voltage between the 2 voltages. As
the Vdac rise time is shorter than the MAKO voltage rise time in micro seconds, the initial rise time of
the output of smaller picker will be picked by the Vdac. However, when the MAKO voltage exceeds
the Vdac voltage, the output of the smaller picker will be picked from the MAKO voltage. The final
output voltage from the smaller picker will then be filtered by a Sallen Key active low pass filter
before feeding it to the Integrator voltage reference pin.
The integrator will then integrate the voltage feed into its Voltage reference pin versus the Voltage of
the second order low pass filter. The output voltage of the integrator will be double up by an op-amp.
This control voltage is then feed into the Power amplifier and Driver amplifier to control the current of
the active devices.
3.1.3.7.3 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
MHz) and then to the PA driver and final.
Theory of Operation: Main Board