4/18/11
888-2587-001
4-3
WARNING: Disconnect primary power prior to servicing.
Section 4 Boost Pro Theory of
FM HD Radio System
One of the requirements of the Boost Amplifier is to have adjustable phase for aligning
multiple cabinets. For this reason, the controller board features a 90° hybrid in a voltage
controlled phase shifter circuit. The phase shifter has the capability of adjusting the
phase of the RF from 0º to -90° compared to the input.
Several analog voltages are needed throughout the Boost Pro. The phase adjustment
circuit and the APC circuit both require an analog voltage to operate correctly. Analog
voltages representing the power amplifier voltage, current, forward power, and reflected
power are also needed for setting external limits. For this reason three AD5327s are
used.
4.2.1
Signal Descriptions
•
PA_APC_INTERNAL: This is the internally generated APC voltage used for power
control.
•
PHASE_ADJUST: Voltage used to control the phase of the RF signal.
•
RFLD_PWR_TRIP: This voltage will set a trip point to mute the PA if the reflected
power becomes too great. This voltage will change when the modulation mode
changes.
•
FWD_PWR_TRIP: Same concept as RFLD_PWR_TRIP except for forward power
is monitored.
•
RFLD_PWR_SAMPLE: This analog voltage represents the reflected power sampled
on the analog-to-digital converter, scaled for the Z-interface.
•
FWR_PWR_SAMPLE: Same concept as RFLD_PWR_SAMPLE except forward
power is sampled.
•
TX_RFLD__PWR_SAMPLE: Same concept as RFLD_PWR_SAMPLE except the
output is on the user I/O connector and may be scaled differently then the
RFLD_PWR_SAMPLE.
•
TX_FWR_PWR_SAMPLE: Same concept as FWR_PWR_SAMPLE except the out-
put is on the user I/O connector and may be scaled differently then the
FWD_PWR_SAMPLE.
•
PA_CURR_SAMPLE: Analog voltage representing the PA current, sampled on the
analog-to-digital converter.
•
PA_VOLT_SAMPLE: Analog voltage representing the PA voltage, sampled on the
analog-to-digital converter.
•
APC_SCALING_V: This voltage will be used to determine how much of the APC
voltage will be used in the variable attenuator.
