User's Manual

68P80801E35-A 9/1/2001 15
EBTS System Manual - Vol 2 Power Amplifier (PA)
PA Theory of Operation
1500 MHz
Both LFM outputs are input into this module where they are combined, with a
branchline coupler, for a single output signal. The RF signal is first coupled to the
Exciter module, via the DC/Metering Board, so that it can be monitored. The RF
output signal is then passed through a circulator that acts as a protection device
for the PA in the event of reflected power.
A power monitor circuit monitors the forward and reflected power of the output
signal. This circuit provides the A/D converter on the DC/Metering board with
an input signal representative of the forward or reflected power levels.
For forward power, a signal representative of the measured value is sent to the
BRC module via the SPI bus. The BRC determines if this level is within tolerance
of the programmed forward power level. The programmed forward power is set
through the use of MMI commands. If the level is not within certain parameters,
the BRC will issue a warning to the site controller and may shut-down the Exciter
module.
Reflected power is monitored in the same manner except that the BRC determines
an acceptable reflected power level. The BRC calculates the reflected power
through an algorithm stored in memory. If the reflected power is determined to be
excessive, the forward power is rolled back. If the reflected power level is
extremely excessive, the BRC will issues a shut-down command to the Exciter
module.
A thermistor is located on the RF Combiner/Peripheral module to monitor the
operating temperature of the Power Amplifier. A voltage representative of the
monitored temperature is sent from the A/D converter to the BRC. The BRC
issues a cut-back command to the Exciter module if the monitored temperature is
greater than 121˚ F (85˚ C).
RF Combiner/Peripheral Module (800 MHz QUAD and 900 MHz QUAD)
This module consists of two parts: an RF combiner and a Peripheral module. The
RF combiner combines three RF signals from the Combiner Board into a single
signal using a Wilkinson coupler arrangement. Following the combiner circuit,
the single combined RF signal is then passed through a directional coupler, which
derives a signal sample of the LFM RF power output. Via the coupler, a sample of
the RF output signal is fed to the Exciter, via the DC/Metering Board, as a
feedback signal. Following the coupler, the power output signal is passed through
a dual stage circulator, which protects the PA in the event of high reflected power.
The Peripheral module provides a power monitor circuit that monitors the
forward and reflected power of the output signal. This circuit furnishes the A/D
converter on the DC/Metering Board with input signals, representative of the
forward and reflected power levels.
For forward power, a signal representative of the measured value is sent to the
BRC via the SPI bus. The BRC determines if this level is within tolerance of the
programmed forward power level. If the level is not within tolerance, the BRC
will issue a warning to the site controller, which, in turn, will shut down the
Exciter, if required.