User's Manual

14 68P80801H45-1 1/20/2002
Power Amplifier (PA) EBTS System Manual - Vol 2
PA Theory of Operation
RF Combiner/Peripheral Module (40- 800 MHz, 70W- 800 MHz PAs)
This module consists of two portions: an RF combiner and a peripheral module.
The RF Combiner portion of the module combines the three RF signals from the
RF Interconnect Board (40W- 800 MHz PA) or the Combiner Board (70W- 800
MHz PA) 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 single stage circulator, which protects the PA in
the event of high reflected power.
The peripheral portion of the 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 parameters, the BRC
will issue a warning to the site controller which, in turn, will shut down the
Exciter if required.
Reflected power is monitored in the same manner. The BRC uses the reflected
power to calculate the voltage standing wave ratio (VSWR). If the VSWR is
determined to be excessive, the forward power is rolled back. If it is extremely
excessive, the BRC issues a shut-down command to the Exciter.
A thermistor is located on the RF Combiner/Peripheral module to monitor the
operating temperature of the PA. The thermistor signal indicating excessive
temperature is applied to the A/D converter and then sent to the BRC. The BRC
rolls back forward power if the monitored temperature is excessive.
900 MHz PA
This module consists of two parts: an RF combiner and a peripheral module. The
RF combiner combines the two RF signals from each LDM into a single signal,
using a branchline coupler arrangement. Then, the RF signal passes through a
directional coupler which derives a signal sample of the LFMs 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, thereby allowing the Exciter to accordingly
adjust signal drive. Following the coupler, the power output signal is passed
through a circulator, which protects the PA in the event of high reflected power.
A power monitor circuit 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 parameters, the BRC
will issue a warning to the site controller which, in turn, will shut down the
Exciter if required.