4-4 MTR3000 Power Amplifier: Functional Theory Of Operation
A combination of software (SW) and hardware (HW) controls are used to regulate the PA RF output
power level. By way of software control, the SCM programs RF output power through the SPI bus.
Once programmed, the PA power control loop will level the output power to the requested level, if
possible. The requested output power may not be achievable if an alarm condition is present, and
output power may be reduced by way of SW or HW control if alarm conditions are present.
Within the base station/repeater, the modulated RF signal passes from the Exciter Module to the
driver amplifier within the PA. The driver amplifier is located on the RF Board and is a multi-stage
amplifier circuit that amplifies the RF input signal up to a maximum of approximately 4 W, as needed
to achieve the software requested base station/repeater RF output power. The RF signal from the
driver amplifier is further amplified by the final amplifier stage, which is also located on the RF Board
and consists of multiple paralleled RF devices with associated splitting and combining circuitry.
This RF signal then passes through a circulator and harmonic (low pass) filter, through an RF
directional coupler, and is output to the site transmit antenna port. The circulator prevents a high
Voltage Standing Wave Ratio (VSWR) at the PA output from damaging the final amplifier devices,
and also improves intermodulation attenuation (IMA) performance by providing isolation to the final
amplifier. The harmonic filter attenuates the harmonic levels generated by the preceding RF amplifier
devices.
The RF directional coupler is a calibrated wattmeter, with its calibration information stored in the PA
NVM. The forward and reverse power wattmeter provides a signal corresponding to the measured
output power to the ADC for metering purposes so that SW can monitor the PA or base station/
repeater output power. Also, this voltage is provided to the power control circuitry within the PA to
serve as the feedback signal in the power control loop for setting output power. The forward power
meter along with the reverse power meter is used by SW to measure the VSWR presented to the PA
output and to determine when an alarm condition is present.
The SW requested power level may include one or more SW controlled adjustments based on
various alarm conditions, if present, generated from monitored PA metering signals which are fed
back to the SCM via an ADC converter. Using the ADC metering circuitry, SW monitors the PA for
alarm conditions, such as high reflected power/VSWR or high/low supply voltage, and takes action to
reduce output power or dekey the station accordingly in order to protect the HW from damage or
improper operation.
Temperature monitoring circuitry is also contained within the PA. When the temperature reaches a
certain threshold, the PA fan turns on to reduce the operating temperature of the PA. If the
temperature continues to rise and reaches another predefined threshold, the base station/repeater
output power may also be reduced in order to protect the HW from thermal failure. Both the fan
control and the power control manipulation for thermal protection is fully HW controlled, and requires
no SW interaction.
Note
For procedure to replace the PA fan, refer to Section 14.7.2.1 on page 14-6.
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