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Functional Description / Operation of Modules (Draft) 
 

The following table shows the specifications of MTR Power Amplifier (PA). 

Specifications of Power Amplifier 
Parameter Specification 
Operational Frequency Range 

851–870 MHz (800 MHz), 935 – 941 MHz (900 MHz) 

Minimum Input Return Loss (TX mode) 

10 dB 

Rated RF Output Power Range 

20–75 W 

Rated RF Input Power Range 

12.8–18.5 dBm 

28.6V Supply Nominal Voltage (AC Power) 

28.6V (+/- 4%) 

14.2V Supply Nominal Voltage 

14.2V (+/- 3%) 

28.6V Supply Maximum Current Draw 

12.6A 

14.2V Supply Maximum Current Draw 

2.0A 

Maximum Standby Power Consumption 

1.5 W 

 

Functional Theory of Operation 

A low-level modulated RF signal (approximately +16 dBm) from the Exciter Module enters the PA Input board via a 

coax cable.  The signal is input to the IPA and amplified to approximately 0 to 5.5 W (depending on the dc power 

control voltage (V_CNTL) from the PA output board).  The IPA output is then fed to the Driver PA module and 

amplified to greater than 10 Watts (depending on the dc power control voltage).  The Driver output is then fed to 
the Dual Device Module for further amplification. 

The output of the DDM is then transferred to the circulator and then to the harmonic filter/coupler via the PA output 

board.  The signal then exits from the Power Amplifier Module via an N-type coax connector. 

A single circulator is built into the Power Amplifier Module to provide isolation between the PA module and the 

transmit antenna. 

A thermistor mounted on the PA printed circuit board outputs a variable resistance proportional to the heat sink 

temperature.  This signal is fed to the power control circuitry on the PA output board, which monitors the signal and 

reduces the PA output power (and enables a fan) if the PA temperature exceeds set limits. 

A feedback and control loop configuration is used to regulate the PA output power.  The Directional Coupler 

generates a dc voltage proportional to the PA Module output power.  This voltage is fed to power control circuitry 
on the PA output board where it is compared to reference voltages to generate a dc power control voltage.  The 

Station Control Module sends current and power limit signals to a D/A converter (via the SPI bus) to control the 

maximum current/control voltage levels.  The control voltage is fed to the IPA module where it controls the IPA 

output, thus controlling the overall output from the PA Module. 

Analog signals from various strategic operating points throughout the PA module are fed to an A/D converter, 
which converts them to a digital signal and, upon request by the Station Control Module, outputs the signal to the 

Station Control Module via the SPI bus. 

For example, the directional coupler generates a dc voltage proportional to the reflected power.  This signal is 

converted to a digital signal and sent to the SCM.  If the sense line indicates an impedance mismatch (high 

VSWR), the PA is either cut back in power or shut down completely.