TB8100 Service Manual Power Amplifier Circuit Description 115© Tait Electronics Limited September 20067.1 Microprocessor Control CircuitryRefer to Figure 7.2 on page 117.The PA has a microprocessor which performs the following functions:■ monitors the operating conditions of the PA■ controls the RF power level■ reports faults (by generating alarms)■ takes preventative action to protect the PA under fault conditions.The alarms and diagnostics are accessed through I 2 C bus messages via thereciter, control panel and Service Kit.There are no manual adjustments in the PA because all the calibrationvoltages and currents required to control and protect the PA are monitoredby the microprocessor. The software also automatically detects the PAconfiguration and controls the PA accordingly.7.1.1 Diagnostics and AlarmsThe diagnostic functions and alarms monitored by the PA include:■ the temperature of the 6W board■ the temperatures of 60W board 1 (50W and 100W PAs) and 60Wboard 2 (100W PA)■ the ambient temperature of the intake air (ambient temperature isdefined as the temperature of the air from the cooling fan; the PA fanmust be in position and running to get an accurate reading in the ServiceKit)■ forward and reverse power■ load VSWR■ the transmit current of the 6W driver transistor■ the transmit current of the final transistors on 60W board 1 (50W and100W PAs) and 60W board 2 (100W PA)■ the current imbalance between the final transistors on 60W boards 1 and2 (100W PA)■ the supply voltage■ the summary alarm.Most alarms have two thresholds. The first threshold activates the alarm andis usually set by the user via the Service Kit. The second threshold definesthe limit of operation of the PA and the software will either foldback the RFtransmit power or suspend transmission until the fault condition isrecovered. The values for the second threshold are set in the factory andcannot be changed. In all instances there is a small amount of hysteresisbetween the value at which the alarm, foldback or shutdown activates andthe value at which the alarm or shutdown is cleared.
