118 Power Amplifier Circuit Description TB8100 Service Manual© Tait Electronics Limited September 20067.1.4 ShutdownThe PA software will prevent transmission under the following conditions:■ the following calibration procedures were not completed successfully:■ 6W driver transistor, 60W final transistor 1 or 60W finaltransistor 2 bias■ detector bias■ supply voltage■ output power■ the supply voltage drops below 24VDC or exceeds 32VDC■ the transmit current of the 6W driver and 60W final transistors does notreduce to a safe level after foldback■ the configuration of the PA is invalid.7.1.5 Power ControlThe PA has a power control circuit which keeps the output power constantand protects the PA from VSWR mismatch conditions. The control circuitmonitors forward and reverse power and these measurements are used to:■ determine forward and reverse power■ keep the output power constant■ foldback the power under mismatch conditions■ calculate VSWR.The power level is set by requesting the power level in watts via an I 2 C busmessage. The minimum size of a power step is 1W and the range of outputpower settings is:The forward power, reverse power and load VSWR measurements may alsobe requested via I 2 C bus messages.The foldback loop is analogue until the VSWR reaches 10:1, at which pointthe microprocessor initiates foldback.The microprocessor and control loop are also used to shape the RF signalenvelope during key-on and key-off transients.Model Output Power Range5W 1W to 5W50W 5W to 50W100W 10W to 100W
