28 Functional Description TB7100 Service Manual© Tait Electronics Limited October 20053.2.3 RF Power AmplifierRF Power Amplifierand Switching The RF PA (power amplifier) is a four-stage line-up with approximately42dB of power gain. The output of the frequency synthesizer is first bufferedto reduce kick during power ramping. The buffer output goes to a broad-band exciter IC that produces approximately 200mW output. This isfollowed by an LDMOS driver producing up to 2 W output that is power-controlled. The final stage consists of two parallel LDMOS devicesproducing enough power to provide 25W at the RF connector.Output of RFPower Amplifier The output of the RF PA passes through a dual-directional coupler, used forpower control and monitoring. Finally, the output is low-pass-filtered tobring harmonic levels within specification.Power Control The steady-state power output of the transmitter is regulated using ahardware control loop. The forward power output from the RF PA is sensedby the directional coupler and fed back to the power control loop. The PAoutput power is controlled by varying the driver gate bias voltage that has acalibrated maximum limit to prevent overdrive. The power control signal issupplied by a 13-bit DAC driven by custom logic.Ramping Power ramp-up consists of two stages:■ bias■ power rampingThe timing between these two stages is critical to achieving the correctoverall wave shape in order to meet the specification for transient ACP(adjacent channel power). A typical ramping waveform is shown inFigure 3.4.Figure 3.4 Typical ramping waveformsPowerrampHigh powerpowerLowPowerTimeBiasrampBiasramp Powerramp
