4-16 888-2595-001 12/20/11WARNING: Disconnect primary power prior to servicing.Section 4 Theory of Operation ZX Series8. PWR_CTL line to exciter or IPA bias control (U28C)9. RF amplification by IPA-PA modules with power set by exciter or IPA bias.10. Transmitter output (loop complete).Comparators U25C and U25D compare the forward and reverse power samplesentering the APC circuit against a fixed +2.25V reference from voltage divider R107-R35 and signals a fault when the forward sample is too low (low gain) or the reversesample is too high (reverse foldback).Potentiometer R13 adjusts the gain applied to the reverse power sample by variablegain amplifier U24A, thereby setting the threshold where the reverse power foldback isinvoked by the APC circuit.4.12.3 Operating Mode and Bias Level ControlThe transmitter is capable of operating in any one of three modes based on incomingcommand lines from the exciter. The transmitter determines its operating state via theFM_ON_STATUS and HD_ON_STATUS lines available at the transmitter interfaceconnector of the Harris FlexStar HDX exciter. These lines are pulled low by the exciterwhen the FM carrier or HD carriers are present at the exciter output. These lines arepassed via the I/O flter board to the controller via the FM_ON_BUS and HD_ON_BUSsignal lines.The grounding of the FM_ON_BUS status line causes the gate of transistor Q2 to golow, thereby applying a logic high signal to input A of 4-bit decoder IC U12. A similaroperation takes place with the HD_ON_BUS line, Q3, and U12-B input. The status ofthese two lines can select on of three possible transmitter operation modes: FM,FM+HD, or HD/SL according to the following truth table:The FlexStar exciter split level mode is mapped to the transmitter HD mode via steeringdiode CR8. This was done because a transmitter operating in split level mode (FM+HDTable 4-1 Operating Mode TableFM_ON_BUS HD_ON_BUS FlexStar Mode TX Modefalse (high) false (high) Split Level Mode HD/SL Modetrue (low) false (high) FM Mode FM Modefalse (high) true (low) HD Mode HD/SL Modetrue (low) true (low) FM+HD Mode FM+HD Mode
