Harris ZX2500 Technical Manual

Contents

Harris ZX2500 Technical Manual Manual pdf 103 page image

12/20/11 888-2595-001 4-23WARNING: Disconnect primary power prior to servicing.Section 4 Theory of OperationZX Seriesalong their length, the exciter main/alt switcher will respond by changing its exciterselection. This ground connection could come from the optional web remote card inany chassis, or directly to the J10 USER REMOTE connector from an external remotecontrol system installed by the end-user. Steering diodes inside each PA chassis on thelines going to the J10 connector protect the web remote card from potentially damagingvoltages should an external remote control by connected incorrectly. This allows thepins of J10 on the exciter main/alt switcher to be shared by two connections in completesafety.4.14.2 Power Control SharingConsult Figure 4-2, below.In each PA chassis, an AGC comparator (U25A on page 3 of 8010203541) compares aforward power sample from the amplifier’s output to a power setting reference. Theoutput of the comparator adjusts the power control circuits of the transmitter to increaseor decrease the output power level.NOTE:In reality, the APC comparator reference voltage remains fixed at 2V, and the for-ward power sample is scaled up and down to raise or lower power by making itappear higher or lower than the 2V reference. Figure 4.2 employs the abstractionof showing an adjustable reference voltage for the purpose of simplifying the dis-cussion.When multiple PA chassis are connected together via the system bus, the outputs fromall APC comparators are diode-ORed together (CR11 on page 3 of 801-0203-541) suchthat the power control for the transmitter – either the internal IPA bias voltage or theexciter APC voltage – will respond only to the highest output signal. The APC circuitsfrom the other chassis will remain dormant, with their output(s) being blocked by theirCR11 being in a non-conducting state.When the APC circuits are aligned per the procedures in Section 5 – Maintenance ofthis manual, a “dominant” PA chassis is chosen to perform power control. Byconvention and for convenience, this is usually the PA chassis closest to the exciters.The APC setting in the dominant PA is set to achieve the desired per-chassis power toachieve full transmitter output. All other PA chassis are set approximately 10% lower.This ensures that the power control function for the transmitter will always track thedominant chassis, while the APC circuits in the remaining chassis lie in reserve. Anyfine trim adjustments are done on the dominant chassis only, and from a top-level userstandpoint, the whole transmitter behaves as if there is only the single APC circuit inoperation. The reserve APC circuits spring into action if the dominant PA chassissuffers a failure or its chassis is shut down by the user.