Power Control Sharing - Harris ZX2500 Technical Manual

ZX Series
along their length, the exciter main/alt switcher will respond by changing its exciter
selection. This ground connection could come from the optional web remote card in
any chassis, or directly to the J10 USER REMOTE connector from an external remote
control system installed by the end-user. Steering diodes inside each PA chassis on the
lines going to the J10 connector protect the web remote card from potentially damaging
voltages should an external remote control by connected incorrectly. This allows the
pins of J10 on the exciter main/alt switcher to be shared by two connections in complete
safety.

4.14.2 Power Control Sharing

Consult Figure 4-2, below.
In each PA chassis, an AGC comparator (U25A on page 3 of 8010203541) compares a
forward power sample from the amplifier's output to a power setting reference. The
output of the comparator adjusts the power control circuits of the transmitter to increase
or 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 it
appear higher or lower than the 2V reference. Figure 4.2 employs the abstraction
of 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 from
all APC comparators are diode-ORed together (CR11 on page 3 of 801-0203-541) such
that the power control for the transmitter – either the internal IPA bias voltage or the
exciter APC voltage – will respond only to the highest output signal. The APC circuits
from the other chassis will remain dormant, with their output(s) being blocked by their
CR11 being in a non-conducting state.
When the APC circuits are aligned per the procedures in Section 5 – Maintenance of
this manual, a "dominant" PA chassis is chosen to perform power control. By
convention 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 to
achieve 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 the
dominant chassis, while the APC circuits in the remaining chassis lie in reserve. Any
fine trim adjustments are done on the dominant chassis only, and from a top-level user
standpoint, the whole transmitter behaves as if there is only the single APC circuit in
operation. The reserve APC circuits spring into action if the dominant PA chassis
suffers a failure or its chassis is shut down by the user.
12/20/11
WARNING: Disconnect primary power prior to servicing.
Section 4 Theory of Operation
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