Operating Mode And Bias Level Control - Harris ZX2500 Technical Manual

Section 4 Theory of Operation
8. 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 samples
entering 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 reverse
sample is too high (reverse foldback).
Potentiometer R13 adjusts the gain applied to the reverse power sample by variable
gain amplifier U24A, thereby setting the threshold where the reverse power foldback is
invoked by the APC circuit.

4.12.3 Operating Mode and Bias Level Control

The transmitter is capable of operating in any one of three modes based on incoming
command lines from the exciter. The transmitter determines its operating state via the
FM_ON_STATUS and HD_ON_STATUS lines available at the transmitter interface
connector of the Harris FlexStar HDX exciter. These lines are pulled low by the exciter
when the FM carrier or HD carriers are present at the exciter output. These lines are
passed via the I/O flter board to the controller via the FM_ON_BUS and HD_ON_BUS
signal lines.
The grounding of the FM_ON_BUS status line causes the gate of transistor Q2 to go
low, thereby applying a logic high signal to input A of 4-bit decoder IC U12. A similar
operation takes place with the HD_ON_BUS line, Q3, and U12-B input. The status of
these two lines can select on of three possible transmitter operation modes: FM,
FM+HD, or HD/SL according to the following truth table:
Table 4-1
FM_ON_BUS
false (high)
true (low)
false (high)
true (low)
The FlexStar exciter split level mode is mapped to the transmitter HD mode via steering
diode CR8. This was done because a transmitter operating in split level mode (FM+HD
4-16
WARNING: Disconnect primary power prior to servicing.
Operating Mode Table
HD_ON_BUS
false (high)
false (high)
true (low)
true (low)
888-2595-001
FlexStar Mode
Split Level Mode
FM Mode
HD Mode
FM+HD Mode
ZX Series
TX Mode
HD/SL Mode
FM Mode
HD/SL Mode
FM+HD Mode
12/20/11