NX15/NX25 Operations and Maintenance Manual
Description
Issue 6.0 2019-01-31
Page 1-5
The digital PDM component consists of six phased PDM signals, each separated by 60 electrical
degrees. These PDM drive signals determine the transmitter output power level as well as the
output modulation level. Three of these phases, each separated by 120 electrical degrees (e.g., 1,
4 and 7), are applied to a given RF power module. To achieve optimal harmonic cancellation,
three different phases (e.g., 2, 5 and 8) are applied to the adjacent module.
Samples of the RF output voltage, RF output current, SCR rectifier temperature and RF power
module temperature are monitored. If a parameter exceeds an acceptable limit, the active exciter
attempts to compensate by decreasing its output power to restore the parameter to an acceptable
level
.
A sample of the B+ supply voltage is monitored. A B+ compensation circuit adjusts the PDM
duty cycle to compensate for variations in the B+ supply in order to maintain constant
transmitter output power and minimize hum.
A PDM inhibit (A or B) input is applied from the control/interface PWB to inhibit the PDM
drive for a specific exciter during certain alarm/fault conditions.
RF Drive Distribution
The control/interface PWB (A11A1) accepts the RF drive (+ and -) signals from the digital AM
exciter PWBs (A and B) and splits the active exciter’s RF drive 1 and 2 signals for application to
the RF drive distribution PWB (A7). The RF drive distribution PWB splits the signal from the
control/interface PWB and buffers the individual outputs that are provided to the RF power
stage.
PDM Distribution
The control/interface PWB (A11A1) accepts the PDM signals from the digital AM exciter
PWBs (A and B) and splits the active exciter PDM signals for application to the PDM
distribution PWB (A8). The PDM distribution PWB accepts each signal from the control/
interface PWB and splits it into two (+ and -) differential outputs that are provided to the RF
power stage.
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