User's Manual

Digital ATSC Exciter-Modulator System Chapter 2,
Installation and Operating Instructions
Axcera Axciter, Rev. 0 10
Setup and Operation Procedures
The initial setup and operation of the ATSC modulator should be performed
after the unit has been connected to an external SMPTE 310M source and the
Axciter modulator has been connected to the upconverter or to the
driver/amplifier chassis. Also check that the output of the upconverter or the
driver/amplifier is connected to the transmitter and that the pre-filter and
post-filter sample inputs from the transmitter are connected to the
upconverter or the reay as detailed in Tables 2-1, 2-2, 2-3, & 2-4. A detailed
description of the operational modes, menus, and setup of the Axciter
modulator can be found in rest of Chapter 2 of this manual. A description of
the upconverter/ downconverter tray and sled-based upconverter and
downconverter modules can be found in Chapter 3.
Modulator Operational Overview
The Axciter 8-VSB modulator accepts an ASTC transport stream in SMPTE
310MfFormat input and outputs an 8-VSB IF signal centered at 44 MHz.
The 44 MHz IF signal is upconverted to the desired channel by the upconverter
located in the upconverter tray or, in a sled-based system, as an upconverter
sled mounted in the driver/amplifier chassis assembly.
The signal generation function of the Axciter is also referred to as the “forward
signal path” in this manual. There is also a “reverse signal path” that is used
for automatic adaptive equalization.
Two transmitter output samples are taken from directional couplers located
before and after the channel mask filter and are applied to the upconverter
tray or the external Relay. The upconverter tray also contains a
downconverter that converts the RF sample to an IF output. In a sled-based
system, the downconverter sled is mounted in the driver/amplifier chassis
assembly.
The downconverted IF transmitter sample is digitized by the Axciter
modulator. It is then demodulated in non-real time software. The result is
analyzed to calculate linear and nonlinear adaptive equalizers to improve the
transmitted signal quality by compensating for the nonlinear compression of
the power amplifier, and the linear distortions (mostly group delay effects) of
the channel filter.
When a linear adaptive equalizer is being calculated, the transmitter sample is
taken after the channel mask filter so that its linear distortions can be “seen.”
When a nonlinear adaptive equalizer is being calculated, the transmitter
sample is taken before the channel mask filter, so that the distortion
sidebands being generated by the power amplifier can be seen (the channel
filter would remove the out-of-band sidebands).
The Axciter modulator contains a standard Personal Computer. The PC
performs user interface functions and the numerical processing necessary for
the adaptive linear and nonlinear equalization. The PC is not in the forward
signal path; if it fails the exciter will continue to generate a signal with the
most recently calculated equalization.