Specifications

5/9/13 888-2857-001 5-17
WARNING: Disconnect primary power prior to servicing.
Section 5 Maintenance and Alignments
Platinum VAX-C Series
5.5 Pre-correction for Digital TV & DAB
Digital transmitters are typically operated in adaptive nonlinear predistortion and linear
equalization mode. The precorrection and equalization are applied in the modulator
section (exciter). The procedure described below will automatically calculate the
precorrection for an installed system. Once the precorrection has been calculated, it can
be stored in the setup memory for later use in Stored mode. The precorrection is
normally applied by setting both the Linear and Nonlinear RTAC mode to ADAPT.
Assumptions:
• Transmitter average power level is known
• An signal sample is available from a forward coupler after the amplifier and
prior to the mask bandpass filter. This sample is inside the VAX-C chassis.
• A signal sample is available from a forward coupler after the mask bandpass
filter.
Equipment and Tools Required:
• Spectrum or TV analyzer
5.5.1 Precorrection Setup Procedure
Nonlinear Predistortion:
STEP 1 With the transmitter operating at the desired power level, set the
nonlinear RTAC mode to ADAPT via the POWER menu on the VAX-C
front panel LCD display or by using the VAX-C web browser to
navigate to Exciter Home > Setup > DUC/RTAC screen.
STEP 2 Verify that the nonlinear RTAC attempt and success counters increment
about every 20 seconds on the Exciter Home > Setup > DUC/RTAC
screen.
STEP 3 Use a spectrum analyzer to measure the adjacent-channel IMD shoulder
levels. Verify compliance with any prevailing spectrum mask
requirements.
STEP 4 Save this current nonlinear correction in one of the four slots (See
Storing Precorrection Section 5.5.2).