User's Manual

Innovator CU0TD-1/CU0RD-1 – CU4TD/CU4RD 8VSB ATSC Modulator
ATSC Transmitter/Regenerative Translator
Instruction Manual, Rev. 2 37
Manual Digital Linear and Non-linear Pre-correctors
The modulator is equipped with digital linear and non-linear pre-correctors (pre-
distorters) that can significantly improve the performance of a transmitter. The Pre-
corrector function can be switched on and off from the Web GUI or CLI.
The Non-linear pre-corrector compensates for power amplifier non-linearity and is able
to provide a separate adjustment for the low and high frequency shoulders of the wide
channel spectrum. Additionally the non-linearity corrector may serve as peak limiter in
order to control peak to average power ratios. The Linear pre-corrector compensates for
the group delay created by an output filter.
The input-gain of the Pre-corrector can also be used to adjust the power-level of a
transmitter.
CAUTION!
Adjusting the input-gain of the Pre-corrector and thereby the power-level of
the transmitter must be carried out with the utmost care, since higher than
normal levels may cause damage to the transmitter.
The construction of the Pre-corrector curve is aided by a software application that
provides a graphical user interface to the Pre-corrector curve. Combined with a
spectrum analyzer on the transmitter output, a repetitive process can be carried out to
optimize the pre-corrector curve to the actual transmitter.
In addition to being a highly effective tool for creating pre-correction curves, the UBS-
Axcera Corrector GUI has extensive features for storing and retrieving pre-correction
curves to and from the PC storage medium (for example hard disk drive). The UBS-
Axcera Corrector GUI also allows storing and retrieving up to ten linear and non-linear
curve presets to and from the modulator.
The Manual for the Digital Pre-Corrector GUI Software describes in detail the
operation of the Pre-corrector.
Adaptive Non-linear and Linear Digital Pre-correction
The modulator is equipped with Adaptive Non-linear and Linear Digital Pre-correction
that provides compensation for the non-linear distortions produced by a high power
amplifier and the group delay introduced by an output filter.
High power amplifier and band pass filter feedback (reference) signals are
downconverted to 25 MHz IF before the “adaptation process” or “cycle” begins.
The “adaptation process”, or “cycle”, includes set-up of the capturing cycle, the
capturing of data, signal measurements, the calculation of NLP/LP coefficients and the
transfer of coefficients and signal measurement to the modulator board.
Once the capturing cycle is set up, the downconverted HPA or BPF reference signal is
sampled by an ADC and stored in a local memory buffer for the processor to read upon
completion of the data capturing process.