Specifications

APEX™ Exciter Incorporating FLO™ Technologyr
APEX Exciter Digital Assembly Overview Theory of Operation
2604s400.fm
03/08/07 888-2604-001 Page: 4-5
WARNING: Disconnect primary power prior to servicing.
4.3.2 RTAC and Adaptive Precorrector Board Theory
Adaptive precorrection is a process where an RF sample from somewhere along the trans-
mitter system is down converted to the 1st IF frequency in the down converter board,
converted to a digital signal in the ADC board, and the digitized sample is sent to the
adaptive precorrector board where it is time aligned with and compared to the reference
digital signal from the modulator board. The results of this comparison are used to pre
correct the digital output signal from the adaptive precorrector board so that it will cancel
certain distortions generated in the transmitter system power amplifiers and high power
filter (HPF).
Adaptive precorrection is divided into two general areas.
• Non linear correction: This refers to amplifier non linearity, which is poor linearity
(also referred to as AM to AM conversion) and ICPM (incidental carrier phase modu-
lation), also referred to as AM to PM conversion.
Non linear distortion causes intermodulation products to appear within the signal pass
band and in the adjacent channels, making it difficult or impossible for the transmitter
to pass the output signal mask response test without some type of amplifier lineariza-
tion.
• Linear correction: This generally refers to filter, tuned circuit, or coupling circuit dis-
tortions, which are amplitude response problems and group delay (also called PM to
PM conversion).
Linear distortions increase MER (modulation error rate) and degrades the transmitter
signal to noise performance.
The RTAC correction circuits perform their operations in several sequential steps, which
are as follows.
• The PA feedback signal is used by RTAC to correct non linear distortions generated
by the transmitter PA. This provides a near perfect input signal for the high power fil-
ter.
• The HPF feedback signal is used by RTAC to correct linear distortions generated by
the transmitter high power filter, and the smaller amounts of linear distortions caused
by tunable PAs.
The adaptive precorrection board is also capable of pre distorting the transmitter output
signal to correct for group delay caused by wave guide when it is used to transport the
transmitter RF output signal to the antenna. The waveguide type and length are entered in
exciter setup screen.