Real-Time Adaptive Correction (RTAC™)

RTAC is an advanced type of digital precorrection that enables the Apex M2X exciter to fully utilize the transmitter power amplifier, yet maintain spectral mask compliance of the digital signal. Precorrection can be implemented in many ways. In order of sophistication, they are:

• Analog, fixed precorrection

• Digital, fixed precorrection

• Digital, adaptive precorrection

• Digital, memoryful, adaptive precorrection (RTAC)

There are two types of precorrection used in the Apex M2X: linear and nonlinear.

Linear precorrection corrects for the non-ideal amplitude response and time response of the transmission system, including the power amplifier and any bandpass filters. Linear precorrection is helpful in optimizing the modulation performance of the transmitted digital signal so that less equalization is required at the receiver.

Nonlinear precorrection corrects for the two major types of nonlinearity (AM->AM and AM->PM) in the RF power amplifier that cause undesired spectral components and poor RF mask compliance. AM to AM nonlinearity causes the RF power amplifier’s output amplitude to not exactly track the input amplitude. Typically, this nonlinearity occurs near the peak output power of the amplifier, where saturation effects cause the output response to flatten as the input continues to increase and where the amplifier output goes to zero. AM to PM nonlinearity causes the RF power amplifier’s output phase to not track the input phase. The amplifier acts like a phase modulator as the power output varies to follow the input signal, producing undesirable sidebands.

Fixed precorrection can improve the system linearity for one specific operating point, but has to be manually readjusted for changes in power output, antenna load impedance, temperature or operating point.

Adaptive digital precorrection utilizes an RF sample taken at the output of the RF amplifier to automatically adjust the shape of the precorrection and continuously optimize the linearization of the system. The bandwidth limitation, especially in tube type amplifiers (IOT), and the stored energy in tuned circuits introduce “memory effects.” Digital, adaptive, memoryful precorrection can correct for these memory effects and for simple AM to AM and AM to PM distortions. Even wideband solid-state RF amplifiers have some memory effects that make the shape of the nonlinearities change with the digital modulation data states.

As the only system with simultaneous, linear and nonlinear, adaptive, memoryful precorrection, RTAC provides the best possible total correction to all types of RF amplifiers.