Predistortion adjustment method for a digital TV transmitter power amplifier

A digital signal input 1 to a digital power amplifier is periodically replaced 9 by a short digital or analogue test signal 8. The test waveform may be a ramp or a staircase. The output of the amplifier is synchronously demodulated 11 to provide in-phase and quadrature waveforms of the amplified test signal to monitors 12 and 13. This direct time-domain visual indication of the transfer characteristics of the transmitter allows an operator to adjust the settings of the predistorter while viewing the monitors 12 and 13. The test signal duty cycle is less than 4% and is typically 0.5%. The testing is performed off-line.