Hybrid Harmonic Cancellation Digital Predistortion With a Feedback Loop Compensation

High frequency (HF) transmitter whose operating frequency range is about four octaves is always faced with the problems of harmonics because of the nonlinearity of the power amplifier (PA). The conventional analog harmonic cancellation scheme uses lossy and bulky filter banks. The digital harmonic cancellation scheme requires a high sampling rate. In this brief, a hybrid harmonic cancellation digital predistortion (HHC-DPD) technique is proposed to suppress the interfering harmonics by adding a low-pass filter between the PA and the directional coupler. The proposed HHC-DPD scheme can greatly lower the sampling rate, in turn, decrease the digital power consumption and resource occupation. Further, a feedback loop compensation (FLC) method is proposed to cancel the time misalignment between the harmonics and the fundamental caused by the chromatic dispersion in the feedback loop. With the FLC method, the non-convergence issue in iterations can be avoided. Experimental results on an HF PA with carrier frequency located at 8 MHz validate that the proposed HHC-DPD scheme with the FLC outperforms current harmonic cancellation approaches. The sampling rate was reduced from 300 MHz to 100 MHz. Both the harmonics and the adjacent channel power ratio (ACPR) were cancelled for being below -50 dBc after the HHC-DPD.