Digitally Assisted Harmonic Cancellation for Multi-Octave Filter-Less Transmitter

Abstract

This paper presents and demonstrates the design of a filter-less transmitter architecture with digitally assisted harmonic cancellation. A neural network is used to model the harmonics as well as IMD for digital predistortion applications. This neural network-based harmonic modeling does not require any reference signal to be injected at the input of PA, thereby reducing the complexity in characterizing the harmonics. An in-house 10 W PA operating from VHF to L-band has been designed and characterized for its harmonic suppression. This PA is used along with agile RF transceiver AD9361 from Analog Device and Xilinx embedding platform using Zynq ZC-706 system-on-chip for implementing the entire transmitter. The receiver of AD9361 captures the nonlinearity of the PA in terms of harmonics as well as intermodulation distortion (IMD) components for modeling and predistortion. The proposed architecture can handle all types of distortions due to hardware as well as PA nonlinearity. Besides, it is also able to cancel the harmonics using a harmonic injection in the feed-forward configuration. This transmitter architecture has the advantages of being low cost, filter-less, wideband, frequency agile, reconfigurable and less bulky compared to the conventional scheme. The proposed scheme is demonstrated to transmit 5 MHz LTE signal at different frequencies over the range of 100 MHz to 400 MHz. In such a case, the second and third harmonics appear over the frequency range from 200 MHz to 1.2 GHz, which are within the amplification range of PA, yet they are suppressed without using any filter at the output. More than −40 dBc harmonic rejection is achieved over the entire operating range of this filter-less transmitter. The adjacent channel leakage ratio (ACLR) is always better than −45 dBc after applying digital predistortion.

Simplifying the design of harmonic‐suppressing power amplifiers adopting capacitance‐splitting method

“…The comparisons of this work with other state of the arts are listed in Table 3. Though Singhal and Rawat 8 and Nguyen et al 9 realize the widest fractional bandwidth of harmonic suppression, however the DSP assisted harmonic injection technique employed in Singhal and Rawat 8 rely on complex system architecture, the push‐pull based operation reported in Nguyen et al 9 rely on bulky architecture and can only suppress even‐order harmonics. By using a HS‐OMN, Tuffy et al 15 achieves a comparable harmonic‐suppressing bandwidth as this work, but an effective design methodology for synthesizing the wideband HS‐OMN is not provided in Tuffy et al 15 Overall, this work demonstrates comprehensive advantages of wideband harmonic suppression, easy‐implemented design methodology and the highest power efficiency compared with other works in Table 3.…”

“…Harmonic-suppressing PAs (HS-PA) obtained through various design methods are presented in previous studies. [8][9][10][11][12][13][14] Harmonic cancelation through harmonic injection is reported in Singhal and Rawat, 8 of which the hardware configuration is schematically shown in Figure 1A. The DSP technique renders harmonic suppression over large bandwidth; however, the hardware overhead is Index Terms: Efficiency, harmonic suppression, matching network, output power, power amplifier complex.…”

“…It would be great progress if the PA itself possesses good harmonic suppression, which benefits to simplify the system design, reduce system volume/cost, and enhance system efficiency. Harmonic‐suppressing PAs (HS‐PA) obtained through various design methods are presented in previous studies 8–14 . Harmonic cancelation through harmonic injection is reported in Singhal and Rawat, 8 of which the hardware configuration is schematically shown in Figure 1A.…”

Nonlinear modeling and digital predistortion for HF transmitters with harmonic cancelation

“…The baseband harmonic injection signals were transmitted with additional channels. In addition, 20 adopted a neural network (NN) to adapt the phase of the injected signal. Feed‐forward harmonic cancelation techniques 21–23 . The DPD was used to mitigate the IMD and the harmonics were injected after the linear error amplifier and hybrid coupler.…”

“…Feed-forward harmonic cancelation techniques. [21][22][23] The DPD was used to mitigate the IMD and the harmonics were injected after the linear error amplifier and hybrid coupler. Similar to the previous baseband injection, the additional harmonic channels should synchronize well with the fundamental channel.…”