MULTI-OCTAVE TRANSMITTER DESIGN WITH FILTER-LESS HARMONIC SUPPRESSION SCHEMES FOR POWER AMPLIFIERS

Abstract

This thesis aims to develop filter-less transmitter architecture, which is capable of eliminating the harmonics falling within the operation band in the multi-octave transmitter and suppressing intermodulation distortions near to the fundamental. In this context, the thesis focuses on two different architectures. The first architecture realizes a fully digital assisted harmonic cancellation methodology based on feed-forward cancellation. The second architecture utilizes a hybrid analog/digital harmonic cancellation scheme, where even harmonics are cancelled using push-pull scheme implemented in an analog domain. Both the architecture requires modeling of harmonics and intermodulation distortions, which is performed using the neural network. The first architecture utilizes this harmonic modeling to cancel both even as well as odd harmonics, whereas the second architecture utilizes this only for cancelling the odd harmonics. The neural network based harmonic modelling does not require any reference signal to be injected at the input of the power amplifier, thereby reducing the complexity in characterizing the harmonics. For the demonstration of the first architecture, an in-house 10-watt power amplifier operating from very-high frequency to ultra-high frequency band has been designed and characterized for its harmonic suppression. This power amplifier 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 power amplifier in terms of harmonics as well as intermodulation distortion components for modelling and predistortion. The proposed architecture can handle all types of distortions due to hardware, as well as power amplifier 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 long term evolution signals 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 the power amplifier. 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 is always better than -45 dBc after applying DPD. Later, for simultaneously, 2nd and 3rd harmonic cancellation with DPD is validated and demonstrated using the instrument based testbed. The second architecture utilizes a hybrid analog/digital harmonic cancellation technique where even-order harmonics are cancelled in an analog way, and odd-harmonics are cancelled in the digital domain. For the demonstration of the proposed architecture, an in-house push-pull power amplifier is designed. The main component of the push-pull power amplifier is baluns, which are used at the input and output sides of power amplifier. So, input/output baluns have been characterized for designing a push-pull power amplifier for very-high frequency and ultra-high frequency band applications. The fabricated push-pull power amplifier is tested with a 10 MHz, 9 dB peak-to-average power ratio, long term evolution signal from 70 MHz to 350 MHz. The evenorder harmonics are suppressed by more than -41 dBc over the band. For the cancellation of oddharmonics, particularly 3rd harmonic and intermodulation distortions, a digitally based testbed is used. In this, the push-pull power amplifier is used 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 has been utilized for capturing the nonlinearity of developed push-pull power amplifier in terms of harmonics, as well as intermodulation distortions. The third-order harmonic suppression and adjacent channel power ratio are always better than -40 dBc and -45 dBc, respectively. This proposed transmitter architecture has reduced the number of required channels by more than half for harmonic cancellation, as compared to the feed-forward topology. The thesis also discusses the challenges in the design of the Ku band power amplifier using the hybrid microwave integrated circuit, which can also be used in filter-less transmitters if required in the future. The challenges faced during the design of board-level Ku-band power amplifier using hybrid microwave integrated circuit technology are discussed. Among these challenges, die bonding and wire bonding are critical phenomena, and they have a great impact on the working of the power amplifier at high frequencies. Considering these challenges, the boardlevel power amplifier at 17 GHz has been designed using a Gallium Nitride 0.25 μm process bare die device with a 400 MHz frequency band. In order to omit such limitations, Ku-band power amplifier design using monolithic microwave integrated circuits may be carried out in future work.