The Boolardy Engineering Test Array (BETA) is the first prototype of the Australian Square Kilometer Array Pathfinder (ASKAP), a new radio telescope that is pioneering the use of Phased Array Feed (PAF) technology in radio astronomy to provide instantaneous wide field-of-view imaging. BETA consists of six 12 meter parabolic antennas, each equipped with a 188-element PAF at the focus and a signal processing system capable of imaging an instantaneous 7 square degree field-of-view with a bandwidth of 300MHz at a resolution of 18kHz. In this paper we describe the architecture of the digital signal processing chain and its implementation on reconfigurable hardware based on Field-Programmable Gate Array (FPGA) technology. We present early engineering results taken by the instrument during commissioning trials at the Murchison Radio Observatory (MRO) in remote Western Australia. In particular, we present the first multi-beam image of an astronomical source captured with a PAF-based interferometer.

In a direct-conversion transmitter system, the non-ideal transfer characteristics of analog subsystems in the I and Q channels adversely affect the performance of the system. The study of static errors, including gain imbalance, phase errors, and DC offsets has received considerable attention in the literature. Much less attention has been paid to the effect of frequency-dependent imperfections that arise in the analog reconstruction filters commonly employed in direct-conversion structures. This paper presents results of research into robust methods for automatic digital compensation for these non ideal frequency-dependent characteristics. The design and implementation of FIR and Eigenfilter structures to achieve compensation are presented. As a specific application the paper considers a direct-conversion system used to generate continuous-phase frequency-shift keying (CPFSK) signals. The performance achievable using the proposed digital compensation structures is presented, including measurement results taken on a DSP-based modulator platform.

In a direct-conversion transmitter system, the nonideal transfer characteristics of practical analog subsystems can have significant adverse effects on the performance of the transmitter system. While a significant body of research exists on correcting static errors (including gain imbalance, phase errors, and DC offsets) in the analog subsystems, relatively little attention has been paid to the impact of nonideal frequency-dependent characteristics in the analog reconstruction filters that are employed in direct-conversion structures. This paper presents a new computationally efficient and numerically robust method for automatic digital compensation for nonideal frequency-dependent characteristics in the signal reconstruction filters of a quadrature (IQ) modulator structure. The new compensation technique is used in conjunction with existing compensation approaches that compensate for static errors.

A flexible approach to the implementation of a radio transmitter is to synthesise base-band I and Q signals using a DSP (Digital Signal Processor) followed by a vector modulator to up-convert the base-band signals directly to RF. This approach can be limited, however, by analogue reconstruction filters that are necessary to attenuate digital image components in the base-band signal spectrum before transmission. Recently, methods have been proposed to digitally compensate for these shortcomings. While these methods have been shown to be effective in improving modulator performance they require special attention to numerical issues and they typically result in large filter structures that are computationally demanding to implement. In this paper we present the results of an investigation into reducing the computational load of a digital compensation method on the DSP. The investigation has resulted in a technique that not only significantly reduces the computational burden but also improves the numerical robustness of the solution.

In digital IQ modulators generating continuous phase frequency shift keying (CPFSK) signals, departures from flat-magnitude, linear phase in the pass bands of signal reconstruction filters in the I and Q channels cause ripple in the output signal envelope. Amplitude modulation (AM) in the signal envelope function produces undesirable sidelobes in the FSK signal spectrum when the signal passes through nonlinear elements in the transmission path. A structure is developed for digitally pre-compensating for the magnitude and phase characteristics of signal reconstruction filters. Optimum digital pre-compensation filters are found using least squares (LS) techniques and we propose a method by which the optimum pre-compensation filters can be estimated using test input signals. This method can be used as part of an automatic compensation process.

We present a technique for digitally precompensating for shortcomings in signal reconstruction filters in an IQ modulation system. Optimum digital precompensation filters are found using least squares (LS) techniques. The optimization problem is formulated in the frequency-domain, and the optimum LS solution is characterized using an eigenanalysis approach.

In many adaptive signal processing applications, it is often desired to impose linear and quadratic constraints on the adaptive filter weights in order to meet certain performance criteria. The paper presents a modification of the well-known adaptive RLS or FLS algorithm to achieve this. By way of illustration, the paper considers an adaptive narrowband beamformer with first and second order spatial derivative constraints. The performance of the algorithm is studied via computer simulations.