Studies on Selected Topics in Radio Frequency Digital-to-Analog Converters
Abstract: The network latency in fifth generation mobile technology (5G) will be around one millisecond which is much lower than in 4G technology. This significantly faster response time together with higher information capacity and ultra-reliable communication in 5G technology will pave the way for future innovations in a smart and connected society. This new 5G network should be built on a reasonable wireless infrastructure and 5G radio base-stations that can be vastly deployed. That is, while the electrical specification of a radio base-station in 5G should be met in order to have the network functioning, the size, weight and power consumption of the radio system should be optimized to be able to commercially deploy these radios in a huge network.As the number of antenna elements increases in massive multiple-input multiple-output based radios such as in 5G, designing true multi-band base-station radios, with efficient physical size, power consumption and cost in emerging cellular bands especially in mid-bands (frequencies up to 10~GHz), is becoming a challenge. This demands a hard integration of radio components; particularly the radio's digital application-specific integrated circuits (ASIC) with high-performance energy-efficient multi-band data converters.In this dissertation radio frequency digital-to-analog converter (RF DAC) and semi-digital finite-impulse response (FIR) filter digital-to-analog converter has been studied. Different techniques are used in these structures to improve the transmitter's overall performance.In the RF DAC part, a radio frequency digital-to-analog converter solution is presented, which is capable of monolithic integration into today's digital ASIC due to its digital-in-nature architecture, while fulfills the stringent requirements of cellular network radio base station linearity and bandwidth. A voltage-mode conversion method is used as output stage, and configurable mixing logic is employed in the data path to create a higher frequency lobe and utilize the output signal in the first or the second Nyquist zone and hence achieving output frequencies up to the sample rate.In the semi-digital FIR part, optimization problem formulation for semi-digital FIR digital-to-analog converter is investigated. Magnitude and energy metrics with variable coefficient precision are defined for cascaded digital Sigma-Delta modulators, semi-digital FIR filter, and Sinc roll-off frequency response of the DAC. A set of analog metrics as hardware cost is also defined to be included in semi-digital FIR DAC optimization problem formulation. It is shown that hardware cost of the semi-digital FIR DAC, can be reduced by introducing flexible coefficient precision in filter optimization while the semi-digital FIR DAC is not over-designed either. Different use cases are selected to demonstrate the optimization problem formulations. A combination of magnitude metric, energy metric, coefficient precision and analog metric are used in different use cases of the optimization problem formulation and solved to find out the optimum set of analog FIR taps.Moreover, a direct digital-to-RF converter (DRFC) is presented in this thesis where a semi-digital FIR topology utilizes voltage-mode RF DAC cells to synthesize spectrally clean signals at RF frequencies. Due to its digital-in-nature design, the DRFC benefits from technology scaling and can be monolithically integrated into advance digital VLSI systems. A fourth-order single-bit quantizer bandpass digital Sigma-Delta modulator is used preceding the DRFC, resulting in a high in-band signal-to-noise ratio (SNR). The out-of-band spectrally-shaped quantization noise is attenuated by an embedded semi-digital FIR filter. The RF output frequencies are synthesized by a configurable voltage-mode RF DAC solution with a high linearity performance.A compensation technique to cancel the code-dependent supply current variation in voltage-mode RF DAC for radio frequency direct digital frequency synthesizer is also presented in this dissertation and is studied analytically. The voltage-mode RF DAC and the compensation technique are mathematically modeled and system-level simulation is performed to support the analytical discussion.
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