In this paper the system concept of a radiofrequency digital-to-analog converter (RF-DAC) based multistandard transmitter is presented. The RF-DAC combines DA converter and up-conversion mixer in a single building block. Thereby, the analog baseband blocks are eliminated and replaced by easily reconfigurable digital blocks. Requirements on the digital and mixed-signal blocks are discussed. Spurious emissions are a major issue of this 'direct digital modulator'. This is especially true for multiradio devices with several receivers running simultaneously and in close proximity to the transmitter. Therefore, a maximum emission limit is formulated and especially critical frequency bands are identified. Several techniques for reducing the spurious emissions are discussed and performance requirements for an LTE transmitter and its main building block are found. System simulations demonstrate the feasibility of the concept.

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This work presents a lowpass ∆Σ based transmitter frontend suitable for software defined radios (SDRs). ∆Σ modulation is utilised to generate an RF signal suitable for driving highly efficient switchingmode power amplifiers. The whole transmitter frontend is implemented in the digital domain up to the upconversion. Thus, high reconfigurability and flexibility necessary for SDR is achieved. This work presents two implementations methods for lowpass ∆Σ modulator based transmitter frontends for SDR applications. The circuit design in a 90 nm CMOS process and a discrete implementation utilising a field programmable gate array (FPGA) are discussed. Feasibility of the concept is demonstrated by simulation for the integrated implementation and by measurements for the FPGA solution.Index Terms-Field-programmable gate array (FPGA), lowpass (LP) delta-sigma (DS), RF transmitter, software defined radio (SDR).

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This paper presents a novel concept for direct-pulsewidth/position-modulation (PWPM) in an all-digital transmitter by using a parallel baseband PWPM and high speed digital upconversion. By parallel calculation and subsequent serialization of the digital PWPM signal carrier frequencies above 1 GHz become feasible. PWPM enables the usage of highly efficient switching-mode power amplifiers (SMPA) and direct control of the output power. Because PWPM modulator can be completely implemented in the digital, time discrete domain, all analogue baseband blocks can be dropped and replaced by reconfigurable digital blocks. The only remaining mixed-signal and analogue blocks are the multiplexer and the power amplifier. Requirements on the digital and mixed-signal blocks are discussed. Feasibility of the concept is demonstrated by system and circuit simulations showing the possibility of using digital PWPM in wireless applications.