UC Berkeley

Advances in high-speed DAC implemented in deeply scaled-CMOS processes open up the possibility of direct IF and RF waveform synthesis in a next-generation digitized transmitter architecture. A state of the art CMOS current-steering DAC is capable of providing wide bandwidth and high dynamic range for direct RF waveform generation. This enables the mostly-digital implementation of a direct waveform synthesis transmitter for TV band cognitive radio. RF waveforms with various modulation schemes can be synthesized in the digital domain and directly converted by a high-speed DAC. The transmission characteristics can be dynamically adapted to time-varying spectral environment. This transmitter architecture brings the benefits of reconfigurability and frequency agility. However, conventional waveform synthesis requires a >2GS/s DAC to fulfill the Nyquist requirement for covering the whole TV band (54 ~ 862MHz). Nevertheless, an alternative method is to convert the signals at a sampling rate below the Nyquist requirement and extract the image spectrum in higher Nyquist zones by bandpass filters.

This work demonstrates a proof-of-concept design of sub-Nyquist rate conversion and wideband direct synthesis using image spectrum. By utilizing the proposed multi-mode reconstruction, the DAC can shape the spectral envelope for enhancing the image spectrum located in the target channels. The desired transmission waveforms can be extracted from the second or third Nyquist zone by a bandpass filter. A circuit prototype demonstrating the proposed concept has been designed, fabricated, and measured in a general-purpose 65nm CMOS process.

This work presents the implementation of a 600MS/s 10-bit multi-mode sub-Nyquist rate DAC that enables wideband direct waveform synthesis for TV band cognitive radio transmitters. Measurement results show SFDR >55dB across the first three Nyquist zones and a low power consumption of 30mW. The IM3 is < -60dBc in the first and second Nyquist zones and < -55dBc in the third Nyquist zone.