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Radio transmitters are evolving towards digitalintensive solutions to exploit reconfigurability and benefit from CMOS process scaling. Outphasing has been identified as a suitable candidate for digital wideband transmitters. However, with recent digital-intensive outphasing transmitters the achieved performance in terms of adjacent channel leakage ratio (ACLR) has been limited. This paper identifies the sampling images of the modulating phase signal as the main factor limiting the ACLR of digital outphasing transmitters. We present a new digital interpolating phase modulator architecture, capable of providing significantly better sampling image attenuation. When evaluated in outphasing configuration with a 100 MHz OFDM signal at the carrier frequency of 2.46 GHz, and 10-bit phase resolution, the proposed solution achieves an ACLR of-59 dBc, compared to-43 dBc achievable with the phase modulator architecture utilized in state-of-the-art digital outphasing transmitters. The proposed digital interpolating phase modulator is also capable of custom carrier generation, a straightforward method for generating an arbitrary carrier frequency up to 1.25 times the phase modulator sampling rate.

This paper describes the first purely digital approach to reduce the receive band noise in digitally-intensive RF transmitters. The proposed solution applies bandpass deltasigma modulation and dynamic element matching (DEM) to the receive band (RX-band) instead of the transmit band. This enables selective attenuation of the noise originating from amplitude quantization and static mismatches of the digitalto-analog converter (DAC), which would otherwise reach the transmitter output almost unattenuated. A highly configurable 4 th-order noise transfer function is designed to achieve optimum attenuation in the programmable RX-band, while ensuring negligible degradation of the transmitted signal quality as well as stable operation of the tree structure DEM encoder. A general validation of DEM, independent from the duration of the DAC impulse response, is also presented. The proposed solution is verified through system-level simulations with LTE signals. In the presence of typical amplitude and timing mismatches, the RXband noise can be reduced below-160 dBc/Hz without filtering after the DAC, thus potentially enabling SAW-less operation of all-digital transmitters.

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