Digital Interpolating Phase Modulator for Wideband Outphasing Transmitters

Radio transmitters are evolving towards digital-intensive 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.