An Energy-Efficient Composite Phase Shift 16-QAM Frequency Multiplying TX Achieving a 3.4% EVM in 65nm CMOS

Next-generation bioelectronic/loT devices demand highly-integrated wireless systems at extremely low cost and high energy efficiency, where the transmitter (TX) often dominates the overall power consumption, posing a major design challenge to the entire system. Although sub-GHz TXs with sub-mW power consumption have been reported recently [1–3], the simple modulation scheme (OOK, FSK, etc.) limits the spectral efficiency and data rate, while is not well-suited for applications, such as brain-computer interface, real time image transmission, etc. Therefore, a QAM architecture is usually desired with an expected data rate up to tens of Mbps. State-of-the art QAM-modulated sub-GHz TX architectures can be categorized as follow. Conventional direct-conversion architecture in Fig. 1(a) provides excellent error-tolerable performance at the expense of extra power/hardware overhead. The quadrature structure in [4] modulates phase/amplitude separately by leveraging a multi-phase PLL or injection-locked ring oscillator (ILRO) [Fig. 1(b)(c)], however, requires precise phase/amplitude control to achieve good error vector magnitude (EVM). In [5], the QAM signal is generated by combining two adjacent vectors through a RF-DAC in a polar-like structure. Despite extra power saving, it suffers from significant phase/amplitude mismatch and limited EVM. In this paper, we proposed an ultra-low power 16-QAM TX based on the composite phase shift (CPS) technique. The modulation scheme in this paper demonstrates excellent mismatch-resilient performance as analyzed in the following part, resulting in an improved EVM at the target data rate with high energy efficiency and low hardware overhead.