Digital Calibration of Wide Bandwidth Open-Loop Phase Modulator

摘要

The rapid rise in demand for high data rates has led to communication standards like LTE that use signals with wide signal bandwidth and high peak-to-average power ratio. Since, total power consumption is largely determined by the efficiency of the power amplifier used, high efficiency architectures like polar and out-phasing, are preferable for future transmitter designs. Such architectures require a phase modulator (PM) as one of their key building blocks. The recent developments in phase modulator design have demonstrated superior wide-bandwidth performance of open loop modulation techniques. However, currently the resolution is limited by systematic errors inherent in the phase modulator circuit and random errors due to inevitable component mismatches.;In this dissertation, a phase-interpolator based open-loop phase modulator is proposed, which leverages a digital calibration technique to mitigate the sources of phase errors and achieves excellent phase resolution. At the heart of this technique, a time-to-digital converter performs high resolution measurement of phase errors of the phase modulator. These measurements are used to continuously pre-distort the modulation data, so that the linearity of the overall transfer function is enhanced, thereby resulting in low out-of-band emission and low in-band noise. A prototype IC was implemented in 0.13 mum CMOS process. Measurements on the prototype show that out-of-band quantization noise is 56-dB lower than the signal when transmitting 20-Mb/s GFSK signal and the r.m.s. error is only 3.2%. The power consumption of the phase modulator is 18 mW. Since the IC was implemented in 0.13 mum CMOS process, the power is expected to reduce a lot, if it is implemented in finer process nodes. The dissertation also presents theory and measurement results on frequency synthesis using the open-loop phase modulator. New frequency can be synthesized with very fine frequency step size, by applying a digital phase ramp to the phase modulator. However, the non-linearity of the phase modulator results in strong spurious tones. Digital compensation is proposed to mitigate these spurs, and generate multiple low-jitter clocks.