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This paper presents a low-power high-bit-rate phase modulator based on a digital PLL with single-bit TDC and two-point injection scheme. At high bit rates, this scheme requires a controlled oscillator with wide tuning range and becomes critically sensitive to the delay spread between the two injection paths, considerably degrading the achievable error-vector magnitude and causing significant spectral regrowth. A multi-capacitor-bank oscillator topology with an automatic background regulation of the gains of the banks and a digital adaptive filter for the delay-spread correction are introduced. The phase modulator fabricated in a 65-nm CMOS process synthesizes carriers in the 2.9-to-4.0-GHz range from a 40-MHz crystal reference and it is able to produce a phase change up to ±π with 10-bit resolution in a single reference cycle. Measured EVM at 3.6 GHz is -36 dB for a 10-Mb/s GMSK and a 20-Mb/s QPSK modulation. Power dissipation is 5 mW from a 1.2-V voltage supply, leading to a total energy consumption of 0.25 nJ/bit.
Date of Publication: Dec. 2012