A TDC-Free Mostly-Digital FDC-PLL Frequency Synthesizer With a 2.8-3.5 GHz DCO

This paper presents the first published fully-integrated digital fractional- N PLL based on a second-order frequency-to-digital converter (FDC) instead of a time-to-digital converter (TDC). The PLL´s quantization noise is nearly identical to that of a conventional analog delta-sigma modulator based PLL (ΔΣ-PLL). Hence, the quantization noise is highpass shaped and is suppressed by the PLL´s loop filter to the point where it is not a dominant contributor to the PLL´s output phase noise. However, in contrast to a ΔΣ-PLL, the new PLL has an entirely digital loop filter and its analog components are relatively insensitive to non-ideal analog circuit behavior. Therefore, it offers the performance benefits of a ΔΣ-PLL and the area and scalability benefits of a TDC-based digital PLL. Additionally, the PLL´s digitally controlled oscillator (DCO) incorporates a new switched-capacitor frequency control element that is insensitive to supply noise and parasitic coupling. The PLL is implemented in 65 nm CMOS technology, has an active area of 0.56 mm², dissipates 21 mW from 1.0 and 1.2 V supplies, and its measured phase noise at 3.5 GHz is -123, -135, and -150 dBc/Hz at offsets of 1, 3, and 20 MHz, respectively. The PLL´s power consumption is lower than previously published digital PLLs with comparable phase noise performance.