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A new topology in PLL architecture dual-mode K^subVCO^ (DMK) to reduce in-band noise and reference spurs is presented. The DMK PLL also adopts the technique to shrink the spurs by modulating the control voltage. The prototype DMK PLL is implemented in a 65 nm CMOS and shows about 3 ps RMS jitter, phase noise of 2 107 and 2 109 dBc/Hz at 100 kHz, 1 MHz offset frequency and reference spur of 68.5 dBc.
Introduction: One of the main challenges of PLLs in nano-scaled CMOS processes is a low supply voltage. It enforces higher VCO gain (KVCO) or usage of the capacitor or inductor banks for a wide tuning range of PLLs [1]. However, since a high KVCO is susceptible to in-band noise contributions [2], a low KVCO is preferred to ensure the phase noise in the PLL. A charge pumped PLL with dual-mode KVCO (DMK), which reduces the in-band noise and reference spurs by lowering the KVCO after the loop locked, is presented. To reduce the reference spurs, the frequency modulation technique of control voltage (VC) is also used.
Dual-mode VCO gain topology: Fig. 1 showsthe proposed charge pump DMK PLL architecture with the two modes to control the KVCO similarly to [3]. The coarse-locking mode consists of the normal PLL loop with SWF ¼ LOW and the path passing through the ADC followed by the digital lock detector. The other fine-locking mode includes the normal PLL loop and the R-2R DAC with SWF ¼ HIGH. The three-stage ring VCO has four varactor loads. One of them is fixed to the output of a loop filter (LF) and three reminders are switchable to the outputs of an LF or to a DAC to control KVCO. An 8-bit successive approximation register (SAR) ADC is used to convert VC into digital code. The DAC shown in Fig. 1 is not an additional building block but a part of the SAR ADC. The two phase-frequency detectors (PFDs) and charge pump (CP) circuits are used to modulate the frequency of VC. PFD I is rising and PFD II is the falling edge triggered type. The CP has negative feedback loops to reduce current mismatch.
The oscillation frequency ( fout) of the ring VCO is in inverse...