Low phase-noise VCO utilizing modified symmetric load and partial positive feedback for FDSM

T.-V. Cao, D.T. Wisland, T.S. Lande, Farshad Moradi

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    The voltage-controlled oscillator (VCO) in frequency-based ΔΣ modulator (FDSM) systems behaves as a voltage-to-phase integrator converting an analog input voltage to phase information. Tuning range and phase noise are the most important factors of the basic design of a VCO in FDSM systems. In this paper a novel low phase-noise and wide tuning-range differential VCO based on a differential ring oscillator with modified symmetric load and a partial positive feedback in the differential delay cell is presented. The VCO is combined with a new bias circuit and implemented using 90 nm CMOS process technology. By using modified NMOS symmetric loads and a PMOS tail for delay cells, the VCO phase noise can be reduced with more than 13 dB compared to that of the conventional approach, achieving -125 dBc/Hz at 500 kHz offset from the center frequency of 450 MHz. The wide tuning-range by using two added transistors (parallel with the active loads) increases the operating frequency range by about 22%, while the partial positive feedback provides the necessary bias condition for the circuit to oscillate. The designed VCO operating at a low power supply voltage of 0.6V can achieve low power consumption of 670 μW at oscillation frequency of 800 MHz and good linearity reducing harmonic distortion in the ΔΣ modulator.
    Original languageEnglish
    JournalAnalog Integrated Circuits and Signal Processing
    Pages (from-to)151-160
    Number of pages10
    Publication statusPublished - 1 Oct 2012


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