A Low-Leakage Open-Loop Frequency Synthesizer Allowing Small-Area On-Chip Loop Filter

A frequency synthesizer targeting low-power packet-based frequency-shift-keying (FSK) applications using open-loop modulation of the oscillator is presented. Unlike conventional implementations, the proposed synthesizer is open both when transmitting and receiving data. It is, therefore, possible to use a wide loop-filter bandwidth without violating the noise or spurious requirements. A wideband loop-filter can be implemented using small component values, allowing an on-chip loop filter. To handle the frequency drift associated with open-loop implementations, a low-leakage charge pump is proposed. The synthesizer is implemented using a 0.18-mum CMOS process. The total power consumption is 9 mW, and the circuit area including the voltage-controlled oscillator (VCO) inductors and on-chip loop-filter is 0.32 mm
²
. The measured frequency drift indicates a leakage current of below 2 fA.