A 5-GHz LC VCO with digital AAC and AFBS for 2.4 GHz ZigBee transceiver applications

Abstract

A 5GHz LC VCO (voltage-controlled oscillator) with automatic amplitude control (AAC) and automatic frequency-band selection (AFBS) for 2.4GHz ZigBee transceivers is presented. Instead of continuous feedback loop, an alternative amplitude calibration scheme is proposed in this paper to alleviate the deficiencies inherent in the conventional approach. It helps to keep the VCO at optimum amplitude to avoid saturation of the cross-coupled transistors and therefore stabilizes the phase noise performance over process, voltage and temperature variations. For the ZigBee application with 16 frequency channels, a coarse tuning loop is added in this work to implement the frequency-band selection using the AFBS mechanism. The VCO core and the digital AAC, AFBS modules have been fully integrated in a 2.4GHz ZigBee transceiver which was fabricated in a 0.18μm RF-CMOS technology. The current consumption is 4.7mA at 4.85GHz with 1.8V power supply and a chip area of about 0.285mm2 is occupied. The VCO is capable of operating from 4.67GHz to 5.18GHz and the measured phase-noise level is –120dBc/Hz at 1MHz offset from a 4.85GHz carrier. The tuning sensitivity KVCO of the VCO is about 78MHz/V with 0.9V control voltage.