1. Introduction
The growth in the wireless personal area network (WPAN) market leads to the 802.15.4 IEEE standard (ZigBee) definition. The main goal of 802.15.4 is to reduce the chip cost production and the chip power consumption by relaxing some constraints like bit rate, channel rejection and range. To be around 1$ per unit, such transceivers must be integrated using a low-cost technology as a standard CMOS technology and the occupied silicon area must be reduced as much as possible. Moreover, to reach more than one-year battery's life, the power consumption must be drastically optimized. In the 802.15.4 standard [1], an offset quadrature phase shift keying (O-QPSK) modulation with half-sine pulse shaping, equivalent to a minimum shift keying modulation (MSK with a modulation index h=0.5), is used because of its high bandwidth efficiency and constant envelope properties. In such context, the design of low-cost and low power frequency shift keying (FSK) demodulator is then an important issue.