A low power 2.4 GHz hybrid polyphase filter (PPF) based BFSK receiver with high frequency offset tolerance (FOT) at small modulation indexes (MIs) is presented for medium data rate wireless sensor network applications. A high FOT at low MI is achieved by a frequency-to-energy conversion architecture using PPFs without any frequency correction circuits. Channel selection and interference rejection are performed simultaneously by the PPFs without any extra hardware and power consumption. Furthermore, the proposed hybrid topology of the PPFs provides an improved adjacent channel rejection (ACR) at reduced power. The prototype receiver fabricated in a 0.13-µm CMOS process, including the RF and analog front-ends, consumes 1.97 mW from a 1 V supply. With a data rate of 1 Mb/s, a sensitivity of –84 dBm, a FOT of 450 kHz (180 ppm), and an ACR of 40 dB are achieved for a MI of 2.