A 400-MHz/2.4-GHz combo transceiver (TRX) integrated chip (IC) Supporting simultaneous dual-band communication is designed and fabricated in a 65-nm CMOS technology for compact wireless personal area network applications. The IC is composed of a 3-10 Mb/s quadrature phase shift keying (QPSK)/offset QPSK (OQPSK) TRX working at the 400-500 MHz band, and a 1-3 Mb/s Gaussian frequency shift keying (GFSK)/half sine OQPSK (HS-OQPSK) TRX at the 2.4-2.4835 GHz band for the Bluetooth low energy/IEEE 802.15.4 protocols. The presented combo TRX IC requires one single antenna without any bidirectional RF coupler, to support simultaneous dual-band communication. Two key circuit techniques, namely, the combination biasing technique to suppress the high-order harmonics and the active in-band interference cancellation, are adopted to tackle the crosstalk interference between the two bands during simultaneous dual-band operation. Measurement results show that the in-band interference presented to the 2.4-GHz receiver due to the 400-MHz transmitter is suppressed by 19 dB with the presented design techniques.