A Fully Digital 65nm CMOS Transmitter for the 2.4-to-2.7GHz WiFi/WiMAX Bands using 5.4GHz ΔΣ RF DACs

This paper describes a fully digital CMOS transmitter that meets the IEEE 802.11 b/g and 802.16e standards in the 2.4-to-2.7 GHz band, while taking into account the coexistence requirements when used in 2G/3G mobile handsets. The baseband signal is upsampled first to a sampling rate F_M around 160MHz, using FIR interpolators. I/Q and DC precompensation is then applied, to eliminate the image and LO components at the RF output. A first fine digital gain stage controls the signal level in a 0 to -12 dB range. The baseband signal is then upsampled to F_c/4, where F_c is the carrier frequency in the 2.4 to 2.7 GHz range. As the ratio between F_c/4 and FM is not necessarily integer, a fractional-N interpolator has to be used. The interpolating ratio has 17 bits of resolution, which allows carrier frequencies on a 25 kHz raster, to satisfy all possible WiFi and WiMAX channel allocations.