A low-power tunable-band sub-harmonic direct-conversion receiver covering the whole Unlicensed National Information Infrastructure band is demonstrated using 0.18-μm CMOS technology. The RF band is selected by tuning varactors at the loads of the two-stage low-noise amplifier, while a wideband octet- phase generator is applied at the local oscillator (LO) port. The band tuning of both an LC tank and a transformer and the de- sign of an optimal transformer turn ratio are fully discussed in this paper. Vertical-NPN bipolar junction transistors in a standard CMOS process are used at the mixer switching core for excellent 1/f noise performance. As a result, the receiver achieves a 48/50 voltage gain and 4.5/4.8-dB noise figure with a 1/f noise corner of 70 kHz when the RF band is tuned to 5.2/5.8 GHz, respectively. The dc current consumption of the RF front-end (including the LO buffer) is 8.5 mA at a 1.8-V supply.