A sequential sweeping receiver with fast switching time has been recently shown to be much more effective at spectrum scanning for cognitive radio applications than a compressed sensing receiver under similar hardware constraints. This paper presents a design of such rapid spectrum sweeping scanner. The design leverages the harmonics of hard-switching current-driven passive mixer to sense an arbitrary subband in the spectrum and downconvert it to baseband without the need for a wide-range tunable local oscillator (LO). The design operates as a frequency multiplexer with fast transient switching. The proposed design is implemented in 65nm CMOS technology and achieves 4.8-dB minimum noise figure and total power consumption of 38 mW from 1V supply voltage. The switching time to select a 62.5 MHz subband among 16 in a 1-GHz RF span is approximately 30 nsec.