A 3 to 5GHz fast hopping frequency synthesizer for multi-band OFDM UWB applications is designed using 0.13μm CMOS technology. The frequency synthesizer operates in the "Mode 1" bands (centered at 3432MHz 3960MHz, and 4488MHz). Single-sideband mixer up converts or down-converts the 528MHz quadrature signals of ring VCO mixing 3960MHz LC VCO signals. The sideband suppression is less than -40dBc for 3432MHs and -32dBc for 4488MHz and band switching time from 4488MHz to 3432MHz is measured about 1.5ns. Phase noise of the LC VCO and the ring VCO is about -115dBc/Hz and 93dBc/Hz at 1MHz offset frequency, respectively. The total power consumption of the proposed frequency synthesizer is about 62mW with a 1.5V supply voltage.