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Frequency synthesizers based on sub-harmonic injection-locked oscillators can reduce power and area effectively. However, they suffer from unwanted side-bands caused by undesirable harmonics in the injection pulse. This paper presents a pulse shaping technique that reduces these undesirable harmonics in the injection pulse and produces low-spur synthesized output. The robustness of the proposed technique and the impact of circuit non-idealities are also discussed in depth. A test chip tailored to the first band group of WiMedia UWB is fabricated in 0.13-μm CMOS technology to verify the effectiveness of this technique. Experimental results show that the proposed pulse shaping attenuates the spurs by 22 dB and suppresses them below - 47 dBc. The prototype achieves a switching time of less than 4.7 ns and phase noise of -122 dBc/Hz@1 MHz offset at 4.488 GHz. The chip occupies an active area of 0.15 mm2 and draws only 18 mA from a 1.2-V supply.