This paper describes a quantization noise reduction method in DeltaSigma fractional-N synthesizer design based on a semidigital approach. By employing a phase shifting technique, a low power hybrid finite impulse response (FIR) filtering is realized which is suitable for RF applications. A prototype fractional-N synthesizer is implemented in 180 nm CMOS for WCDMA/HSDPA applications. Experimental results show that the proposed method can effectively suppress out-of-band phase noise to meet the phase noise mask requirements in various RF applications.