High performance fractional-N frequency synthesizer using a two-point channel selection technique

In this paper, we present a 2.4 GHz fractional-N frequency synthesizer with high frequency resolution, low noise, and fast channel switching speed. The performance of the fractional-N frequency synthesizer was dramatically improved using a two-point channel selection (TPCS) technique. The TPCS technique uses two main paths in the synthesizer for channel selection. One is a traditional channel selection path that selects the channel via a delta-sigma modulator (DSM) and a frequency divider. The other one uses the same DSM and a digital-to-analog converter (DAC) to offset the phase error at the voltage tuning input of a voltage-controlled oscillator (VCO). The combination of these two channel selection paths not only reduces the voltage charging time, but also cancels the quantization noise of the DSM. Using this technique, we successfully developed a low noise and high speed fractional-N frequency synthesizer. The results show that the phase noise is improved by 5 to 10 dB improved, and the channel switching time is reduced forty-fold, compared to synthesizers developed using conventional methods.