Reduction of Phase Noise in Fractional-N Frequency Synthesizer Using Self-Injection Locking Loop

Delta-sigma modulation-based fractional-N frequency synthesizers generally suffer from quantization noise, which causes them to exhibit poor phase-noise performance outside their loop bandwidth. In this work, a self-injection locking (SIL) loop is utilized in a fractional-N frequency synthesizer. The SIL loop can reduce the noise that is generated by the injection-locked oscillator (ILO), the reference signal, and the delta-sigma modulator, greatly improving the phase-noise performance of the fractional-N frequency synthesizer. Experimental results demonstrate that the phase noise in the developed 2.4-GHz fractional-N frequency synthesizer is reduced by up to 36.5 dB using a 17-ns SIL loop. This great reduction of phase noise enables the developed synthesizer to have a very low phase noise of -110, -136, and -145 dBc/Hz at the offset frequencies of 100 kHz, 1 MHz, and 10 MHz, respectively.