In this work, a novel self-injection-locked (SIL) loop is incorporated into a CMOS frequency synthesizer for fast frequency switching. The proposed SIL loop is based on an injection-locked oscillator (ILO) which behaves as an active delay unit to suppress the excess phase noise produced owing to the large phase-locked loop (PLL) bandwidth. Consequently, the synthesizer with the ILO-based SIL loop can achieve an ultrashort settling time of 2.3 μs and simultaneously a low phase noise of −123 dBc/Hz at 1 MHz offset frequency.