This paper presents an in-band noise reduction method for 螖危 fractional-N bang-bang phase locked loops (BBPLLs) by using a nested integer-N BBPLL in the feedback path that works as a phase-domain low-pass filter (PDLPF). A prototype 5.4GHz 螖危 fractional-N BBPLL is implemented in 65nm CMOS. The proposed 螖危 fractional-N BBPLL achieves the in-band noise reduction of 19dB when the PDLPF is enabled. Experimental results show that the PDLPF method is useful for the 螖危 fractional-N BBPLL not only to suppress the out-of-band noise but also to mitigate the in-band noise degradation.