This　paper　precented　a　phase-locked　loop　frequency　synthesizer　with　almost　constant　loop　bandwidth.　In　order　to　reduce　the　loop　bandwidth　variations　caused　by　the　LC-VCO　gain,　a　calibration　system　is　introduced,　which　sets　the　charge-pump　current　to　be　inversely　proportional　to　the　square　of　the　divider　ratio.　The　frequency　synthesizer　with　this　technique　was　fabricated　in　0.13μm　CMOS　process.　The　measured　results　show　that　this　method　keeps　the　loop　bandwidth　almost　a　constant　in　frequency　range　of　2.44G∼2.8GHz　while　consuming　less　power　with　better　in-band　phase　noise.　©　2016　IEEE.