Mismatch Shaping Techniques to Linearize Charge Pump Errors in Fractional-N PLLs

A recent charge pump linearization technique demonstrated 8 dB reduction in spurious tones caused by charge pump current mismatch in delta-sigma fractional- N phase locked loops. In this paper, two purely digital mismatch shaping techniques are proposed to modify and further improve the charge pump linearization technique. Both the techniques suppress spurious tones by randomizing the residual charge pump mismatch error power. The second technique further spectrally shapes the residual charge pump mismatch errors to suppress close-in phase noise. No spurs are observed and -120 dBc/Hz phase noise is achieved at frequency offsets lower than 10 kHz in simulation. A theoretical proof of the spectral shaping of the charge pump mismatch error power is also presented.