Phase compensation of intermodulation-induced spurs in fractional-N frequency synthesisers

Intermodulation effect occurs when the nonlinearity of a device or system with multiple frequencies causes undesired outputs at other frequencies. Intermodulation effect may cause high amplitude spurs on some dedicated channels in addition to the spurs introduced by nonlinear distortion in the modulator of fractional-N synthesisers, which severely degrade the signal-to-noise ratio. In this study, a phase compensation technique is proposed to suppress at least part of these fractional spurs - in particular the most dominant sidebands. A small amount of controlled amplitude and phase voltage controlled oscillator (VCO) signal is coupled onto the phase/frequency detector input to cancel-out the potential intermodulation-induced spurious component. A prototype of fractional-N frequency synthesiser based on the proposed technique has been developed. The synthesiser has measured -113 dBc/Hz in-band phase noise benefiting from the high speed sigma-delta noise shaper. A modest degrees of VCO to phase detector cross-coupling suppressed the spurious components by 18 dB. A variety of measured results are shown to demonstrate a substantial improvement in spectra purity.