Apparatus for frequency correction in a frequency synthesizer

摘要

The present invention provides apparatus and a method for determining and correcting frequency offset for use in a frequency synthesizer to provide external control and DC frequency modulation of the synthesizer output frequency. The technique provides DCFM capability by unlocking a phase-lock loop (PLL) which provides the synthesizer reference frequency and making the PLL oscillator (VCO) tune line available for external inputs. Utilizing a frequency counting circuit, the exact output frequency of the VCO is determined. The desired VCO frequency is compared with the actual VCO frequency to derive an error signal proportional to any offset in the VCO frequency resulting from unlocking the PLL. The error signal is then utilized to compensate (the synthesizer output signal) for any frequency change as a result of unlocking the PLL.