FREQUENCY MODULATED SYNTHESIZER USING LOW FREQUENCY OFFSET MIXED VCO

摘要

An improved frequency synthesizer (100) capable of producing true DC frequency modulation without the need to DC fre-quency modulate the reference oscillator. The synthesizer (100) includes a phase comparator (104) which compares the referencesignal (102) and the feedback signal (112). The output of said phase comparator (104) is filtered and coupled to the main VCO(108). A balanced image mixer (200) mixes the two signals from the main VCO (108) and an offset VCO (300). The offset VCO(300) is a low frequency (e.g. 100 to 300 KHz) integrated offset VCO (308) whose center frequency is controlled accurately, bymeans of matching to a VCO (306) which is controlled by a feedback loop, and by using an image balanced mixer (200) to reducethe image response to substantially reduce or eliminate the filtering required at the mixer output. The configuration of the offsetmixer within the synthesizer loop, any residual spurious content due to imperfect image balance or carrier feedthrough of themixer is attenuated by the loop response. The output signal of the mixer (200) is looped back to the phase comparator (104) afterbeing divided by N through a divider (114).