FREQUENCY SYNTHESIZER

摘要

Frequency synthesizers are used for down conversion of RF signals in a lot of applications such as digital and analog radio and television receivers, car radios id. By combining the voltage-controlled oscillator of the phase locked loop demodulator with the voltage controlled oscillator of the frequency synthesizer a major cost reduction can be achieved but measures have to be made to prevent that the two PLLs try to lock the same VCO to different frequencies. The invention provides a frequency synthesizer (FS3) whereby switching means (33) between the phase frequency comparator (31) and the charging means (35) are controlled by using a frequency window detector (371, 373, 375). The phase frequency detector (31) is disconnected from said charging means (35) when it is detected that the frequency difference between the frequency synthesizer input signals is lower than a predetermined value, whereafter said voltage-controlled oscillator (VC03) is only driven by an AFC signal.