Patent ID: 6038268
Filing Date: 2000-03-14
Classification: H04L

Abstract:
A direct conversion FSK signal radio receiver comprising:converter means for directly converting a received FSK signal into a base band signal;local oscillator means for generating an in-phase local carrier;shift means for producing an orthogonal phase carrier on the basis of said in-phase local carrier;switching means coupled to said converter means, said converter means capable of receiving said local carriers of in-phase and orthogonal phase, as well as ground potential, said switching means sequentially switching and supplying one of said in-phase, said orthogonal phase, or said ground potential to said converter means per switch of said switching means;variable gain amplifier means for amplifying the base band signal provided from said converter means, providing an amplified output;distributing means to distribute said amplified output into three outputs;said switching means and said distributing means both being synchronously switched together at a rate which is higher than that of said base band signal;said three outputs from said distributing means each being filtered respectively to provide both base band signals on said in-phase and said orthogonal phase and a detected DC offset component integrated through the switching, converting, amplifying and distributing stages, respectively;said DC offset component being negatively feedbacked to any one of said switching, converting, amplifying and distributing stages to correct any DC offset;negative feedback means to convert said output of said variable gain amplifier means into signal power which is negatively feedbacked to control the gain of said variable gain amplifier means making said both base band signals of in-phase and said orthogonal phase accurate;said base band signals of in-phase and orthogonal phase being multiplied by their differentiated base band signals of orthogonal phase and in-phase, respectively, providing respective multiplied results;said both multiplied results being subtracted for frequency discrimination with linear characteristics, providing a subtracted result;said subtracted result being compared with a reference level to provide a demodulated output of the FSK signal, which is reformed into a two-value logical voltage level; andsaid subtracted result and said demodulated output of the FSK signal being subtracted to provide a difference, said difference is transformed into a control signal representing any frequency error between the center frequency of said input FSK signal and said in-phase local carrier, said control signal controlling said local oscillator means so that an AFC operation is carried out irrespective of the code contents of modulation of said FSK signal.