Base band signal offset correcting circuit for FSK receiving apparatus and method thereof

An FSK receiving apparatus with a base band signal offset correcting circuit for correcting a DC offset of a demodulated base band signal which is obtained by demodulating an FSK signal, comprising:

• integrating circuit for integrating said demodulated base band signal;
• offset detecting means for judging whether an output of said integrating circuit is or not within a predetermined range to thereby detect a DC offset component of said demodulated base band signal;
• counting means for incrementing or decrementing an output of said offset detecting means at predetermined timing to thereby update a count value of said counting means;
• reference voltage generating means for generating a reference voltage on the basis of said count value, said reference voltage being used in a comparator to perform signal judgement upon said demodulated base band signal;
• and
• initial integral value setting means for supplying said reference voltage generated the last time to said integrating circuit to thereby set said reference voltage as an initial output value of said integrating circuit;
• said base band signal offset correcting circuit comprising:
• frequency error judging means for comparing said count value of said counting means with a reference value to thereby detect a frequency error of said FSK signal;
• second counting means for incrementing or decrementing an output of said frequency error judging means at predetermined timing to thereby update a count value of said second counting means;
• and
• local oscillation frequency control means for controlling an oscillation frequency of a local oscillator in accordance with said count value of said second counting means.

A base band signal offset correcting method for correcting a DC offset of a demodulated base band signal which is obtained by demodulating an FSK signal, comprising:

• an integrating step for integrating said demodulated base band signal;
• an offset detecting step for judging whether an integral value of said demodulated base band signal is or not within a predetermined range to thereby detect a DC offset component of said demodulated base band signal;
• a counting step for incrementing or decrementing said DC offset component at predetermined timing to thereby update a count value;
• a reference voltage generating step for generating a reference voltage on the basis of said count value, said reference voltage being used in a comparator to perform signal judgement upon said demodulated base band signal;
• an initial integral value setting step for supplying said reference voltage generated the last time to an integrating circuit for carrying out said integrating step to thereby set said reference voltage as an initial output value of said integrating circuit;
• a frequency error judging step for comparing said count value in said counting step with a reference value to thereby detect a frequency error of said FSK signal;
• a second counting step for incrementing or decrementing said detected frequency error at predetermined timing to thereby update a count value;
• and
• a local oscillation frequency control step for controlling an oscillation frequency of a local oscillator in accordance with said count value in said second counting step.