DC offset compensation for zero if quadrature demodulator
AssigneePhilips Electronics North America Corporation
InventorRishi Mohindra
Life of this patent
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Abstract
The present invention provides dc offset compensation circuit (800) for a zero IF FM quadrature demodulator in an AMPS cellular mobile telephone system. A frequency offset is introduced to remove modulation inherent dc components from supervisory audio tones and Manchester codes from a modulated signal. The local oscillator (608) is operated at a frequency that is slightly different, e.g. 5 kHz from a frequency which would mix down the intermediate frequency signal to zero. A dc offset is produced with a control loop to eliminate fixed dc components caused by leakage from a local oscillator (608). After a given time, the control loop may be frozen and the frequency offset of the local oscillator (608) can be removed.
Claims20
A method of compensating for dc voltages in a quadrature demodulator, comprising:
- receiving a modulated signal with a center frequency f.sub.c ;
- splitting the modulated signal into a first path and a second path;
- mixing the modulated signal of the first path with a local oscillator signal to generate an I path signal and mixing the modulated signal of the second path with the local oscillator signal to produce a Q path signal, such that the I path signal is phase shifted from the Q path signal by approximately 90 degrees, and wherein the local oscillator signal is offset in frequency from the center frequency f.sub.c ;
- removing dc components from the mixed modulated signal;
- and
- demodulating the mixed modulated signal.
A system for compensating dc voltages in a quadrature demodulator, comprising:
- means for receiving a modulated signal with a center frequency f.sub.c ;
- means for splitting the modulated signal into a first path and a second path;
- means for mixing the modulated signal of the first path with a local oscillator signal to generate an I path signal and means for mixing the modulated signal of the second path with the local oscillator signal to produce a Q path signal, such that the I path signal is phase shifted from the Q path signal by approximately 90 degrees, and wherein the local oscillator signal is offset in frequency from the center frequency f.sub.c ;
- means for removing the dc components from the mixed modulated signal using a feedback network;
- and
- means for demodulating the mixed modulated signal.
A system for removing a dc offset from a modulated signal in a communication receiver, comprising:
- an input communication signal, having a modulated signal center frequency of f.sub.c, the input communication signal further comprising a dc offset component;
- an I signal path and a Q signal path, each path coupled to the input communication signal, wherein the I signal path is phase shifted from the Q signal path by approximately 90 degrees;
- a local oscillator coupled to the I signal path and to the Q signal path for mixing with the input communication signal, the local oscillator having an oscillation frequency that is offset from the modulated signal center frequency;
- a cancellation circuit coupled to the I signal path and to the Q signal path for removing the dc offset component from the mixed input communication signal;
- a demodulator coupled to the cancellation circuit for demodulating the mixed input communication signal.
Dependencies are inferred from the claim text.