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.

Abstract: An approach for demodulating a frequency-modulated signal involves processing a frequency-modulated signal with a phase shifter network to provide a demodulated signal that has a relatively constant amplitude around the center frequency of the frequency-modulated signal and that exhibits a relatively linear phase change over an operational frequency range. Embodiments of the invention include a phase shifter network, using N number of cascaded all-pass filters, that receives as an input a limited amplitude signal and outputs a phase-shifted limited amplitude signal that is mixed with the limited amplitude signal. The phase shifter network may also comprise a low-pass bessel filter.

Abstract: A direct conversion receiver for FSK modulated digital signals has two quadrature signal paths (I, Q) and an a.f.c. loop for controlling a local frequency generator. A control signal for the a.f.c. loop is produced by an offset frequency detector, and is in the form of a unipolar series of pulses having an average value which is proportional to the offset of the local frequency with respect to the carrier frequency of the signal being received. The offset frequency detector derives such control signal based on the difference between phase-shifted cross-products of the signals in the I and Q paths. This achieves fast a.f.c. loop response, improving the performance of the receiver, and permits a simpler loop filter for achieving the requisite signal averaging. The receiver also includes an out-of-range detector for detecting when the local frequency is outside a predetermined window with respect to the carrier frequency of the received signal, in which case the a.f.c.

Abstract: A direct conversion receiver, such as a paging receiver includes a local oscillator for mixing down to baseband a received radio frequency signal having a carrier frequency which is digitally modulated (FSK). To provide automatic frequency control, instead of using a control loop, the present invention includes means which at intervals evaluates the recovered digital signal to determine whether a predetermined quality thereof is within a selected D/A conversion range. A microcontroller including a D/A converter generates a control signal for the local oscillator to maintain the oscillator at a frequency such that the signal quality remains within the selected conversion range.