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Abstract

A demodulator of the type employed in a 'zero-IF' system uses a local oscillator for providing quadrature output signals at the center frequency of a received signal to be demodulated. The demodulator has first and second mixers for separately mixing the received signal with the quadrature signals to provide a first and second output signal each in quadrature at the outputs of said mixers. These signals are low pass filtered. The demodulator includes third and fourth mixers with each mixer respectively receiving at an input one of the first and second low pass signals. At another input the mixers receive third and fourth signals. The third and fourth signals are derived from mixing a variable controlled oscillator (VCO) signal with local oscillator quadrature signals the frequency of said is independent of the center frequency of the received signal. The outputs of the third and fourth mixers are applied to the inputs of a difference amplifier, the output of which controls the frequency of the VCO in an automatic frequency control mode (AFC) to cause the output of the difference amplifier to provide demodulation of an FM signal. Additional mixers are provided which mix the first and second low pass signals respectively with the fourth and third signals, with the signals appearing at the outputs of such additional mixers being summed to provide demonstration of an AM signal independently of the FM demodulation.

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