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Abstract

An improved adaptive notch filter for removing undesired cochannel FM interference includes in-phase and quadrature signal processing paths in which the undesired FM signal is translated to zero frequency, i.e. to DC. In each of the in-phase and quadrature phase signal paths, a first multiplier translates the input frequency spectrum in a dynamic manner so that the frequency of the undesired FM signal is always centered on DC. The undesired signal, now at DC, is filtered out by a high pass filter with a sharp rolloff. The output of the high pass filter is remultiplied in a second multiplier to translate the desired FM signal back to the original position in the frequency spectrum. In order to eliminate an unwanted difference frequency remodulation signal at the output of the second multiplier, the quadrature phase signal path performs a parallel operation on the same input signal, but with a quadrature phase (90 degree phase shift) control signal. The output the in-phase and quadrature signal processing circuits are combined in an adder circuit to cancel the unwanted difference frequency remodulation signal.

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First Claim