Narco Scientific Industries, Inc.

John M Ferrara

Frank Patterson Smith, 2nd

A circuit which allows the operation of a VOR receiver in the presence of unwanted interference signals without sacrificing a fast response to omnirange bearing selector changes. A switching-type synchronous filter is used to filter the 30 Hz AM channel signal in the receiver. The narrow bandwidth of this synchronous filter rejects unwanted interference signals without introducing phase shift into the desired signal. The switching signal for the synchronous filter is generated by the FM channel in such a manner that the switching filter center frequency follows the input frequency. The use of a synchronous filter allows rejection of interfering AM signals that lie outside a narrow pass band, while allowing the receiver to follow variations in frequency of the transmitted signal.

Synchronous filter for VOR systems

A known reference signal is applied to a zero crossing detector and normal and inverted outputs from the zero crossing detector are applied to the input control terminals of a pair of bilateral switches. A measured signal of the same frequency but not necessarily of the same phase is simultaneously applied to the input terminals of the bilateral switches. The resultant output of the bilateral switches is connected to a parallel RC network. The capacitor acquires a charge proportional to the deviation from 90.degree., which is applied to a differential amplifier. The output of the differential amplifier is used to drive a meter which indicates the phase relationship of the two signals. The capacitor used to accumulate the net charge is ungrounded to eliminate errors resulting from unequal currents to ground.

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Hardley, PA, United States of America

John M Ferrara