Interference detection and reduction

Claim 2

An interference reducing device: wherein a received signal path is branched into first and second lines, said first line having an interference detector for detecting a narrow band interference carrier mixed in a digital modulated signal; wherein said second line is branched into third and fourth lines, said third line having first and second mixers and a narrow band-elimination filter and a band-pass filter both connected between said first and second mixers, said narrow-band elimination filter having its attenuation at the center frequency set to a fixed value and said band-pass filter permitting the passage therethrough of only a desired frequency-converted signal band component; wherein means is provided whereby the output of a voltage-controlled oscillator whose oscillation frequency is controlled by the output of said interference detector is applied as local oscillator outputs of said first and second mixers; and wherein the center frequency for band elimination between the input and output of said third line is controlled by the output frequency of said voltage-controlled oscillator and the attenuation of the interference carrier by said band-elimination filter is limited to a fixed value.

Claim 3

An interference reducing device according to claim 1 or 2, wherein said interference detector is formed by a phase-lock loop which is synchronized with an input interference carrier, and wherein the elimination frequency of said interference reducing circuit is controlled by the output frequency of a voltage-controlled oscillator of said phase-lock loop.

Claim 4

An interference reducing circuit according to claim 1 or 2, wherein said interference detector is formed by a multiplexer composed of a plurality of narrow band-pass filters of different center frequencies and a circuit for detecting and converting each output of said multiplexer into a digital voltage value, and wherein an input signal band is divided by said multiplexer and a detected voltage value of each divided band is processed to thereby detect the interference carrier and control the oscillation frequency of said voltage-controlled oscillator.

Claim 5

An interference detector for a narrow-band interference eliminating circuit in which a narrow-band filter and a band-pass filter which permits the passage therethrough of only desired frequency-converted signal band components are connected between first and second mixers, and the output of a voltage-controlled oscillator, whose oscillation frequency is controlled by the output of an interference detecting circuit for detecting a narrow-band interference carrier mixed in a digital modulated signal input thereto as an input signal, is provided as the local oscillator output of each of said first and second mixers to thereby control the center frequency of the elimination band of said band-elimination filter, said interference detector comprising: a third mixer; a multiplexer employing a plurality of narrow band-pass filters; a circuit for detecting and converting the output of said multiplexer into a digital voltage value; a CPU for processing the detected digital voltage value; a D/A converter for controlling the oscillation frequency of said voltage-controlled oscillator with the output of said CPU; and means whereby said input signal is frequency-converted by applying it to said third mixer employing said voltage-controlled oscillator as its local oscillator and the output of said multiplexer having its center frequency set in said frequency-converted desired signal band is detected and converted into a digital voltage value for input into said CPU; wherein the oscillation frequency of said voltage-controlled oscillator is swept by a control signal from said CPU, digital voltage values of plural outputs of said multiplexer swept in a desired signal band are processed by said CPU, whereby the presence or absence of said interference carrier, its frequency and the direction of its change are decided to permit said interference carrier to be followed.

Claim 6

An interference wave detector according to claim 5, which further comprises a second line branched from a first input signal line, siad second line having a circuit made up of a fourth mixer, a second multiplexer employing a plurality of narrow band-pass filters of pass bandwidths larger than that of said narrow band-pass filter connected to said first line, a circuit for detecting and converting the output of said second multiplexer into a digital voltage value, a CPU for processing the detected digital voltage value, and a D/A converter for controlling the oscillation frequency of said second voltage-controlled oscillator with the output of said CPU; wherein the oscillation frequency of said second voltage-controlled oscillator is swept by a control signal from said second-mentioned CPU, the digital voltage value of the output of said second multiplexer swept in a desired signal band is processed by said CPU to decide the presence or absence of said interference carrier and its frequency, the oscillation frequency of the voltage-controlled of said first line is controlled by said CPU and digital voltage values of plural outputs of the multiplexer of said first line are processed by said CPU to decide the direction of change of the interference carrier frequency and follow it up, whereby said interference carrier can be quickly detected and the output frequency of said voltage-controlled oscillator of said first line following the interference carrier frequency with high accuracy can be output.

Claim 7

An interference detector according to claim 6, wherein the voltage-controlled oscillator of said first line is used as a local oscillator of said fourth mixer connected to said second line, the oscillation frequency of said voltage-controlled oscillator is swept with a control signal from said CPU, the digital voltage value of the output of said second multiplexer swept in a desired signal band is processed by said processor to decide the presence or absence of the interference carrier and its frequency, and after the detection of the interference carrier, the digital voltage value of the output of said multiplexer connected to said first line is processed by said CPU to decide the direction of change of the interference carrier frequency to be followed, whereby the interference can be quickly detected and the output frequency of said voltage-controlled oscillator connected to said first line following the interference carrier frequency with high accuracy can be output.