Adaptive notch filter for FM interference cancellation

• US 5,307,517 A
• Filed: 10/17/1991
• Issued: 04/26/1994
• Est. Priority Date: 10/17/1991
• Status: Expired due to Fees

• Current Assignee: David A Rich
• Original Assignee: David A Rich
• Inventors: Rich, David A.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): WISLER, MARK
• Application Number:
  US07/778,501
• Time in Patent Office: 922 Days
• Field of Search: 455/295, 455/296, 455/303, 455/305, 455/306, 455/307, 455/340, 455/304, 455/208, 455/209, 375/99, 375/103, 331/2, 333/176, 333/166
• US Class Current: 455/306
• CPC Class Codes: H04B 1/1036 with automatic suppression of narrow band noise or interference, e.g. by using tuneable notch filters H04B1/123 takes precedence; filter circuits H03H
• CPC Class Codes: H04B 1/123 using adaptive balancing or compensation means adaptive filter circuits and algorithms H03H
• CPC Class Codes: H04B 2001/1063 using a notch filter

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.

Claims

1. An FM interference cancellation system comprising:

an input signal terminal for receiving an input signal;

an adaptive notch filter for filtering an input signal coupled to said input signal terminal, in accordance with a control signal input, said adaptive notch filter having a respective input terminal and output terminal, said adaptive notch filter comprising;

means for translating said input signal to form a translated input signal in the frequency domain in accordance with said input control signal;

means for baseband filtering said translated input signal to remove a band of baseband signal frequencies between DC and a given baseband frequency, and to remove the DC component, to form a baseband filtered signal; and

means for translating said baseband filtered signal in the frequency domain in accordance with said input control signal;

a phase locked loop coupled to said input signal terminal for tracking the frequency of an interfering FM signal, said phase locked loop providing an output frequency control signal coupled to said control signal input of said adaptive notch filter; and

means coupled to said output terminal of said adaptive notch filter for demodulating

a desired,

whereby interference caused by said interfering FM signal with said desired signal is substantially canceled.

3. An FM interference cancellation system in accordance with claim 1, wherein said adaptive notch filter includes an input terminal, an output terminal, and a control input terminal, said adaptive notch filter comprising:

a first mixer coupled to said input terminal and to said control input terminal;

a first baseband filter coupled to said first mixer, for filtering out a band of baseband signal frequencies between DC and a given baseband frequency, and to remove the DC component;

a second mixer coupled to said first baseband filter and to said control input terminal;

a 90 degree phase shift circuit having an input terminal coupled to said control terminal, and an output terminal;

a third mixer coupled to said input terminal and to said output terminal of said 90 degree phase shift circuit;

a second baseband filter coupled to said third mixer, for filtering out a band of baseband signal frequencies between DC and a given baseband frequency, and to remove the DC component;

a fourth mixer coupled to said second baseband filter and to said output terminal of said 90 degree phase shift circuit; and

an adder coupled to the output of said second mixer and the output of said fourth mixer, the output of said adder coupled to said output terminal of said adaptive notch filter.

4. An FM interference cancellation system in accordance with claim 1, wherein said adaptive notch filter includes an input terminal, an output terminal, and a control input terminal, said adaptive notch filter comprising:

a first mixer coupled to said input terminal and to said control input terminal;

a first baseband filter coupled to said first mixer, for filtering out a band of baseband signal frequencies between DC and a given baseband frequency, and to remove the DC component;

a second mixer coupled to said first baseband filter and to said control input terminal;

a 90 degree phase shift circuit having an input terminal coupled to said control input terminal, and an output terminal;

a third mixer coupled to said input terminal and to said output terminal of said 90 degree phase shift circuit;

a second baseband filter coupled to said third mixer, for filtering out a band of baseband signal frequencies between DC and a given baseband frequency, and to remove the DC component;

a fourth mixer coupled to said second baseband filter and to said output terminal of said 90 degree phase shift circuit; and

a comparator coupled to the output of said second mixer and the output of said fourth mixer, the output of said comparator coupled to said output terminal of said

5. An adaptive notch filter in accordance with claim 1, wherein each of said mixers comprise a balanced modulator.

6. An adaptive notch filter in accordance with claim 1, wherein said 90 degree shift circuit comprises a synchronous digital divider circuit.

2. An FM interference cancellation system comprising:

an input signal terminal for receiving an input signal;

first and second adaptive notch filters for filtering said input signal coupled to said input signal terminal, in accordance with respective input control signals, and having respective output terminals, each of said first and second adaptive notch filters comprising;

means for translating said input signal to form a translated input signal in the frequency domain in accordance with said respective input control signal;

means for baseband filtering said translated input signal between DC and a given baseband frequency, and to remove the DC component, to form a baseband filtered signal to remove a band of baseband signal frequencies; and

means for translating said baseband filtered signal in the frequency domain in accordance with said respective input control signal;

a first phase locked loop coupled to said output terminal of said second adaptive notch filter for tracking the frequency of a first FM signal, said first phase locked loop providing an output frequency control signal coupled to said input control signal of said first adaptive notch filter; and

a second phase locked loop coupled to said output terminal of said first adaptive notch filter for tracking the frequency of a second FM signal, said second phase locked loop providing an output frequency control signal coupled to said input control signal of said second adaptive notch filter, said second FM signal being stronger than said first FM signal,

whereby interference caused by said first FM signal to said second FM signal is substantially canceled by said first adaptive notch filter, and interference caused by said second FM signal to said first FM signal is substantially canceled by said second adaptive notch filter.

7. An FM interference cancellation system in accordance with claim 2, wherein said adaptive notch filter includes an input terminal, an output terminal, and a control input terminal, said adaptive notch filter comprising:

a first mixer coupled to said input terminal and to said control input terminal;

a first baseband filter coupled to said first mixer, for filtering out a band of baseband signal frequencies between DC and a given baseband frequency, and to remove the DC component;

a second mixer coupled to said first baseband filter and to said control input terminal;

a 90 degree phase shift circuit having an input terminal coupled to said control terminal, and an output terminal;

a third mixer coupled to said input terminal and to said output terminal of said 90 degree phase shift circuit;

a second baseband filter coupled to said third mixer, for filtering out a band of baseband signal frequencies between DC and a given baseband frequency, and to remove the DC component;

a fourth mixer coupled to said second baseband filter and to said output terminal of said 90 degree phase shift circuit; and

an adder coupled to the output of said second mixer and the output of said fourth mixer, the output of said adder coupled to said output terminal of said adaptive notch filter.

8. An FM interference cancellation system in accordance with claim 2, wherein said adaptive notch filter includes an input terminal, an output terminal, and a control input terminal, said adaptive notch filter comprising:

a first mixer coupled to said input terminal and to said control input terminal;

a first baseband filter coupled to said first mixer, for filtering out a band of baseband signal frequencies between DC and a given baseband frequency, and to remove the DC component;

a second mixer coupled to said first baseband filter and to said control input terminal;

a 90 degree phase shift circuit having an input terminal coupled to said control input terminal, and an output terminal;

a third mixer coupled to said input terminal and to said output terminal of said 90 degree phase shift circuit;

a second baseband filter coupled to said third mixer, for filtering out a band of baseband signal frequencies between DC and a given baseband frequency, and to remove the DC component;

a fourth mixer coupled to said second baseband filter and to said output terminal of said 90 degree phase shift circuit; and

a comparator coupled to the output of said second mixer and the output of said fourth mixer, the output of said comparator coupled to said output terminal of said adaptive notch filter.

9. An adaptive notch filter in accordance with claim 2, wherein each of said mixers comprise a balanced modulator.

10. An adaptive notch filter in accordance with claim 2, wherein said 90 degree phase shift circuit comprises a synchronous digital divider circuit.